Bread Staling:
Molecular Basis
and Control
J.A. Gray and J.N. Bemiller
ABSTRACT: The molecular basis of staling is examined by reviewing what is known about the components of wheat flour,
factors that affect staling rate, and the various mechanisms that have been proposed. The conclusion reached is that bread
staling is a complex phenomenon in which multiple mechanisms operate. Polymer crystallizations with the formation of
supermolecular structures are certainly involved. The most plausible hypothesis is that retrogradation of amylopectin
occurs, and because water molecules are incorporated into the crystallites, the distribution of water is shifted from gluten
to starch/amylopectin, thereby changing the nature of the gluten network. The role of additives may be to change the
nature of starch protein molecules, to function as plasticizers, and/or to retard the redistribution of water between
components. Nothing more definite can be concluded at this time.
Introduction
Although it has been studied for more than a century and a
half, bread staling has not been eliminated and remains responsible for huge economic losses to both the baking industry and the
consumer. Bechtel and others (1953) defined staling as “a term
which indicates decreasing consumer acceptance of bakery products caused by changes in crumb other than those resulting from
the action of spoilage organisms”. While an American Association
of Cereal Chemists Approved Method (AACC Method 74-30;
AACC 2000) quantifies staling organoleptically, many researchers
use the 1953 definition as a general definition and describe specific components of the complex staling process with specific
terms such as crumb firming, crust staling, and organoleptic staling (Kulp and Ponte 1981). In fact, the most widely used indicator
of staling is measurement of the increase in crumb firmness (see
“Rheological methods: Uniaxial compression” section), which is
the attribute most commonly recognized by the consumer. In this
review, the term “bread staling” is used to refer to the phenomenon of “crumb firming” in white pan bread.
Bread is an unstable, elastic, solid foam, the solid part of which
contains a continuous phase composed in part of an elastic network of cross-linked gluten molecules and in part of leached
starch polymer molecules, primarily amylose, both uncomplexed
and complexed with polar lipid molecules, and a discontinuous
phase of entrapped, gelatinized, swollen, deformed (wheat) starch
granules. Neither the bread system nor the staling process is understood well at the molecular level. Even simple bread dough
formulations contain several ingredients, which themselves may
contain several components, each of which may undergo changes during the breadmaking process and during aging of the final
product. And just as bread is a complex, heterogeneous system,
the staling phenomenon seems to be complex, because investigation of hypotheses involving changes in 1 or 2 components have
failed to fully explained the process.
Because the literature on bread staling is so extensive, any re© 2003 Institute of Food Technologists
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view of bread staling confined to a limited space cannot discuss
all available information, hypotheses, or conclusions; nor can it
give in-depth treatment to the aspects covered. It is believed, however, that most important pieces of known information, concepts,
principles, hypotheses, and conclusions are presented here.
Several previous reviews on staling (the process, the mechanism, its measurement, and factors that affect it) have appeared
(referred to elsewhere and in the references), and 2 books (Hebeda and Zobel 1996; Chinachoti and Vodovotz 2000) are available
for a more thorough treatment. Discussions in the literature referenced in this review will lead readers to additional information. A
brief review not elsewhere referenced in this review is that by
Guilbot and Godon (1984).
Physical and mechanical mixing, chemical reactions (including
enzyme-catalyzed reactions), and thermal effects (baking time and
temperature) are factors that influence the nature and properties of
the final product. This review focuses on antistaling agents, using
what is known about the mechanism of staling and factors that affect staling rate as a basis for the discussion. It also focuses on
crumb staling, because crumb staling is of much greater concern
to the consumer than is crust staling and has been studied more.
Four things are called to the attention of the reader before beginning: (1) Experimental work done on staling to date has involved looking for correlations between staling (by whatever definition and measurement employed) and a change in the formulation or process, but correlations do not necessarily prove a direct
cause-and-effect relationship; for example, addition of a surfactant
known to form a complex with amylose may increase shelf life,
but that does not necessarily mean that amylose complexation is
responsible for the increase in shelf life. The critical effect could
be on the structure of water, for example. (2) There is much information on the effects of various additives and conditions on
starch gelation and retrogradation and complex formation in dilute and concentrated starch pastes. For the most part, that literature is neither presented nor discussed, even though the mecha-
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nisms of retrogradation in concentrated amylopectin gels and
bread crumb are believed to be very similar, if not identical (Slade
and Levine 1987.) For a review of this literature as it pertains to
staling and that of the crystal structures in bread, the reader is referred to the review of Zobel and Kulp (1996). (3) When an
amount of an additive is stated, it is a percentage of the weight of
flour. (4) Abbreviations used include CP MAS (cross-polarization
magic-angle spinning), DSC (differential scanning calorimetry),
DTA (differential thermal analysis), MRI (magnetic resonance imaging), NMR (nuclear magnetic resonance), and Tg (glass transition temperature).
Molecular Basis of Staling
Components of wheat flour
To understand the mechanism of staling in breads, it is important to understand the natures of the major components that make
up the system. Relationships of these components to staling are
described in Section 2.2. The role of water and additives in staling
are discussed in Sections 3 and 4.
A typical bread formula consists of the following ingredients:
flour (wheat), water, sugar, shortening, nonfat dried milk (or a substitute), salt, yeast, malt, a dough strengthener, a crumb softener, a
mold inhibitor (sodium propionate), and an oxidant (Hoseney
and Seib 1978). Wheat flour consists primarily of gluten, starch,
and “pentosans” (primarily arabinoxylans), all of which are important contributors to the characteristics of the process and the final
product. Native flour lipids play an important role in breadmaking
(Morrison 1976), especially in their interaction with added shortening (Rogers and others 1988). Wheat flour has considerable ␣amylase activity and a minor amount of ␣-amylase activity.
States of the starch, gluten, and polar lipids in the 3 main stages
in the life of aged bread are outlined in Table 1.
Protein. Hydrated gluten is the continuous phase of wheat flour
doughs (Ponte and Faubion 1985; Davies 1986). During baking,
gluten is denatured, and protein-protein crosslinking occurs via
formation of disulfide bonds (Schofield 1986). The resulting network, combined with partially gelatinized starch granules, is most
certainly responsible for the semirigid structure of baked products
(Blanshard 1988; Hoseney 1989).
Starch. Wheat flour contains 84 to 88% (db) starch. During
baking of bread dough, the starch granules are generally gelatinized (Table 1, footnote c), but little else other than restricted
swelling followed by collapse happens to them because of the
limited amount of water present in the dough system (Schoch
1965), so deformed wheat starch granules can be isolated from
the crumb (Hoseney and others 1978). [Note: When starch granules are heated in excess water, granules swell and some portion
of the amylose diffuses from the granules, concentrates in the interstitial water between granules, and undergoes retrogradation.
The small amount of amylose that leaches from granules during
baking in the limited moisture system of bread dough retrogrades
upon cooling and rapidly becomes unextractable (Kim and
D’Appolonia 1977b,c); so even if amylose does leach from granules, by the time bread has completely cooled, any interstitial
amylose will have retrograded (that is, become insoluble) and is
unlikely, therefore, to play a major role in subsequent staling
events.] Even in the presence of excess water, monoglycerides
block the leaching of amylose molecules (Schoch 1965; see “Surface-active lipids: Surfactants” section), so it can be assumed that
other surfactants would act in the same way, especially in the limited moisture system of bread. Therefore, freshly baked and
cooled bread is an elastic system containing swollen wheat starch
granules that are still largely intact, but may be deformed.
On the other hand, observations made with transmission elec2
tron microscopy, led Bechtel and others (1978) to conclude that,
after baking, most starch granules were destroyed and most starch
molecules were part of the continuous phase, but separate from
protein strands.
Nonstarch polysaccharides. Arabinoxylans and arabinogalactans (arabinogalactan-proteins) are the “pentosans” (more properly pentoglycans) of wheat flour. Arabinoxylans are divided into 2
classes (“water-soluble” and “water-insoluble”) and have been
much more extensively studied than have the arabinogalactans
(Loosveld and others 1997), because they are present in greater
concentrations and are believed to play a more important role in
both the preparation and the shelf-life of bakery products. Both
classes of arabinoxylans of hard wheat flours have been investigated with regards to structure (Izydorczyk and others 1991; Izydorczyk and Biliaderis 1995, 2000) and to differences in structure
as a function of cultivars (Izydorczyk and others 1991; Cleemput
and others 1993; Izydorczyk and Biliaderis 1993; Rattan and others 1994). Their influence on breadmaking and bread quality is
still being debated (see “Mechanisms of staling: Role of pentosans” section).
Mechanism of staling
Attention is called to another review on the mechanism of staling (Schiraldi and Fessas 2001). Bread staling falls into 2 categories: crust staling and crumb staling. Crust staling is generally
caused by moisture transfer from the crumb to the crust (Lin and
Lineback 1990), resulting in a soft, leathery texture and is generally less objectionable than is crumb staling (Newbold 1976).
Crumb staling is more complex, more important, and less understood. The firmness of bread varies with position within a loaf,
with maximum firmness occurring in the central portion of the
crumb (Short and Roberts 1971).
The key hindrance to development of a preventive strategy for
bread staling is the failure to understand the mechanism of the
process. Many investigations have examined the phenomenon of
crumb-firming, and many theories have been proposed and discussed in previous reviews (Herz 1965; Willhoft 1973; Zobel
1973; Maga 1975; Knightly 1977; Kulp and Ponte 1981; Zobel
and Kulp 1996). A cursory overview of the major theories on the
subject is presented here.
Amylopectin retrogradation. Katz (1928) proposed that starch
polymers retrogradation was responsible for staling of bread because his x-ray diffraction patterns of fresh bread were similar to
those of freshly gelatinized wheat starch, while the patterns of
stale bread were similar to those of retrograded starch. This finding led to the hypothesis that a gradual change in the starch components from amorphous to crystalline forms is important to the
staling process. Hellman and others (1954) provided evidence
that the rate of development of crystallinity in starch gels was similar to the rate of bread firming; but Dragsdorf and Varriano-Marston (1980) obtained evidence that the degree of crystallinity of
bread crumb was inversely related to its firmness and, therefore,
concluded that starch crystallization and bread firming were separate processes.
Vodovotz and others (2002) detected no increase in molecular
rigidity, that is, decrease in molecular mobility, in an aged bread
sample (proton cross-relaxation NMR spectroscopy) that was concurrent with an increase in the amylopectin retrogradation endotherm (DSC). They concluded that “differences in molecular mobility could not be, therefore, due to recrystallized amylopectin and
may be attributed to the role of gluten [see “Mechanisms of staling: Role of flour protein” section] and/or redistribution of water
[see “Moisture migration: Moisture redistribution among components” section] in the amorphous regions of the samples”.
Whether the fraction of starch that contributes to bread firming
is amylose or amylopectin also has been debated. The linear,
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Bread staling. . .
Table 1—States of critical components in various stages in the life of breada
Stage
Starch
Gluten
Polar lipid
Dough
Hydrated, intact granules.
Apb partially crystalline.
Amb amorphous
Hydrated. In the form of
fibrils with adhering starch
granules in a continuous network.
Free. Perhaps some
protein-lipid interactions.
Fresh-baked,
but cooled,
bread
Granules in a spectrum of states.
Some rather intact. Most gelatinizedc
and deformed/collapsed.
Denatured. Crosslinked. Possible
formation of starch-gluten associations
(starch-gluten fibrils) during baking.
Some complexed with Am
(inside and outside of granules).
Some free. Possible protein-lipid
interactions (Sections 2.2.6, 4.2).
Loss of water of hydration from gluten
network via transfer to starchd, which
enables crystallization of Ap (Section 3.2).
Unchanged from fresh-baked bread
(?)
Starch-starch interactions both
within and between granules
Double-helical structure of Ap at least
partially lost. Perhaps some Ap molecules
partially or completely outside of granules
(Section 2.2.1).
Some Am partially or completely leached
from granules, putting some of it in the
continuous phase, where it is largely
insoluble. Some complexed with polar
lipid molecules. (Sections 2.2.2, 4.2).
Aged bread
Retrograded Ap inside gelatinized
granules. Perhaps some outside of
granules (Sections 2.2.1, 4.1.1).
Am retrograded. Some complexed with
lipid. Probably little changed from freshbaked bread (Sections 2.2.2, 4.1.1).
a Based on best evidence available. Other views have been stated; see discussion.
bAp = amylopectin, Am = amylose
cGelatinization is the disruption of molecular order within starch granules as they are heated in the presence of water (Atwell and others 1988).
d Both macro-and microscopic redistribution of water occurs during aging.
more readily retrograded fraction, amylose, was suspected first
(see “Mechanisms of staling: Role of amylose” section). Evidence
from Katz (1928) suggested formation of side-by-side associations
of linear starch molecules in the B-type x-ray patterns of staled
bread and retrograded starch. Hixon (1943) speculated that, if a
waxy wheat variety were available, then bread made from that
flour might not stale since it would be essentially void of amylose.
Alsberg (1927, 1928) pointed out the well-known fact that heating stale bread above 50 °C can restore the loaf to its original
freshness. Since retrograded amylose will not melt at this temperature (Knightly 1977), amylopectin was suggested to be the fraction
of starch responsible for staling. Supporting evidence was presented when bread prepared from a synthetic flour composed of
waxy maize starch and nondevitalized gluten exhibited a normal
tendency to stale (Noznick and others 1946). Further, Schoch and
French (1947) found that the water-soluble material that could be
leached from bread crumb at 30 °C was predominantly amylopectin. They hypothesized that progressive spontaneous aggregations of amylopectin molecules was responsible for bread firming. Furthermore, they suspected that the contribution of the amylose fraction to staling was negligible, since they believed it to be
retrograded/insolubilized during cooling. The important role of
amylopectin in starch retrogradation was confirmed by calorimetry (Russell 1983a, b).
However, Hoseney and Miller (1998) have pointed out that
stale bread must be heated to about 100 ºC before its compressibility approaches that of fresh bread (Ghiasi and others 1984)
and that, since retrograded amylopectin should have melted by
the time the temperature reached 60 ºC, retrogradation of amylopectin cannot be the only factor affecting firming. Retrograded
waxy corn starch (5%) was added to a bread formula and found
to decrease gelatinization and to reduce the firming rate (Hibi
2001). [Note: As pointed out above, the retrograded material
should have melted during baking so the effect would be one of
adding corn amylopectin.]
Toufeili and others (1999) found that an all-amylopectin Arabic
bread (made with waxy barley starch and cross-linked waxy barley starch in place of wheat starch) staled at a significantly faster
rate than did Arabic bread made with normal wheat starch, that a
low degree of starch crosslinking promotes recrystallization of
amylopectin [possibly by keeping polymer chains in close proximity to one another], and that a higher degree of crosslinking decreased the staling rate [possibly by restricting granule swelling
and separation of polymer chains].
Most agree that there is at least a correlation between amylopectin retrogradation/crystallization and staling, even though
the 2 events may not be part of the same process. Our conclusion
is that amylopectin retrogradation is part of the staling process,
but is not solely responsible for the observed changes in texture.
For information on associations of starch polymer molecules in
concentrated wheat starch (and other starch gels), see Keetels and
others (1993, 1995, 1996 a,b,c,d; Vodovotz and others 2002).
Role of amylose. While Schoch and French (1947) believed
that the linear fraction of starch had a negligible influence on
bread staling, there is evidence that amylose is involved in some
way. Due to its rapid rate of retrogradation, Hoseney and others
(1978) proposed that amylose was responsible for setting the initial crumb structure, but not involved in the staling process. Erlander and Erlander (1969) theorized that amylose-amylopectin
aggregation was responsible for the changes that occur during aging of bread crumb. Kim and D’Appolonia (1977c) found that the
solubility of amylose decreased markedly during the 1st d of
bread storage, while the solubility of amylopectin decreased
steadily over 5 d of storage. They also found that the amount of
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soluble amylopectin in fresh bread was 5 to 24 times the amount
of soluble amylose, indicating that little amylose was leached from
granules or that, by the time the bread had cooled to room temperature, much of the amylose had become insoluble by retrogradation, probably the latter. Ghiasi and others (1984) changed the
ratio of amylose to amylopectin in flour by using waxy barley
starch and also found that the amylose fraction was involved in
staling of bread through 1 d only. It has been suggested that the
role of amylose in bread staling may be merely one of diluting
amylopectin (Inagaki and Seib 1992), a conclusion reached from
a study of breads made with cross-linked waxy barley starch,
which staled at a faster rate than did control breads, even though
the experimental bread had less firmness after 6 h. Evidence
against a role for amylose in staling is that, while stale bread can
be refreshed by heating, amylose crystals (either of the V-type or
B-type) do not melt at the temperatures employed (Knightly 1977).
From an interesting microscopic examination, Hug-Iten and
others (1999) reported that, during baking, there was a separation
of amylose and amylopectin where amylose accumulated at granule centers, and that, upon aging, gelatinized granules regained
birefringence, with the most intense birefringence being observed
in the amylose-rich granule centers. [Note: It may have only appeared that amylose accumulated at granule centers. Another
possible explanation is that amylose was lost by leaching from the
outer area.] They hypothesized that reorganization of intragranular amylose enhances the rigidity of starch granules during staling.
See also sections “Mechanisms of staling: Role of native lipids”
and “Surface-active lipids” for more on the role of amylose in
bread staling.
Relationship between crumb firming and starch retrogradation. Alsberg (1927, 1928) proposed that bread staling could not
be completely attributed to starch retrogradation, since retrogradation in pastes is a slower process than is staling. [Note: There is
reason to believe that retrogradation might occur more easily in
bread than in pastes, which are more often studied, because in
our opinion, since granules in baked bread are still largely intact,
although deformed because their swelling is limited by a deficiency of water, the molecular chains in them are not completely disengaged. Therefore, although there is some degree of crystalline
packing order disruption, it is much easier for chains, which are
still close to one another and still aligned similarly to what they
were in the native granule, to reassociate than it is for amylopectin
molecules in a cooked paste to realign and form an ordered structure. However, it is not known whether intragranular recrystallization is related to staling. In this regard, surfactants that inhibit
granule swelling/gelatinization from occurring in the first place are
effective as antistaling agents.] Others have also questioned the
concept that amylopectin crystallization and bread firming are
one and the same, even though both may occur simultaneously
(Dragsdorf and Varriano-Marston 1980: Baik and Chinachoti
2000).
Dragsdorf and Varriano-Marston (1980) concluded that there is
not a cause-and-effect relationship between starch crystallization
and bread firming. Their results agreed with those from earlier
work by Zobel and Senti (1959), who observed an increase in
crystallinity from bacterial a-amylase addition along with the typical reduction in bread firming (see “Enzymes” section), and postulated that the observed antistaling/antifirming effects of bacterial aamylases were the result of cleavage of interconnecting (amorphous) chains in the crystalline starch network.
Ghiasi and others (1984) also stated that the degree of retrogradation/crystallization of starch molecules was not closely related
to the staling rate of bread. Neither did changes in starch crystallinity upon reheating bread (DSC monitoring) correlate well with
changes in staleness. Furthermore, it was suggested that the degree of softening of stale bread was temperature-dependent, and
4
because the relationship was biphasic, that at least 2 mechanisms
were responsible for the staling and refreshening of bread (Ghiasi
and others 1984).
However, using DTA to examine retrogradation in starch
(source unstated) pastes, McIver and others (1968) determined
that the calculated Avrami exponent and time constant were in
general agreement with values found for bread (Axford and Colwell 1967) and, therefore, concluded that starch retrogradation is
the major factor in bread staling. Colwell and others (1969) found
that the role of starch crystallization in the firming of bread becomes progressively less important at storage temperatures above
21 °C (70 °F).
Others have concluded that starch plays a role in strengthening
the structure of bakery products that is at least equivalent to that
of gluten (Gambus 2000) and that starch retrogradation alone is
sufficient to cause bread firming (Morgan and others 1997).
Although considerable evidence has been presented that there
is not a direct cause-and-effect relationship between starch polymer molecule retrogradation and crumb firming, most researchers
believe that starch retrogradation is part of the staling process. As
stated earlier, our conclusion is that amylopectin retrogradation
plays a significant, but not the only, role in the stalling process.
See also the “Surface-active lipids” section for evidence on the
relationship between starch polymer retrogradation and bread
staling.
Role of flour protein. Protein is another component that has
been studied for its role in bread staling. Kim and D’Appolonia
(1977b) and others have reported that flour protein content is an
important factor in the rate of bread staling. It has been suggested
by different investigators that protein (gluten) reduces the firming
rate of bread during staling, has no effect on the firming rate, and
is required for firming; that is, that staling is dependent on starchgluten interactions. It is now generally believed (Martin and others
1991) that starch-gluten interactions are somehow involved in the
firming process.
Steller and Bailey (1938) reported an inverse relationship between protein content and bread staleness upon storage, although the 2 were not linearly correlated. Others have also found
that increasing the protein level resulted in decreased crumb firmness and crumb firming rate (Bechtel and Meisner 1954a; Prentice and others 1954; Callejo and others 1999). Bechtel and Meisner (1954a) concluded that staling is a result of 2 separate processes: staling during the 1st 2 to 3 d of storage is a result of
changes in the organization of starch polymer molecules; thereafter, staling is caused by loss of moisture from gluten. Prentice and
others (1954) explained that increasing the protein content would
tend to decrease any association between starch granules (swollen and embedded in the gluten network), thereby retarding
crumb firmness development. They also suggested that gluten
may serve as a moisture reservoir to buffer any changes in the hydration capacity of starch. However, since high-loaf-volume
breads are generally softer than those of low volume, their results
are difficult to explain (Kulp and Ponte 1981).
Willhoft (1973) suggested that the antifirming activity of gluten
was due to either a dilution of starch or the effect of gluten enrichment on loaf volume. Erlander and Erlander (1969) suggested that
starch-gluten interactions could prevent staling of bread, possibly
via hydrogen bonding between the amide groups of wheat gliadin, glutenin, and possibly albumin and hydroxyl groups of
starch. They concluded that the ratio of starch to protein in the
dough is important in determining the rate of staling and suggested that some staling will occur no matter how much protein is
added.
Kim and D’Appolonia (1977b) also reported that the rate of
bread staling is inversely related to the protein content of the flour.
However, Avrami exponent values suggested that the basic staling
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Bread staling. . .
mechanism was not affected by protein content, suggesting that
the rate of staling is independent of protein quality. So they concluded that the primary effect of protein in reducing staling is dilution of starch.
Several investigators have concluded that crumb firmness is not
significantly correlated to flour protein type or concentration (Ponte and others 1962; Leon and others 1997; Gerrard and others
2001). By examining a novel starch bread that contained no gluten, Morgan and others (1997) suggested that starch retrogradation alone is sufficient to effect bread firming.
Every and others (1998) suggested that, qualitatively, starchstarch and starch-protein interactions are of equal importance to
the staling mechanism, but that quantitatively, starch-starch interactions are more important since conventional wheat flour contains about 85% starch (db). They hypothesized that gluten is not
essential to the firming process and that increasing bread firmness
results from chains of partially leached amylose and amylopectin
attached to swollen, partially gelatinized starch granules interacting via hydrogen bonds with other starch granule remnants and,
to a lesser degree, with gluten fibrils. Reconstitution experiments
revealed that breads of equivalent specific loaf volume staled at
the same rate irrespective of protein type or concentration, but
other bread properties were altered by changes in the type or
concentration of protein (Gerrard and others 2001), lending support to the above hypothesis.
Maleki and others (1980) postulated that the flour component
primarily responsible for differences in staling rate is gluten and
that its role in staling is something other than dilution of starch.
Furthermore, they proposed that starch and water solubles were
not involved significantly in determining the rate of staling.
Martin and others (1991) proposed that bread firming is a result
of hydrogen bonding between gelatinized (partially pasted) starch
granules and the gluten network in bread tying together the continuous protein network and discontinuous granule remnants.
They theorized that the crosslinking interactions originate during
baking; then during aging, the crumb loses kinetic energy, and
both the number of interactions and their strength increases.
When reheated, bread freshness is restored because the
crosslinks (hydrogen bonds) and entanglements between gluten
and starch polymer molecules are easily broken. This theory is
congruent with results of Dreese and others (1988), who reported
that starch and gluten molecules interact during baking.
Gerrard and others (1997) suggested a modification to the hypothesis of Martin and others (1991). They agreed with the hypothesis that staling is a result of increasing interactions between
swollen starch granules and the gluten network. However, they
put forth the opinion that the decrease in firming rate in breads
made with a-amylase (see “Enzymes: ␣-amylases and debranching enzymes” section) as a dough additive is not the direct result
of starch hydrolysis products (dextrins and maltooligosaccharides), some of which, they suggest, are nonspecifically associated
with the protein matrix, but a result of modification of swollen
starch granules in such a way that their interaction with the protein network is reduced (presumably either qualitatively or quantitatively).
Rogers and others (1988) reported that, even though shortening
and native lipids have significant effects on bread staling, neither
have major effects on starch retrogradation. They suggested formation of protein-lipid interactions.
Role of pentosans. As mentioned in the “Nonstarch polyosaccharides” section, the influence of the so-called pentosans on
breadmaking and bread properties is not clear, although the subject has been examined extensively (Kulp 1968; D’Appolonia
1971, 1980; Hoseney 1984; Meuser and Sukow 1986; Jankiewicz and Michniewicz 1987; Roels and others 1993: Rattan
and others 1994; Krishnarau and Hoseney 1994; Izydorczyk and
Biliaderis 1995; Biliaderis and Izydorczyk 1995; Cleemput and
others 1997). Water-soluble and -insoluble pentosans have been
reported both to retard staling and to have no effect on the staling
rate.
“Water-insoluble” pentosans. No differences in staling rate of
breads made with or without tailings (starch fraction containing
9% water-insoluble pentosans) was observed by a sensory panel
(Bechtel and Meisner 1954b). Neither did Prentice and others
(1954) observe any effect on crumb firming rate due to tailings, although initial crumb firmness was decreased, probably due to the
high hydration capacity of pentosans. However, others found that
addition of water-insoluble pentosans resulted in a considerable
increase in loaf volume (Kulp 1968) and retardation of bread staling (Casier and others 1972, 1973; Denli and Ercan 2001). To
add more confusion, addition of insoluble pentosans was reported to reduce bread quality, which could be overcome with addition of an optimum amount of pentosanase (Krishnarau and
Hoseney 1994). Such variable results may result from differences
in type, molecular weight, and/or concentration of the pentosans
present in the formulation.
“Water-soluble” pentosans. Contrary to the reports of less beneficial effects of water-soluble pentosans (as compared to water-insoluble pentosans), reports which were not confirmed by Kulp
and Bechtel (1963) or Hoseney and others (1971), Michniewicz
and others (1992), like Jelaca and Hlynka (1972), found that water-soluble pentosans had a significant positive effect on loaf volume and that water-insoluble ones did not, that water-soluble
pentosans retarded amylose aggregation, and that addition of water-insoluble pentosans decreased susceptibility of bread crumb
to a-amylase. They suggested that the contradictory results obtained when studying the effects of pentosans on loaf volume may
have originated in differences in baking characteristics of the
flours of various wheat cultivars, differences in chemical composition of pentosans, and/or the way pentosans were incorporated
into the dough. They further suggested that the reported reduction
in bread firmness upon storage when the dough was supplemented with pentosans, as observed by Kim and D’Appolonia (1977d)
and others, may have been a direct consequence of a higher
moisture content of the system.
Interaction of pentosans with protein. It is possible that pentosans can interact with wheat-flour components other than
starch. Jelaca and Hlynka (1972) proposed that pentosan-gluten
interactions were responsible for baking improvement effected by
pentosans. Based on the effects of actions of arabinoxylanases,
Cleemput and others (1997) suggested that there are associations
of arabinoxylans with proteins and/or other wheat components in
doughs.
Pentosans and starch retrogradation. The effect of pentosans
on starch retrogradation has been investigated using both starch
gels and bread itself. Gilles and others (1961) reported that watersoluble pentosans found in the “soluble starch” extract of bread
crumb inhibited retrogradation of amylose and that, although the
pentosans affected some characteristics of the bread, staling rate
was not one of those characteristics. Kim and D’Appolonia
(1977a) found that pentosans had a definite effect on retarding
starch retrogradation in wheat starch gels, with the effect of waterinsoluble pentosans being more pronounced. They reported that
water-soluble pentosans reduced retrogradation by acting on
amylopectin, while water-insoluble pentosans reduced the degree
of retrogradation of both amylose and amylopectin. Similar results
were found when the effect of pentosans on staling was studied in
a bread system (Kim and D’Appolonia 1977d). Results indicated
that the basic mechanism of bread staling was unchanged; thus, it
was suggested that pentosans decreased the staling rate by reducing the amounts of starch components available for retrogradation
(Kim and D’Appolonia 1977d). However, others have concluded
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(based on calorimetry) that arabinoxylan-fortified breads exhibited
a greater rate of starch retrogradation (Biliaderis and Izydorczyk
1995), because of their higher moisture content (Rogers and others 1988), while having softer crumbs than did controls.
Role of native lipids. Most reports on effects of lipids in preparing baked products discuss their effects on baking characteristics
(MacRitchie and Gras 1973; MacRitchie 1981), rather than on
crumb firming. The effects of native lipids are discussed briefly
here; added surfactants used as antistaling agents are discussed in
the “Surface-active lipids” section.
Flour lipid content has been shown to be inversely related to
loaf volume (Rogers and others 1988). Protein-lipid interaction
has been suggested as the mechanism. Shortening is known to
lower the firming rate of bread, but does not react with starch
(Rogers and others 1988). Therefore, the results suggest that native
flour lipids have an effect on the antifirming action of shortening.
While both native lipids and shortening affect firming rates significantly, neither have significant effects on starch retrogradation.
Davidou and others (1996) reported that complexes between native lipids and amylose were formed within the 1st 2 d of storage
and that such complex formations appeared to reduce the maximum amount of starch retrogradation. However, thermodynamic
considerations indicate that amylose-lipid and amylose-surfactant
(see “Surface-active lipids” section) complexes are formed during
baking, since they form at temperatures higher than 60 °C and
melt at temperatures higher than 100 °C (Zobel and others 1988).
Summary. Bread staling is unquestionably a complex process.
While the mechanism of staling is still not understood, certain
ideas have been accepted, such as the important role of starch
retrogradation, specifically amylopectin retrogradation. Even so, it
is becoming increasingly evident that amylopectin retrogradation
alone is not responsible for bread staling, but it is unclear what
other bread components and processes contribute to the overall
staling process. Evidence has accumulated that gluten proteins
are important and that gluten-starch interactions play a role. Moisture transfer (discussed in the “Moisture migration” section) seems
also to be involved in staling. In conclusion, it is probable that
several factors play a role in the bread firming process, but the
large volume of data that implicates amylopectin retrogradation as
a key factor, and the information that gluten is also involved cannot be ignored.
Other Factors Affecting Staling Rate
Storage temperature
An interesting feature of bread is that the rate of staling has a
negative temperature coefficient (Colwell and others 1969). Thus,
the rate of bread staling is accelerated at lower storage temperatures. Bread staling was correlated with starch recrystallization at
storage temperatures of –1, 10, and 21 °C, while the role of starch
crystallization in staling was diminished at higher temperatures
(32 and 43 °C).
Processes have been developed to quick-chill bakery products,
then allow them to stabilize to ambient conditions in order to reduce staling when the product is held at room temperature (Williams and others 1995).
Freezing retarded firming, the effect being greater the longer the
frozen storage time. The effect of freezing was additive with the effect of monoglyceride addition (Malkki and others 1978).
Polymer crystal growth theory states that there are 3 phases to
polymer crystallization: nucleation, propagation, and maturation.
Slade and Levine (1987) and Marsh and Blanshard (1988) have
determined that amylopectin recrystallization, at least in concentrated pastes, is a nucleation-limiting process occurring at a temperature above the glass transition temperature (Tg), or the glass
6
transition temperature of the maximally freeze-concentrated
starch (Tg’) (about –5 °C) when the starch concentration is < 70%,
and below the melting temperature (Tm) of crystalline amylopectin
(about 60 °C). The maximum rate of nucleation occurs at temperatures slightly greater than Tg (or Tg’ depending on concentration),
while the maximum rate of propagation occurs at a temperature
slightly less than the Tm of crystallized amylopectin. The retrogradation rate of starch pastes held under isothermal conditions is
greatest at a temperature between the optimal temperatures for
nucleation and propagation (about 5 °C for a 50% paste) (Slade
and Levine 1987; Marsh and Blanshard 1988). The situation may
be somewhat different in bread, but temperature cycling is used to
accelerate the staling of bread in the production of croutons
(Slade and others 1987). The very fact that proper temperature cycling is so effective in accelerating bread firming is strong support
for the involvement of starch polymer crystallization. The fact that
staled bread can be resoftened by reheating is additional support.
Slade and Levine (1987) also come to the conclusion that 4 °C
(refrigerator temperature) is the single optimum temperature between Tg and Tm that balances nucleation and crystallization and
that the melting temperature involved implicates amylopectin as
the polymer crystallizing.
Moisture migration
Water is involved in the following changes in the bread system:
drying out, moisture equilibration between crumb and crust, and
moisture redistribution between and among bread components
(Kulp and Ponte 1981). Drying out of the bread, as demonstrated
by Boussingault (1852), does not explain staling, but may accelerate reactions leading to staling (MacMasters 1961). Thus, moisture
relationships within the crumb are important considerations when
studying bread staling.
Breadmakers in the U.S.A. are limited to 38% water for white
pan bread even though breads containing higher levels of moisture generally stale more slowly (Kulp and Ponte 1981). This inverse relationship between moisture content and staling rate was
confirmed (Rogers and others 1988; He and Hoseney 1990),
even though the rate of starch retrogradation in bread was found
to be directly proportional to the moisture content (Rogers and
others 1988). Zeleznak and Hoseney (1986) confirmed that retrogradation in wheat starch gels was a function of the amount of
water present. They also reported that the moisture content of
bread is about optimal for amylopectin retrogradation and that
addition of either monoglycerides or shortening did not alter the
available moisture content.
Schiraldi and Fessas (2001) focus their review on water content
(on which the mobility of polymer chains is dependent), water activity, water migration between phases, and the alveolar crumb
structure of bread. Their conclusion is that “The overall picture of
the crumb could be described as interpenetrated gels separated
by aqueous interphases which contain most of the low molecular
weight solutes. This water is rather mobile and can facilitate mutual displacement of the incompatible gel phases, thus behaving as
a plasticizer, and can enhance the crumb-to-crust migration of
moisture. This local drying makes the walls of the crumb alveoli
more rigid, while the concurrent moisture increase within the
crust region is accompanied by a reduction of crispness even
when overall moisture loss is prevented by packing bread in
sealed bags (Piazza and Masi 1995). Along its way toward the
crust, water can contribute to a closer packing of the structure
through which it is moving, either within a given phase or at the
interphases, by tightening the sites able to form H bonds. This
would explain why refreshed bread softens when its temperature
has been raised above Tg, but then becomes harder than the starting staled product, and why microwave-cooked or refreshed
bread shows a fast firming without significant enhancement of
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Bread staling. . .
amylopectin crystallization”.
Crumb-crust redistribution of moisture. As baked bread begins to cool, a moisture gradient forms in the loaf (Piazza and
Masi 1995). Differences in vapor pressures between the crust and
the internal region of the loaf result in moisture migration from the
crumb to the crust (Stear 1990). Over time, the moisture content
in the center of the loaf decreases, while that in the external region
increases (Bechtel and others 1953).
Baik and Chinachoti (2000) found that bread stored with its
crust became significantly firmer than bread stored without its
crust and contained more recrystallized amylopectin, indicating
that moisture redistribution from crumb to crust plays a significant
role in firming, a conclusion confirmed by a loss in freezable water in the crumb of bread stored with crust, which correlated with
changes in its thermomechanical profile.
Several NMR parameters correlate with crumb firming and are
believed to be related to both microscopic and macroscopic redistribution of water (Chen and others 1997b). Using NMR techniques, it has been found that, as staling proceeds, the water in
bread becomes less and less mobile (Leung and others 1983;
Wynne-Jones and Blanshard 1986; Kim-Shin and others 1991;
Chen and others 1997a,b; Engelsen and others 2001). However,
Ruan and others (1996), using MRI, found that, as storage time of
sweet rolls increased, mobility of the less-mobile water fraction
decreased, while mobility of the more-mobile fraction increased.
Moisture redistribution among components. Transfer of moisture from one constituent of the bread crumb to another is generally accepted as a contributing factor in staling, possibly being responsible for the perceived dryness of stale bread (Senti and Dimler
1960). Water is a plasticizer, making the bread components more
flexible. Thus, as water is removed (from either gluten or starch or
both), increasing crumb firmness should occur. Whether staling involves dehydration of gluten or starch has been studied extensively,
but is still unclear. However, the majority of evidence suggests a
gluten to starch transfer of water as the starch crystallizes.
Katz (1928) first suggested that, during staling, moisture was released from starch and taken up by gluten. Senti and Dimler (1960),
by studying equilibrium relative humidities, also suggested that
moisture transfer would likely occur from starch to gluten. Cluskey
and others (1959) reported a progressive drop in moisture-sorption
capacity for starch and lack of a change for gluten, indicating a
transfer of moisture from starch to gluten during aging.
In contrast, Alsberg and Griffing (1927) and Alsberg (1936) postulated that it was the gluten that hardened as result of moisture
loss to starch. This concept is supported by data of Bachrach and
Briggs (1947), who observed an increase in moisture-sorption capacity of gelatinized starch upon aging [contrary to the results of
Katz (1928) and Cluskey and others (1959)]. Further evidence
came from investigations by Willhoft and coworkers (Breaden and
Willhoft 1971; Willhoft 1971; Kay and Willhoft 1972), who reported that gluten undergoes a 1st-order transformation resulting
in the release of water from gluten and absorption of this water by
retrograding starch.
The notions of “free” and “bound” water have been reported to
be of importance in altering the rate or extent of staling in bread
(Knjaginciev 1970). More recently, the use of NMR and a greater
understanding of the role and mechanism of starch polymer crystallization have led to the conclusion that starch takes up water
from gluten upon aging of bread. Leung (1981) and Leung and
others (1983) proposed that, as starch changes to a more crystalline state, more water molecules become immobilized due to their
incorporation into crystal structures. Chen and others (1997a,b)
reported a decrease in water mobility in bread upon staling, in
agreement with results of others (Wynne-Jones and Blanshard
1986; Slade and Levine 1991), and concluded that the decrease
in water mobility was due to incorporation of water molecules re-
leased from gluten into crystalline structure of starch that developed upon staling. [Note: The B structure has 36 water molecules
in the unit cell, whereas the A structure has only 8 (Sarko and Wu
1978).] Conversely, Kim-Shin and others (1991) proposed that the
redistribution of water occurs in the amorphous phase. The ratio
of starch to gluten (6:1) in bread crumb ensures that moisture
transfer to the starch would result in firming of the continuous gluten phase (Willhoft 1971). It is important to keep in mind at all
times, however, that the change in the state of water cannot be
correlated directly to the retrogradation process (Wynne-Jones
and Blanshard 1986).
Levine and Slade (1990) and Slade and Levine (1991) present
thorough and well-documented evidence for the role of water in
the staling process. Their arguments are based upon the mechanism of polymer crystallization, polymer crystallization kinetics as
a function of glass transition and melting temperatures, water as a
plasticizer, and sugars as antiplasticizers in the system. In their review, Slade and Levine (1991) state essentially that “if adequate
packaging prevents simple moisture loss, the predominate mechanism of staling in bread crumb is the time-dependent recrystallization of amylopectin from the completely amorphous state of a
freshly heated product to the partially crystalline state of a stale
product, with concomitant formation of network junction zones,
redistribution of moisture via both microscopic and macroscopic
migration (Czuchajowska and Pomeranz 1989), and increased
textural firmness (Kulp and Ponte 1981; Russell 1983b; Russell
1987).” They further point out that there is evidence from studies
of starch gels/pastes that the rate and extent of amylopectin crystallization depends on the mobility of its outer branches (Ring and
others 1987; Russell 1987; Marsh and Blanshard 1988; Slade
and Levine 1989, 1991) and on sample history, since the processes that occur both during heating/baking and during aging/
storage are nonequilibrium processes (Ring and others 1987;
Slade and Levine 1989, 1991). [Note: Slade and Levine refer to
recrystallization of amylopectin, and indeed it is a recrystallization. We have not used the term elsewhere in this review so as to
make it clear that amylopectin molecules do not recrystallize to
the same crystalline state that they were originally in nongelatinized granules.]
Amylopectin crystallization results in a partially crystalline, supermolecular structure containing disperse B-type crystalline regions (Slade and Levine 1987). Incorporation of water molecules
into the crystal lattice occurs during formation of the B-type polymorph (Imberty and Perez 1988) and, thus, a redistribution of
moisture is effected. This process was demonstrated by a progressive decrease in the percentage of “freezable” water as bread was
stored over 11 d (Slade and Levine 1991). The water molecules
that are part of the crystal lattice are not available for plasticization, so the result is the perceived drier, firmer texture characteristic of stale bread. So, all in all, amylopectin crystallization in bread
requires both microscopic and macroscopic redistribution of water so that there is sufficient moisture present at the locus where
crystallization takes place to plasticize polymer chains so that
they are mobile enough for crystallization to occur and for incorporation into B-type crystal latices (Levine and Slade 1990; Slade
and Levine 1989, 1991).
It seems clear that moisture transfer between bread components, specifically between gluten and starch, occurs as bread
ages. However, like other measurable changes in the nature of
bread components, the role, if any, of moisture and moisture redistribution in the staling process remains undetermined. (See
also “Mechanisms of staling: Role of pentosans” and “Carbohydrate ingredients” sections).
Processing factors
Effects of technological factors, which include manufacturing
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methods, formulas, and operational steps, on both loaf characteristics and bread staling have been compiled by Kulp and Ponte
(1981) with information from the American Institute of Baking.
Swortfiguer (1971) and Maga (1975) also discuss these variables
in reviews.
Giovanelli and others (1997) showed that baking temperature
significantly affects bread staling. Bread baked at lower temperatures stales at a slower rate in terms of both crumb hardening and
starch retrogradation. Higher baking temperatures led to increased protein denaturation and starch granule disruption.
[Note: This is a little puzzling, since as long as there is water
present in the bread, the temperature inside the loaf cannot go
above the boiling temperature of that water, no matter what the
oven temperature. The oven temperature can, however, affect the
rate of temperature rise and, thus, the time at the maximum temperature.] The authors suggested baking under slight vacuum to
achieve crumb cooking at temperatures < 100 °C, which may enhance the shelf life of bread.
In a study of the effects of processes, Axford and others (1968)
found that the rate and extent of staling decreased as the loaf volume increased in bread stored at the same temperature and that
breads made with the same dough ingredients, but by different
processes (and stored at the same temperature), underwent staling
at different rates because of differences in loaf volume.
Antistaling Additives
Enzymes
One strategy to reduce the rate of bread staling employs enzymes. The enzyme supplements labeled as amylases and proteases are most commonly used in commercial baking (Miller and
others 1953; Waldt 1968, 1969; Martinez-Anaya 1998; Bowles
1996). The most useful enzymic approach to staling rate reduction has been the use of ␣-amylases, which catalyze a small
amount of hydrolysis of the starch. Proteases depolymerize gluten
proteins and modify baking characteristics. Nonamylolytic enzymes may also be active in the enzyme supplements (van Eijk
and Hille 1996). [Note: While many enzymes are useful in aspects of breadmaking other than in reducing crumb firmness,
only enzymes useful as antistaling agents are discussed below.]
␣ -Amylases and debranching enzymes. Numerous studies
have reported that the rates and degrees of firming in baked
goods can be reduced; and the texture, flavor, aroma, and general
qualities improved; by use of a-amylases. Fungal, cereal, and bacterial ␣-amylases all appeared to improve softness retention of
bread to an extent related to their heat stability (Conn and others
1950; Miller and others 1953). Fungal ␣-amylase was inactivated
by heat before acting on the starch. Although cereal (wheat or barley) a-amylases did not survive the baking process, they had time
to act on the swollen starch. A bacterial a-amylase was able to
partly survive the heat treatment (Amos 1955). [Note: after this
work was reported, intermediate thermostable bacterial ␣-amylases became available. See below.] In any case, major ␣-amylase
activity takes place during baking after the starch is gelatinized
and becomes more susceptible to the enzyme (Ghiasi and others
1979); there is a specific temperature range and time in the breadmaking process when the enzyme is most active in degrading
starch (Martin 1989).
Waldt and Mahoney (1967) reported that, when bacterial aamylase was used, the freshness of 4-d-old bread was equivalent
to that of 2.0 to 2.5-d-old untreated bread, but it has been reported that, when bacterial ␣-amylase derived from Bacillus subtilis is
used in a bread formulation, a gummy texture results (because it
can survive baking) (Hebeda and others 1991). Fungal a-amylases
(such as that from Aspergillus oryzae) are less thermostable than
8
are bacterial ␣-amylases (Miller and others 1953).
Commercial ␣-amylases with intermediate thermostability characteristics, known as intermediate-temperature-stable (ITS) enzymes, are now available. Though obtained from different microbial sources, the various ITS enzymes exhibit similar thermostability profiles (Hebeda and others 1991). ITS enzymes have thermostabilities and temperature optima between those of fungal ␣-amylases and conventional bacterial ␣-amylases. Fungal ␣-amylases
have temperature optima of 50 to 55 °C; bacterial ␣-amylases
have optimum activity near 75 °C. The maximum activity of ITS
enzymes occurs at about 65 to 70 °C. Thus, ITS enzymes have
optimal activity at or slightly above the gelatinization temperature
of wheat starch, but are inactivated by the 100 °C baking temperature (Hebeda and others 1991).
Addition of Aspergillus ITS ␣-amylase increased the shelf life of
bread 38 to 75%. When the point in the process where enzyme is
added was optimized, the Aspergillus ITS enzyme increased shelf
life by as much as 200%. B. megaterium ITS ␣-amylase increased
shelf life by 15 to 33% (Hebeda and others 1991).
Rosell and others (2001) determined that commercial ␣-amylases from different sources (wheat flour, malted barley, fungi, bacteria) were strongly affected to different degrees by process conditions and the presence of other ingredients in the dough.
Lent and Grant (2001), in a comparison of bagel ingredients (␣amylase, a modified food starch, xanthan, and a hydrated
monoglyceride), found that the a-amylase was the most effective
in retarding staling as determined by DSC analysis.
The mechanism of the antistaling effect of ␣-amylases has been
debated. At first, ␣-amylases were thought to affect the staling rate
of baked products via modifications of the structure of starch
(Maga 1975). Various techniques have shown that use of commercial “antistaling” ␣-amylase preparations reduces both the rate
of starch retrogradation and the rate of crumb firming (Morgan
and others 1997; Champenois and others 1999). However, results from several studies indicate that the degree of starch crystallinity and the degree of firmness are not correlated (Champenois
and others 1999). Results from use of an “antistaling” a-amylase
and characterization of the properties of the resulting crumb by a
variety of techniques led Hug-Iten and others (2001) to conclude
that the antistaling effect of the enzyme preparation was due to its
ability to produce a partially degraded amylopectin that is less
prone to crystallize, and that its ability to produce partially degraded amylose is responsible for rapid formation of a partially
crystalline polymer network (in fresh bread) that resists later rearrangements.
Schultz and others (1952) suggested that the beneficial effect of
␣-amylase in reducing staling was due to production of low-molecular-weight dextrins that interfered with the retrogradation of
starch. Zobel and Senti (1959) also proposed that dextrins disrupted the continuity of the starch network and reduced its rigidity. Akers and Hoseney (1994) agreed that dextrins produced from
␣-amylases are important in controlling the rate of bread firming.
They reported that ␣-amylases from different sources reduced the
rate of crumb firming to different degrees. They also extracted the
water-soluble hydrolysis products from aged crumb of breads
made with the different enzyme preparations, examined them by
HPLC, and found that certain peak areas were highly correlated
with a reduced rate of crumb firming and that other peaks were
highly correlated with an increased rate of crumb firming.
Leon and others (1997) also attributed the antifirming effect of
␣-amylases to hydrolysis products. Finding that incorporation of a
mixture of ␣-amylase and pullulanase caused bread to firm at a
faster rate, while use of the ␣-amylase alone retarded firming, Martin and Hoseney (1991) concluded that hydrolysis products of a
particular size were responsible for the reduced rate of firming.
Lin and Lineback (1990) found that a bacterial a-amylase pro-
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Bread staling. . .
duced mainly low-molecular-weight, branched dextrins of DP 1924 that either had less ability to retrograde, interfered with amylopectin retrogradation, or interfered with whatever other interactions are responsible for crumb firming.
Duran and others (2001) attributed the antistaling effect of ␣amylases to the production of maltooligosaccharides. Biliaderis
and Prokopowich (1994) found that maltose and maltotriose had
antiretrogradation effects on starch gels and proposed that the
chain ordering of amylopectin in sugar-containing starch gels is a
function of the compatibility of the sugar with the structure of water. Solutes that fit well in the water structure retarded chain reordering. On the other hand, solutes that disturb water structure
promoted ordering and aggregation of starch molecules. Maltotriose, which was reported to be the most effective maltooligosaccharide in impeding retrogradation, disturbs the structure of water
only slightly (Biliaderis and Prokopowich 1994).
Defloor and Delcour (1999) reported that starch hydrolysis
product preparations with average DPs of from 4 to 66 reduced
DSC staling endotherms in baked and stored bread doughs. They
attributed their antistaling effect to a reduction in starch recrystallization but did not speculate about a mechanism.
Martin and Hoseney (1991) proposed that low-molecularweight dextrins (maltooligosaccharides) produced by a-amylases
were directly responsible for the antistaling phenomenon observed by enzyme addition. Their explanation was that the lowmolecular-weight products inhibited cross-link formation between
starch and gluten.
Min and others (1998) studied the effect of 2 novel antistaling
amylases. When added to bread, they produced selectively either
maltose and maltotriose or maltotetraose and maltotriose. Based
on the results, they postulated that maltotriose and maltotetraose
were directly responsible for retarding retrogradation in bread,
suggesting that these oligomers were of the right size to interfere
with starch-gluten interactions [theory of Martin and others (1991)
and Martin and Hoseney (1991) on the mechanism of staling].
Maltose was found to be less effective in bread staling prevention,
and it was suggested that its relatively small size and its ability to
diffuse easily might be the reason why it was less effective than
maltotriose or maltotetraose (Min and others 1998). Donnelly and
others (1973) reported that there is a slight decrease in moisture
adsorptive capacity as the molecular size of maltooligosaccharides increases from DP 3 to DP 11, and that maltose was the exception, being less hygroscopic than was the DP 11 maltooligosaccharide. This led Min and others (1998) to conjecture that
maltotriose and maltotetraose might hold water around starch
molecules and inhibit starch-starch interactions more than maltose does.
Despite conclusions that dextrins directly affect staling in bread,
considerable evidence has been published to the contrary. Salem
and Johnson (1965) found, from experiments in which starch hydrolysis products were added to a bread dough formula, that certain maltooligosaccharides (such as maltohexaose and -heptaose,
as compared to glucose, maltose, and maltotriose, -tetraose, and pentaose) and dextrins increased the rate of crumb firming, in
contrast to results obtained when ␣-amylase was incorporated as
an additive. However, in contrast, Every and others (1992) found
that maltooligosaccharides of DP 3-10 correlated with a reduction
in firming rate, and Akers and Hoseney (1994) implied that starch
hydrolysis products of a size greater than maltoheptaose might
have antifirming properties.
There is a 3rd conclusion. Because added maltooligosaccharides did not survive fermentation and because the presence of
maltooligosaccharides of a specific size class could not be correlated with the firming rate of bread, Gerrard and others (1997)
concluded that maltooligosaccharides (DP 3-8) produced by ␣amylases are not themselves responsible for antistaling, but that
their presence is simply correlated with a key modification of
starch granules that is related to reduced staling, possibly by reducing gluten-starch interactions. Duedahl-Olesen and others
(1999) also reported that maltooligosaccharides of average DP up
to 20 had no effect on formation of the staling endotherm. Interestingly, they did find that amylopectin recrystallization was reduced significantly when ␥-cyclodextrin (3%) was incorporated.
Slade and Levine (1987) and Levine and Slade (1990) studied
maltooligosaccharides of DP 3 to 8 at relatively high concentrations (1:1, oligosaccharide:water). Their conclusion was that the
reported antistaling effect could be explained by an impact on the
Tg, resulting in a smaller ⌬T above Tg, which retarded the starch
crystallization process. Further, they reported a relationship between an increase in Tg and the degree of staling (Slade and Levine 1991). The correlation of an increase in Tg during staling with
the firming of bread was confirmed by Jagannath and others
(1999a).
Dragsdorf and Varriano-Marston (1980) studied the effects of
barley malt, fungal ␣-amylase, and bacterial a-amylase on starch
crystallization and organization in staling breads using x-ray diffraction (see also Akers and Hoseney 1994). Comparing stored
and fresh breads, they found that the degree of crystallinity of
breads baked with different enzyme sources was in the order bacterial ␣-amylase > cereal ␣-amylase > fungal ␣-amylase > control.
These results were in opposition to bread firming data, suggesting
to them that starch crystallization and bread firming are different
and separate processes. Their results agreed with those obtained
by Zobel and Senti (1959), who suggested that bacterial ␣-amylases inhibit staling by breaking interconnecting chain associations
in the network of starch crystallites.
Retardation of bread staling, while avoiding a gummy mouthfeel, was achieved by incorporating pullulanase with a cereal or
bacterial ␣-amylase in the dough (Carroll and others 1987). A
product produced by action of pullulanase or isoamylase on
starch, which the inventors refer to as low-molecular-weight amylose, but which is in reality a mixture of released branch chains,
when added to a dough formulation, was reported to have an antistaling action (Yoshida and others 1972).
All in all, it appears that starch hydrolysis products are involved
in inhibition of staling, but that the products must be of a unique
type, perhaps either maltotriose and maltotetraose or products
larger than those present in traditional maltodextrin preparations.
However, that the presence of such products is only correlated to
some other modification in starch (or another component) that is
the real determinant cannot be ruled out.
Lipases. Although Johnson and Welch (1968) patented lipase
formulations that retard staling in bread, the use of lipases for
breadmaking was virtually unknown until recently (Qi Si 1997).
Depending on the type of flour and the formula, addition of some
1,3-specific lipases resulted in more uniform crumb structure and
thus an improvement in crumb softness during storage (among
other dough conditioning improvements). Furthermore, these lipases were shown to be a replacement for shortening, although
no improvement in crumb elasticity was found.
Siswoyo and others (1999) found that, while use of a purified lipase alone retarded retrogradation in bread crumb, use of a combination of a purified lipase and a purified a-amylase reduced retrogradation to a much greater extent. [Note: Since commercial enzyme preparations are rather crude and probably contain both
activities, this combination could unknowingly be involved in the
antistaling activity.]
Qi Si (1997) suggested that the mechanism of retrogradation retardation does not involve the most obvious explanation: hydrolysis of lipids to monoglycerides, which are reported to have antistaling characteristics. [Note: Tri- and diacylglycerols do not decrease crumb firmness, but monoacylglycerols/monoglycerides
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do (see “Surface-active lipids” section). However, monoglycerides
lack the positive dough-conditioning effects produced by lipases,
and insufficient lipids are present in wheat flour to result in
enough monoglycerides to achieve an antistaling effect. Finally,
only 1st and 3rd position monoglycerides can complex with
starch, thus retarding staling; and the 1,3-specific lipase would
produce 2nd position monoglycerides (Qi Si 1997). There is no
evidence that an effect produced by the released free fatty acids
has been considered.]
Lipoxygenases. Lipoxygenase is reported to have a crumb softening effect when active in bread (van Eijk and Hille 1996). A major source of lipoxygenase is from enzyme-active soy flour, a
common ingredient in breads. While action of the enzyme on the
structure of lipids could explain the crumb-softening effect, it is
also likely that changes in protein conformation via oxidation of
gluten are partially responsible (Daniels and others 1970; Frazier
and others 1973). Lipid peroxide intermediates produced by the
action of lipoxygenase on polyunsaturated lipids may react with
protein sulfhydryl groups to produce protein-bound lipids, which
may subsequently be released by oxidation of the protein.
Nonstarch polysaccharide-modifying enzymes. As discussed
in the “Mechanisms of staling: Role of pentosans” section, the influence of pentosans on bread properties, including the rate of
staling, is unclear. It is also unclear whether enzymes that degrade
nonstarch polysaccharides in bread have any effect on bread staling (van Eijk and Hille 1996). “Pentosanses” (hemicellulases) are
well-known dough conditioners in Europe and have reportedly
been used to increase loaf volume through improved dough machinability and overspring (Qi Si 1997). Fungal enzyme preparations with high endoxylanase, ␤-xylosidase, and ␣-L-arabinosidase activities delayed bread staling considerably without affecting porosity or loaf volume (Rodionova and others 1995).
Proteases. The role, if any, of proteases in the mechanism of
bread staling has not been investigated thoroughly. The purpose
of adding them to breads is to improve flavor profiles, flow characteristics, machining properties, gas retention, and mixing time
(Barrett 1975; Mathewson 2000). However, given evidence that
protein has a significant role in the bread staling mechanism (Martin 1989; Martin and others 1991; Martin and Hoseney 1991), it
is likely that modification of the gluten network structure via enzyme-catalyzed proteolysis would have an effect on bread staling.
It is also possible that liberation of water molecules concurrent
with protein hydrolysis could enhance amylase activity (Schwimmer 1981). Alternatively, proteases could inhibit amylolysis if they
catalyzed the degradation of a-amylase molecules.
Sahlström and Bråthen (1996) reported that addition of a commercial a-amylase product with protease activity resulted in a softer crumb over a shorter time period compared with breads made
with ␣-amylase addition alone. [Note: Both ingredients most likely
contained proteases, though proteolytic activity was not tested.
The former product was marketed as a dual-function enzyme, and
it probably contained a significantly greater level of proteolytic activity. Whether or not the reduction in crumb firmness was due to
its proteolytic activity is unknown].
Techniques for differential inactivation of ␣-amylase and protease from malted wheat and fungal sources were developed by
Miller and Johnson (1949). Results from their use led them to conclude that ␣-amylase alone might be less effective in creating improvements in texture as compared to addition of both protease (in
small amounts) and ␣-amylase activities (Johnson and Miller 1949).
They also concluded that, while ␣-amylase was the component of
malt mainly responsible for increasing crumb compressibility after
66 h of storage, protease alone (at low concentrations) increased
the compressibility of crumb over that of the controls.
Van Eijk and Hille (1996) concluded that, while the addition of
excess concentrations of proteolytic enzymes would certainly be
10
detrimental to the bread loaf, adding optimal levels of proteases to
breads might increase their shelf life. If so, the presence of contaminant levels of proteases in commercial ␣-amylase preparations might partially explain the currently unresolved mechanism
of antistaling (Gray and BeMiller 2001). In fact, since commercial
enzymes, including ␣-amylases, and enzyme blends have activities in addition to the stated one(s) (Silberstein 1961; Hebeda and
others 1991), the possibility that the presence of other activities
(that is, lipases, xylanases, and so on) in commercial enzyme
preparations might have an effect on bread staling cannot be
ruled out. Identification and characterization of such contaminant
activities would be useful.
Surface-active lipids
Most studies with lipids have been concerned with improving
functional properties of bread (D’Appolonia and Morad 1981).
Emulsifiers of various types are widely employed in the baking industry as dough strengtheners and/or crumb softeners (Kulp and
Ponte 1981), but their role in staling has not been established. Examples of surfactants used in breads as antistaling agents are presented briefly, followed by a review of research on the mechanism
of surfactants in reducing the rate of staling. Several more detailed
reviews of the use of emulsifiers in breadmaking, including a discussion on the role of emulsifiers as antistaling agents, have been
published (Knightly 1968, 1973, 1996; Morrison 1976; Krog
1981; Stampfli and Nersten 1995).
Most studies of amylose-lipid complexes involve complexes
formed in dilute solutions of amylose (see, for example, Biliaderis
and others 1985, 1986; Biliaderis and Galloway 1989; Biliaderis
and Seneviratne 1990; Seneviratne and Biliaderis 1991) and occasionally in concentrated starch gels (see, for example, Biliaderis
and Tonogai 1991). Details of the structures of amylose-fatty acid
complexes, based on date from solid-state 13C CP/MAS and deuterium NMR, x-ray powder diffraction, and DSC analysis, have
been proposed (Lebail and others 2000).
Surfactants. Diacetyl tartaric acid esters of monoglycerides
(DATEM). DATEM surfactants (0.05%) were reported to be as effective as antistaling agents as SSL (see “Sodium stearoyl lactylate
(SSL)” section, below) or ethoxylated monoacylglycerols over 5 d
of storage (Rogers and Hoseney 1983), but to be less effective
[compared to monoglycerides (see “Polyoxyethylene monostearate (POEMS)” section below)] in reducing retrogradation of amylopectin and in forming complexes with amylose, while at the
same time reducing crumb firming (Krog and others 1989). It was
suggested that the antifirming properties of DATEM may be due to
changes in cell wall thickness and elasticity effected by it. It was
further reported that optimal reduction in firmness increase over
extended periods of storage can be achieved when DATEM is
used in combination with monoglycerides.
Lecithins. Lecithins have been reported to reduce staling and to
have the advantage of being amenable to modification for specific
applications (Forssell and others 1998). Soy lecithin hydrolyzate
effectively retarded crystallization in starch gels and bread staling.
Oat lecithin retarded staling significantly more than did soy lecithin, but did not affect crystallization in starch gels (Forssell and
others 1998).
Monoglycerides (MG). Most bakeries use mono- or diacylglycerols to delay staling in bakery products (Huang and White 1993).
While Schoch and French (1947) first proposed the use of
monoglycerides (properly termed monoacylglycerols) in the form
of “superglycerinated shortening” to inhibit staling of bread, Hopper (1949) first reported their efficacy. Ofelt and others (1958)
confirmed the action of monoacylglycerols in decreasing crumb
firmness. Diacylglycerols (commonly called diglycerides) had no
effect on crumb firmness when added alone to replace lard and
showed no synergistic effects with monoacylglycerols. While ad-
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Bread staling. . .
dition of monoglycerides may counteract staling of breads during
storage, an increased tendency to crumble may result (Malkki and
others 1978).
The mechanism of the antistaling effect of monoglycerides is
still unknown, but it is thought to be different from that of shortening (Rogers and others 1988), for monoacylglycerols can replace
shortening, but shortening cannot replace monoacylglycerols.
Krog and others (1989) concluded that reductions in crumb
firmness brought about by addition of monoglycerides were
probably the result of interactions with amylose rather than with
amylopectin. When relatively large amounts of monoglycerides
are used, essentially all released amylose can be complexed (as
measured by DSC); interactions with amylopectin are also increased. At lower concentrations, monoglycerides interact primarily with amylose because of competition between the 2 polymers.
Polyoxyethylene monostearate (POEMS). POEMS, a reaction
product of ethylene oxide and stearic acid, was one of the first additives reported to retard staling (Maga 1975). Favor and Johnson
(1947) demonstrated that POEMS (0.5 to 1.0%) dramatically reduced the firming rate of bread between the 1st and 3rd d. Other
results (Freilich 1948; Edelmann and Cathcart 1949; Edelmann
and others 1950; Skovholt and Dowdle 1950) confirmed that POEMS was effective in reducing the rate of firming. Carson and others (1950) theorized that POEMS retarded staling by 2 mechanisms: (1) by insolubilizing amylose and (2) by interacting with
starch granules via hydrogen bonding.
Sodium stearoyl lactylate (SSL). Pisesookbunterng and
D’Appolonia (1983) found that, among various surfactants studied, SSL had the greatest binding affinity to starch. The anionic surfactant might also prevent protein denaturation. Calcium stearoyl
lactylate is less effective as a crumb softener, but is active.
Glycerol monostearate (GMS). GMS is used in many starchbased food products to improve physical characteristics, including the degree of softness after storage (Krog 1971).
Other surfactants. Other surfactants that are effective as antistaling agents include polyoxyethylene sorbitan monostearate
(Polysorbate 60), succinylated monoglycerides, and glycerol.
Novel surfactants or surfactant blends have been formulated for
use as antistaling agents in bakery products. A blend developed
by Knightly (1987) consisted of a hydrophilic lecithin and at least
one of the following: monoglyceride, lactic acid esterified
monoglyceride, succinic acid esterified monoglyceride, maleic
acid esterified monoglyceride, or edible salts of stearoyl lactylic
acid. This blend was claimed to both inhibit staling and to act as a
dough conditioner. Other antistaling surfactant blends were developed by Vidal and Gerrity (1979).
Mechanism of antistaling effect of surfactants. The mechanism
by which surfactants influence crumb firmness has been debated
and is discussed briefly in the following sections. Amylose-surfactant, amylopectin-surfactant, and protein-surfactant interactions
have all been investigated, as has starch swelling in the presence
of added surfactants. Whether surfactants actually decrease the
rate of firming or produce softer breads that then stale at the same
rate as the control has been debated. Surfactants have multiple
properties, resulting in multiple functionalities, so definitive experiments examining a cause-and-effect relationship with regards to
staling are difficult, if not impossible, to design.
In excess water, surfactants do not change the gelatinization
temperature, but they do delay pasting (Miller and others 1953).
Whether this is related to their functionality in breadmaking is unknown. Knightly (1977) reported that surfactants had little to no
effect on initial crumb firmness, but did affect the firming rate during storage, a finding in agreement with earlier reports (Favor and
Johnson 1947; Skovholt and Dowdle 1950; Hopper 1949; Edelmann and others 1950). Based on unpublished results from in-
vestigations by Ponte and Titcomb (1971), Kulp and Ponte (1981)
concluded that a surfactant’s ability to retard firming is more important than an initial softening of crumb in freshly baked bread.
Amylose complexes. Details of fatty acid-amylose complexes
have been examined using x-ray diffraction, DSC, and electron
microscopy (Godet and others 1993b, 1995a, 1996). Using molecular modeling techniques, Godet and others (1993a,b, 1995b)
concluded that the hydrocarbon tails of complexed fatty acid
molecules are indeed inside the hydrated V helix (see below) with
the polar head group outside the lumen. Interactions of amylose
with over 20 surfactants were studied, and a “complexing index”
was calculated and assigned to each one (Krog 1971). Morad and
D’Appolonia (1980) demonstrated that incorporation (0.5%) of 5
commercial surfactants resulted in amylose-surfactant complexes.
It was found by Eliasson (1985) that the amount of amylose
leached from starch granules decreased in the presence of emulsifiers.
Numerous studies have dealt with the ability of polar lipids to
inhibit bread staling. Mikus and others (1946) suggested that a helical complex formed between amylose and MG, thus effecting a
softer crumb, but without affecting the firming rate. Schoch (1965)
reached a similar conclusion.
Data from Lagendijk and Pennings (1970) provided evidence of
the relationship between amylose-lipid complexation and the inhibition of staling. They reported maximum complexation with
monopalmitin, which corresponded with the softest crumb after
48 and 72 h of storage and concluded that complexation reduces
the flexibility of amylose molecules and thereby reduces their retrogradation.
Pisesookbunterng and D’Appolonia (1983) reported that surfactants (SSL, MDG, and 40% Poly-60 / 60% MDG blend) adsorbed to the starch granule surface, preventing moisture uptake
by the starch from gluten during aging of bread. However, water
was able to migrate from crumb to crust. Firmness of fresh bread
was not affected by the surfactant, although firming rate during
storage was slowed. Xu and others (1992) confirmed these results.
On the other hand, no apparent relationship between amylosesurfactant complex formation and reduction of crumb firming was
found by Osman and others (1961).
X-ray diffraction can be used to detect complex formation, as
well as crystallinity and crystal types in general (Zobel 1973). The
amylose V complex helix hydrate can be detected by measuring
the intensity of the characteristic 4.4Å diffraction line. Formation
of the B-type crystal structure is followed by measuring the intensity of the 5.25Å spacing. The B structure, typical of retrograded
starch, is extended, unlike the tight V-form helix. Molecules in
these 2 forms do not cocrystallize (Zobel and Senti 1959). With
no surfactants present, bread that is freshly baked shows only Vcrystallinity due to amylose-lipid complexes formed with the native fatty acids in the starch granules (Zobel 1973), during dough
heating (Zobel and others 1988). Thus, a portion of the amylose is
insolubilized during the overall baking process. The remaining
pattern is an amorphous halo produced by gelatinized starch.
During bread aging, the amorphous starch crystallizes into (Btype) crystals, while the V-type intensity (amylose-lipid complexes)
remains unchanged (Zobel and Senti 1959). With surfactant added, breads were softer after 3 d as measured by a compressimeter
(Zobel and Senti 1959). V-lines increased with surfactant addition,
indicating complexation between amylose and the adjuncts. But,
once again, complexation does not necessarily correlate with surfactant effectiveness in retarding staling (Osman and others 1961;
Zobel 1973).
Dragsdorf and Varriano-Marston (1980) suggested that surfactants that produce a V complex hydrate structure (for example,
SSL) may either prevent migration of starch polymer molecules
from granules during baking, thus retarding crumb firming, or re-
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duce the redistribution of water from gluten to starch and thus
prevent contraction and firming of the gluten phase.
Amylopectin complexes. If one accepts the considerable
amount of data pointing to amylopectin crystallization as a key
factor in bread staling, it would make sense that, to be effective
antistaling agents, surfactants must also interact with the amylopectin fraction; but until recently, there was no direct evidence
for amylopectin-surfactant complexes. Zobel (1973) and Eliasson
and Ljunger (1988) alluded to possible amylopectin interaction
with surfactants, since breads with adjuncts showed diminished
B-type crystallinity. Similarly, Knightly (1973) explained that surfactants retarded staling by forming complexes with amylopectin,
but later (Knightly 1996) stated that complex formation is unrelated to bread firming.
While Krog (1971) reported limited interactions of monostearin
with amylopectin, Lagendijk and Pennings (1970) detected formation of amylopectin-monoglyceride complexes, the degree of
which increased in a linear fashion with fatty acyl chain length,
while still being much less than the amount of amylose complex
formation. Kulp and Ponte (1981) mentioned that the effects of
complexation could be intramolecular (outer branch associations)
or intermolecular (aggregation of polymers).
DeStefanis and others (1977) reported that SSL, succinylated
monoglyceride, and glyceryl monostearate complexed equally
with amylose and amylopectin. Interestingly, no binding of the 3
adjuncts was found during the sponge stage. As the dough developed, increasingly strong binding of adjuncts with protein was
discovered. After baking, however, the 3 surfactants were found
strongly associated with starch polymers, the temperature at
which most translocation took place being above 50 °C.
Finally, Biliaderis and Vaughan (1987) obtained direct evidence
of complexes of amylopectin (and amylose) with labeled fatty acid
molecules using electron spin resonance. Then, Gudmundsson
and Eliasson (1990) obtained additional evidence for amylopectin-surfactant complexes using DSC and x-ray diffraction techniques. They also found that the amount of complexed amylopectin was a function of the amylopectin:amylose ratio, since amylose molecules were more effective in forming complexes in competition with amylopectin molecules for the surfactant molecules.
Finally, they determined that surfactant-starch polymer complexation prevented amylose-amylopectin cocrystallization. Using
mutant corn starches, Villwock and others (1999) also provided
DSC evidence for the existence of amylopectin-surfactant interactions in pastes and additional evidence that both hydrocarbon
chain length and the nature of the polar group affect complex formation.
Effect on starch swelling. According to Ponte and others
(1973), the softening effect of surfactants is related to a reduction
in starch granule swelling, and the degree of granule swelling is
inversely related to crumb firmness. They concluded that surfactants restrict granule swelling during baking by complexing with
amylose at the periphery of starch granules. Polar surfactants that
form strong complexes with amylose (for example, long-chain fatty acids, MG, POEMS) restricted granule swelling and solubilization of various starches over the pasting range 60 to 95 °C (Gray
and Schoch 1962). Sodium lauryl sulfate repressed hydration of
starches below 85 °C, but the complex dissociated at higher temperatures.
Infrared spectroscopy was used to investigate whether surfactants adhered to granule surfaces or entered granules (Finn and
Varriano-Marston 1981). SSL did not appear to interact with the
granule surface, while PGMS did.
Lord (1950) concluded that POEMS retarded staling by 2 mechanisms: (1) by a “shortening” action that softened the crumb and
(2) by reducing granule swelling, which resulted in an initial increase in firmness, which changed little during storage. [Note: As
12
mentioned earlier, less swelling means less disruption of crystallinity and other order within the granule, so there is less “gelatinized” starch to recrystallize, but that which is disordered can recrystallize more easily. Another possible effect is less leaching/migration of amylose so that there is less in the intergranular space
to retrograde. Even in excess water, monoglycerides prevent the
leaching of amylose molecules (Schoch 1965). Surfactants prevent dissolution and leaching of amylose molecules, which may
be the factor responsible for the reduction in granule swelling,
and also complex with amylopectin molecules; and their antistaling effect, which is correlated with a reduction in granule swelling, is likely due to a reduction in starch polymer mobility after
complexation so that less crystallization can occur.
Interaction with protein. Willhoft (1973) hypothesized that the
antistaling effect of monoglycerides might be due to interaction
with gluten, and there is experimental evidence that supports this
hypothesis (Hoseney and others 1969; De Stefanis and others
1977; Quail and others 1991). It has been suggested that surfactant molecules associated with gluten are released during baking
(DeStefanis and others 1977) and complex with leached starch
polymers in intergranular spaces (Conde-Petit and Escher 1994.)
Physical properties of surfactants. According to Kulp and Ponte (1981), the physical state of surfactants is an important factor in
their performance. Krog (1973) reported that amylose-monoglyceride complexation ability decreased in the descending order of
monoglyceride physical states: ␣-type crystalline gel > ␤-type
crystalline hydrate > nonhydrated powder. ␣-Type monoglyceride
crystals pack so that polar groups are exposed to the water phase,
and thus have a greater tendency to form effective aqueous adjuncts (Wren 1968; Larsson 1968). ␤-Type crystals show no
marked antifirming effect unless first hydrated before use. Hydrated ␤-crystals are commonly known as a “coagel-foam” (Krog
1968).
Shortening
Shortening is quite effective in retarding bread crumb staling
and has, for many years, been used as an antistaling ingredient in
breads. Since shortening was shown to have no effect in defatted
bread, it was speculated that its effect is related to the native flour
lipids. Since shortening does not complex with starch, its mechanism of antistaling action differs from that of monoglycerides (Rogers and others 1988).
Carbohydrate ingredients
Roles of dextrins and maltooligosaccharides in staling were discussed in the section “Enzymes: ␣-amylases and debranching enzymes”. Roles of native water-soluble and water-insoluble pentosans were discussed in the section “Mechanisms of staling: Role
of pentosans”. Use of hydrocolloids and modified starches and
effects of damaged starch are covered in this section. If it is accepted that moisture redistribution is a requirement for staling to
occur (see “Moisture migration” section), then it follows that any
ingredient that inhibits movement of moisture is a candidate for
reducing staling (Swortfiguer 1971).
Hydrocolloids/gums. Davidou and others (1996) found that,
among locust bean gum, alginate [presumably sodium alginate],
and xanthan, only locust bean gum reduced the rate of dehydration. However, any increased moisture content of breads, if the
moisture is available to the starch molecules, increases the rate of
retrogradation (Rogers and others 1988) (see sections “Mechanisms of staling: Role of pentosans” and “Moisture migration:
Moisture redistribution among components”).
Schiraldi and others (1996a) studied the effects of added hydrocolloids (pentosans, modified pentosans, galactomannans, whey
protein) and reported that guar and locust bean gums retarded
starch retrogradation, but did not have any clear antistaling activi-
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Bread staling. . .
ty. They also found that all the hydrocolloids they used generally
improved quality and that those with higher water-holding capacity increased crumb firmness. In contrast, Davidou and others
(1996) reported that both degrees of crumb firmness and the rate
of staling during storage were reduced by addition of locust bean
gum, alginate [presumably sodium alginate], and xanthan. They
proposed that the gums modified the organization of the amorphous part of the crumb, perhaps by inhibiting gluten-starch interactions, perhaps in the same manner as proposed for dextrins
(Martin and others 1991). They also reported that only locust
bean gum (of the 3 gums) effected water retention. Carboxymethylcellulose (CMC) and hydroxpropylmethylcellulose (HPMC)
(0.3%) also decreased initial firmness (Armero and Collar 1998).
No increases in the Avrami value were found. Hydrocolloid-gluten entanglements or linkages were suggested.
Addition of psyllium husk powder/gum (2, 4, or 8%) decreased
the staling rate of bread as measured by compressibility and DSC
(Czuchajowska and others 1992). Moisture content remained
constant. Bread softness improved without increasing the possibility of microbial deterioration. Lent and Grant (2001) found that
bagels containing added xanthan had slightly higher crumb moisture contents and staled at a somewhat reduced rate (DSC). Addition of pectin increased the specific volume of bread and reduced
the rate of firming during storage (Kegoya-Yoshino 1997).
Replacement of 10% of the wheat flour with steamed oat flours
retarded bread staling without adversely affecting the loaf volume
(Zhang and others 1998). The reduction in the rate of staling was
attributed to the high water absorption capacity of the ␤-glucan in
oat flour, but oat starch has been found to retrograde at a slower
rate compared to other starches (White and others 1989).
Patents have been issued for the use of karaya gum (Andt 1966)
and what is called low-molecular-weight amyloses, but which in
reality are the branch chains of amylopectin released by the action of an a-1,6-glucan hydrolase as antistaling agents (Yoshida
and others 1972).
It has also been reported (although not supported in the article
with published experimental data) that methylcellulose and hydroxpropylmethylcellulose extend the shelf life of baked products
via prevention of water loss during baking (Dziezak 1991). The
same report states that guar gum and xanthan gum function as antistaling agents.
Damaged and modified starch. Modified starches have been
investigated as antistaling agents (Maga 1975). Tipples (1969) reported that the use of 25 to 35% damaged wheat starch decreased the rate of staling, especially when malt was added and
the sponge-dough method was used.
On the 4th d of storage, breads containing 5% of a phosphorylated waxy maize starch were as fresh as a 1-d-old control bread
(Bergthaller and Stephan 1970). The water-holding capacity of the
bread was not affected by the starch phosphates.
Miscellaneous
Results of use of dairy ingredients in breads for antistaling purposes have been inconsistent (Mannie and Asp 1999).
D’Appolonia (1984) reported that milk solids have little to no effect on bread staling, but do soften the crumb initially. Conversely,
others have suggested that nonfat dry milk solids retard staling
(Dubois and Dresse 1984). Acidic whey (concentrated or unconcentrated) retarded staling in Hamam (French-type) bread at 1%
whey solids (Yousif and others 1998). Neither acid casein or sweet
whey powder were found to reduce staling in bread significantly
(Erdogdu-Arnoczky and others 1996), while acid whey powder
did. Despite its high water-holding capacity, succinylated whey
protein concentrate did not prevent bread staling (Thompson and
Baker 1983).
L-Leucine n-alkyl esters slowed the staling rate of bread more
effectively than did monostearin (Watanabe and others 1981).
Propylene glycol was found to be superior to glycerol, maltodextrins, gelatin, commercial ␣-amylase, and poly(propylene glycol) in antistaling activity (Jagannath and others 1998). 1,3-Butanediol and 1,3-heptanediol (0.5%) reduced the staling rate of
bread significantly, with 1,3-butanediol having the greatest effect
(Frankenfeld and others 1977).
Incorporation of durum wheat flour (25%) into a bread wheat
flour did not improve initial firmness, but did retard staling
through 4 d of storage (Boyacioglu and D’Appolonia 1994).
Breads made with triticale flour staled twice as fast as did bread
made with wheat flour (Tsen and others 1973).
Sato and others (1989) reported staling retardation during longterm storage when methyl 3,6-anhydro-␣-D-glucopyranoside was
added to the formula.
Flavor Changes
Staled bread is considered unacceptable due to changed flavor.
A review on bread flavor is available (Lorenz and Maga 1972).
Processes for Acceleration of Staling
A process by which the staling rates of bread products can be
accelerated using a time-temperature-moisture protocol to produce croutons and dry crumbs at a faster rate has been patented
(Slade and others 1987). The staling process is accelerated via
temperature cycling. Bread is exposed alternately to a temperature
just above the glass transition temperature (maximum rate of nucleation) and to a temperature just below the melting temperature
(maximum rate of crystal growth). Other patents for preparing
bread crumbs and/or croutons have been developed (Tu and others 1986; Dyson and others 1980).
Summary of the Basis of Staling and Factors Affecting the
Rate of Staling
Bread staling is a complex phenomenon, certainly involving
multiple factors. Much has been learned about bread staling, and
application of this knowledge has led to considerable improvements in shelf life. However, without knowledge of the precise
mechanism, addressing the problem of bread staling remains a
process of formulating and testing more and more hypotheses. It
is difficult to determine cause-and-effect relationships because involvement of a constituent may be indirect and additives, other
changes in formulation, and process changes may alter more than
one property and the effects may cancel each other.
Retrogradation of starch molecules remains the most widely accepted factor contributing to bread staling, but it must be remembered that there is also good evidence that there is no cause-andeffect relationship between retrogradation and staling. While amylopectin retrogradation is believed to play the major role, amylose
is now also thought to be involved. And while amylose-surfactant
complex formation has been a widely used strategy for reducing
bread staling, amylopectin complexes may also be important, not
necessarily related to an inhibition of retrogradation because additives that retard starch retrogradation may not retard staling.
Moisture content and moisture transfer among bread components is believed by many to be a significant factor contributing to
bread staling. Most evidence supports the concept that gluten
serves as a moisture reservoir from which water is transferred to
retrograding starch molecules. But the relative effects of dehydration of gluten and hydration of starch can only be surmised, as
can the degree of benefit from prevention of this type of moisture
transfer.
Evidence has accumulated for a major role of gluten in bread
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staling. As mentioned above, moisture transfer from gluten to
starch might be involved in the staling process. Beyond this, it has
been proposed that gluten-starch crosslinks are responsible for
staling, but it has become increasingly clear that multiple mechanisms operate during staling. Supermolecular structures of some
sort, perhaps both starch-starch and starch-protein interactions,
are certainly involved, with one or more of the starch or protein
components, including gelatinized granules, being incorporated
in the structures, and with either or both components of the structure possibly modified by interaction with polar lipid molecules.
As has been suggested, it is likely that both starch and gluten contribute to staling with the process weighted towards starch retrogradation, since there is much more starch than protein in bread.
One theory states that bread firming is a result of hydrogen bonding between gelatinized starch granules and the gluten network. It
could also involve hydrogen bonding between retrograded starch
molecules and the gluten network with retrogradation occurring
either before or after association of amylopectin and/or amylose
molecules with the protein network.
Additives that seem to have the greatest effect in reducing staling in bread are (in no special order) surfactants (complexing
agents), ␣-amylase, and hydrocolloids/gums, including modified
starch. The effect of adding ␣-amylase is most certainly indirect;
that is, the antistaling effect is due to in situ formation of starch
dextrins and/or maltodextrins. Processing protocols are also important.
There is the unmistakable conclusion that polymer crystallization is involved in the staling process and that some, perhaps the
majority, of the crystallization involves amylopectin. Gluten, may
also be involved. The most plausible hypothesis is that amylopectin retrogradation involves incorporation of water molecules into
the crystallites and that this requirement shifts the distribution of
water molecules between components, reducing the water associated with gluten and thereby changing the nature of the gluten
network. The role of surfactants may be to change the chemical
(for example, by ionic bonding to protein molecules) or physical
(for example, by complexing with starch polymer molecules) nature of components involved in forming supermolecular structures so that associations are prevented or so that only less perfect
associations are formed. They might also function primarily as
plasticizers, lowering the glass-transition temperature (Tg) so that
the structure is not in a glassy state at room temperature. The fact
that propylene glycol is quite effective supports this latter idea.
Water is an effective plasticizer; and the fact that low-molecularweight carbohydrates, which hold water, are effective plasticizers
and antistaling agents, as are ␣-amylases which produce them, is
further support for the important role of Tg lowering. A water-holding effect of carbohydrates (such as maltodextrins, dextrins, pentosans, and other gums/hydrocolloids that do not themselves become involved in retrogradation or other polymer-polymer interactions) may be involved; retardation of the movement/redistribution of water may be their mode of action. Although considerable
progress in dissecting the staling process has occurred, bread staling remains an intensively studied, yet not well understood, phenomenon.
Methods for Measuring Degrees of Staleness
Probably because of the mystery that still surrounds the staling
process and because there appear to be so many facets to a presumably complex process, a variety of techniques have been employed to measure staling and/or to investigate the changes that
accompany it. Characteristics of bread crumb that have been
used as bases to determine the degree of staling are changes in
taste and aroma, increased hardness, increased opacity, increased
crumbliness, increased starch crystallinity, decreased absorptive
14
capacity, decreased susceptibility to ␣-amylase, and decreased
soluble starch content (Geddes and Bice 1946). It is obvious that
no one method will completely measure or describe the degree of
staling as noticed by the consumer (Sidhu and others 1996). All
investigations of the mechanism and control of staling reported in
this review have employed one or more of the methods covered
briefly in this section to measure the rate and/or degree of staling.
Other reviews of bread staling measurement methods can be
found in Maga (1975), Kulp and Ponte (1981), and Ponte and
Ovadia (1996). Many of the methods used to measure bread staling are based on principles used to determine the extent of starch
retrogradation. Methods for measuring starch retrogradation have
been reviewed by Karim and others (2000).
Rheological methods
Uniaxial compression. As bread stales, the texture of the crumb
changes from a relatively soft, spongy texture to one that is firm
and crumbly. Hence, numerous compressibility methods have
been developed to quantify the firming of bread, which has been
shown to correlate with bread staling as measured by consumer
acceptability. Hence, compressibility measurements are most
commonly used to determine the degree of bread staleness. Compressibility methods were used in most of the investigations mentioned throughout this review and include 2 of the 3 AACC-approved procedures to measure staleness (Maga 1975). Most measure the force applied to compress a sample a specific distance.
AACC Method 74-10A (AACC 2000) measures crumb firming
changes with a Baker’s Compressimeter, determining the force applied by use of a plunger to ensure uniform compression (Baker
and others 1987; Baker and Ponte 1987). AACC Method 74-09
(AACC 2000) uses the Instron Universal Testing Machine to determine the degree of firmness in white pan bread crumb. Baker and
others (1988) confirmed that a 25% compression depth (as specified in AACC Method 74-09) was the most effective method for
detecting significant differences in bread firmness due to staling.
Instron-type systems have advantages over the Baker’s Compressimeter because the compression rate is linear, and thus,
force-time relationships can be directly converted to force-compression curves (Kamel and others 1984). Most important in this
regard is that a correlation coefficient of 0.98 was found between
firmness measured as compressibility and sensory assessments of
the degree of staleness (Axford and others 1968).
Other instruments that measure compressibility, such the Precision Penetrometer (Kamel and others 1984), Texture Analyzer, QTest, Wheat Research Institute Chomper (Baruch and Atkins
1989), Bloom Gelometer (Baker and others 1987), and the General Foods Texturometer (Szczesniak and Hall 1975), can also be
used to quantify the extent of bread staling. The squeeze test,
which gives the perception of freshness of bread and is a reflection of textural properties of the crumb, is popular with consumers (Kamel 1987).
Dynamometric methods have been developed to characterize
the rheological properties of bread slice surfaces (Kulp and Ponte
1981). Young’s modulus can be determined from results of studies
of compressive stress-strain relationships determined with instruments such as the Instron Universal Testing Machine. Baruch and
Atkins (1989) found that, in a dynamic stress-strain curve, the initial slope, which is a measure of crumb flexibility, increased and
the peak height, which is an indicator of the strength of the gluten
network, decreased as staling progressed. Stress-strain results
were correlated to results of thermal analyses by Schiraldi and
others (1996b). The relationship between mechanical properties
of bread and crumb staling has been reviewed in detail (Vodovotz
and others 2001).
Pasting properties. Under the hypothesis that starch retrogradation plays a significant role in bread staling, the Brabender Visco-
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Bread staling. . .
Amylo-Graph, Rapid Visco Analyzer, and related instruments
have been used to measure the extent of starch gelatinization in
bread crumb slurries (Yasunaga and others 1968). Peak viscosity
changes were suggested as an index of staling because it was
thought that peak viscosity would decrease with age due to a
toughening effect on partially gelatinized starch granules during
staling. Based on results in which the outer 1-cm and 2-cm portions of the crumb produced a lower peak viscosity than did the
center portion, it was concluded that starch granules in the crumb
center were less gelatinized than those in the crumb exterior. Despite reporting amylograph data that agreed with those of Yasunaga and others (1968), Varriano-Marston and others (1980) concluded that the amylograph does not indicate the degree of starch
swelling accurately in bakery products, but rather shows the sum
of the contributions of all macromolecules to the viscosity of the
bread slurry. Toufeili and others (1994) found that, as staling increased, pastes made from Arabic bread changed from being viscoelastic solids (G” < G’) to elastoviscous liquids (G” > G’).
Thermal analysis
Thermal analysis has been used extensively to study starch retrogradation as well as bread staling (Russell 1983a, b;
Czuchajowska and Pomeranz 1989; Le Meste and others 1992;
Schiraldi and others 1966a,b; Champenois and others 1995;
Vodovotz and others 1996; Baik and Chinachoti 2000), and its
use has been mentioned throughout this review. Of the thermoanalytical methods, differential scanning calorimetry (DSC) and differential thermal analysis (DTA) have proven to be the most useful
in providing basic information on starch retrogradation (Karim
and others 2000). Because both measure the differential temperature or heat flow to or from a sample versus a reference material
as a function of time, both can be used to monitor such changes
as phase transitions, molecular conformational changes, interactions with other components, and pyrolytic degradation of the
sample. Specialized DSC instruments, including modulated DSC
and polarization DSC, are also available (Schenz and Davis
1998).
When aged bread samples are heated in a DSC pan, an endotherm is observed as reordered amylopectin reaches its glass transition and/or melting temperature, and the enthalpy change associated with this transition can be measured. Because the time
scales for endotherm development and for the increase in crumb
firmness are broadly similar in magnitude, DSC can be used to
measure the rate of bread staling quantitatively (Jagannath and
others 1999a). However, there are overlapping transitions over a
wide temperature range because of the variety of components
and range of structures present, which cause difficulty in analysis
(Vodovotz and others 2001).
DTA was used to investigate bread staling by Axford and Colwell (1967). An endotherm peak, which was absent in fresh bread
samples, developed during storage, and the increases in peak
area were proportional to increases in bread firmness (Cornford
and others 1964). Because an increase in glass transition temperature (Tg) of bread crumb stored for different times was correlated
(96.53%) with an increase in the degree of bread staling as measured by compression analysis, it was concluded that the measurement of Tg during storage could be used to quantitatively predict the rate of staling (Jagannath and others 1999a). DSC studies
of starch can approximate gelatinization during baking, since in
both cases the gelatinized starch granules are swollen, but nondisrupted (Jacobson and BeMiller 1998). Thus, the conditions of
gelatinization in the calorimeter more closely approximate those
encountered during baking than those encountered during starch
pasting. Unlike compressibility measurements, endotherm peak
development does not appear to be dependent on specific loaf
volume (Fearn and Russell 1982).
Isothermal microcalorimety, a technique which is much more
sensitive and requires larger samples sizes than does conventional DSC (Karim and others 2000), has been used to study the early
stages of starch retrogradation and has been demonstrated to be
effective for examination of the antistaling effects of lipids and surfactants (Silverio and others 1996).
Other thermoanalytical instruments include thermogravimetric
analysis (TGA), thermomechanical analysis (TMA), and dynamic
mechanical analysis (DMA). TGA measures changes in the weight
of a sample as a function of temperature (Schenz and Davis
1998). While events such as volatization, dehydration, and chemical reactions can be observed using TGA, other simple transitions
can be missed if no weight changes occur (Sperling 1992).
Schiraldi and others (1996b), using TGA, found that the release of
water upon heating bread corresponded to 2 main binding states,
and that the 2 fractions were dependent on the age of the bread.
TMA measures changes in penetration, extension, expansion,
or contraction as a function of temperature (Schenz and Davis
1998) and can be used to determine the Tg of a substance by detecting a change in the thermal expansion coefficient (LeMeste
and others 1992). The deformation of a substance is measured
under nonoscillatory (static) load as the substance is subjected to
a controlled temperature program (Flynn 1990). LeMeste and others (1992) developed a TMA method to measure the glass transition of white pan bread.
DMA measures the dynamic moduli and damping of a substance under oscillatory load as a function of temperature and frequency as it is subjected to a controlled temperature program
(Flynn 1990). DMA has also been referred to as forced oscillatory
measurements, dynamic mechanical thermal analysis (DMTA),
dynamic thermomechanical analysis, and dynamic rheology (Menard 1999). In DMA, as oscillatory stress is applied to the sample
in the bending or tensile mode of deformation, the lag of the resulting oscillatory strain is measured. DMA is 1000 times more
sensitive in observing thermal transitions than is DSC (Vodovotz
and others 1996). DMA has been used to study staling profiles of
Indian unleavened breads by Jagannath and others (1999b), to investigate the effects of added hydrocolloids, pentosans, and soluble proteins on bread staling (Schiraldi and others 1996a), and to
examine the effect of aging and drying on thermal transitions of
bread (Vodovotz and others 1996).
Infrared spectroscopy
Fourier transform infrared (FTIR) spectroscopy and near-infrared
(NIR) spectroscopy, which have the advantage of being noninvasive methods, have been used to monitor staling in bread (Wilson
and others 1991).
Fourier transform infrared (FTIR) spectroscopy. Because FTIR
spectroscopy measures the degree of short-range ordering in a
system, conformational changes brought about by starch retrogradation can be monitored by analyzing the band-narrowing,
which is caused by a reduction in the range of conformations and
a smaller distribution of bond energies due to the system becoming more ordered upon staling (Wilson and others 1991; Karim
and others 2000). Changes in band intensities in the 1300 to 800
cm-1 region correlate to conformational changes during starch retrogradation. Peaks at 1047 cm–1, which relate to crystalline regions of starch, and at 1022 cm–1, which are characteristic of
amorphous regions of starch, are of particular interest (Karim and
others 2000). Thus, starch retrogradation can be defined (in terms
of FTIR data) as an increase in the ratio of peak intensities at 1047
and 1022 cm–1 (Smits and others 1998).
Near infrared (NIR) reflectance spectroscopy. Radiation scattering, which in the case of bread relates to the degree of crystallinity of amylopectin, can be measured by NIR absorbance, so
NIR can be used to follow the progress of bread staling (Wilson
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and others 1991). Starch molecules in bread are extensively hydrogen bonded (both intramolecularly and to water). Because the
absorption bands in reflected NIR give information about hydrogen bonding, NIR reflectance data can be used to detect changes
in the hydrogen bond network of a bread system during staling
(Iwamoto and others 1987; Wilson and others 1991).
Nuclear magnetic resonance (NMR) spectroscopy
NMR techniques that have been used to study bread staling
and NMR techniques that have been used to examine changes in
molecular mobilities in breads, include solid-state proton NMR,
deuterium NMR, 13C NMR with cross polarization and magic-angle spinning (CP MAS), and pulsed NMR (Ruan and Chen 2000).
Many have used NMR methods to determine the states of water
in bread and to relate them to bread firming (Leung and others
1983; Wynne-Jones and Blanshard 1986; Kim-Shin and others
1991; Chen and others 1997a,b; Engelsen and others 2001).
Low-field proton NMR has been used preferentially to examine
bread staling since it can provide rapid determination of proton
mobility associated with different molecules (Ruan and Chen
2000). Theoretically, there is an equilibrium state in bread where
mobile (liquid phase) and immobile (solid phase) protons coexist.
Since physiochemical changes can effect a new equilibrium state,
NMR can be used to determine mobility changes during bread
staling. However, bread is always in a nonequilibrium state, and
therefore, its nature changes continuously.
A pulsed-NMR method was used to monitor molecular changes
that resulted in increases in firmness during aging of starch gels
and starch-based products (Seow and Teo 1996). Morgan and
others (1992) used 13C CP MAS NMR to determine crystalline solid, amorphous solid, and liquid-like phases of fresh and stored
wheat starch gels. Other NMR techniques have also been used,
including the 17O NMR method developed by Kim-Shin and others (1991), to monitor water mobility in bread.
Magnetic resonance imaging (MRI) maybe useful in investigations of the mobility of protons in breads during staling. Ruan and
others (1996) monitored moisture migration from crumb to crust
in sweet rolls by MRI during 5 d of storage and found that, with an
increase in storage time/staling, the mobility of the less-mobile
fraction of water decreased and the mobility of the more-mobile
fraction of water increased.
Using proton cross-relaxation NMR spectroscopy, Wu and
Eads (1993) determined that the starch polymer molecules in concentrated waxy maize starch gels could be divided into 3 classes,
characterized by their degree of molecular mobility, and that the
percentage of immobile molecules increased with time, while the
percentage of mobile molecules decreased. Using the same technique, Vodovotz and others (2002) found no change, that is, no
increase in rigidity, of an aged bread sample, even though there
was an increase in amylopectin retrogradation enthalpy (DSC).
X-ray crystallography
X-ray crystallography has been used to examine bread staling
(Zobel 1973), specifically the crystalline nature of the starch in the
system, which can be related to the firmness of the product
(Champenois and others 1995). Starch in freshly baked bread is
mostly amorphous, but slowly reorders during storage. The recrystallization is reflected in x-ray diffraction patterns (Karim and
others 2000). Therefore, x-ray crystallography can be used to determine the molecular organization of starch in bread (VarrianoMarston and others 1980). However, powder x-ray diffraction is
not particularly sensitive as compared with other techniques,
such as NMR and FTIR, which are able to detect even minor extents of recrystallization (Smits and others 1998).
X-ray crystallography has been compared with DSC for determining the increase in crystallinity during storage of Arabic bread
16
(Sidhu and others 1997) and used in conjunction with DSC in the
analysis of the effect of various antistaling additives on wheat
bread (Jagannath and others 1998).
It has been concluded that there is not necessarily a cause-andeffect relationship between starch crystallization and bread firming (Dragsdorf and Varriano-Marston 1980; Zobel and Senti
1959), emphasizing the need, when investigating bread staling,
for methods that are not limited to measuring changes in only 1
component.
Jagannath and others (1998) used wide-angle x-ray scattering
(WAXS) to measure the degree of staling.
Conductance and capacitance
It has been established that changes in bread resulting in staling
of the crumb are at least correlated with starch retrogradation and
moisture redistribution between gluten and starch, whether or not
there is any cause-and-effect relationship. Since free and bound
water differ in their dielectric constants, changes during staling
could cause a change in the electrical properties of bread crumb.
Kay and Willhoft (1972) found that retrogradation was accompanied by changes in conductance and capacitance, indicating that
changes accompanying bread staling could be detected electrically and, furthermore, could be described by an empirical equation identical in form with the Avrami equation. Zaussinger and
others (1975) obtained bread staling data in a similar fashion.
Microscopy
Transmitted and polarized light microscopies. Transmittedlight and polarized-light microscopy have been utilized to monitor changes in starch granules from bread before and after staling
(Hug-Iten and others 1999, 2001). Native starch granules are birefringent and possess ‘Maltese crosses’ when viewed under polarized light. Upon gelatinization, starch crystallites melt and order,
and birefringence is lost. During bread baking, starch granules
lose their Maltese crosses, but retain slight birefringence (Varriano-Marston and others 1980) and granular identity. Upon aging,
the bread crumb regains some birefringence (which is not the
usual native starch granule birefringence, but does indicate
biopolymer ordering in the long, thin birefringent structures) due
to molecular reordering, except in ␣-amylase-containing bread
crumb, which contained more of the birefringent structures (as
compared to a control crumb made without a-amylase) initially,
which changed little with aging (Hug-Iten and others 2001).
Confocal laser scanning microscopy (CLSM). The advantage of
confocal laser scanning microscopy over other microscopies is its
ability to produce an image of the focal plane of interest (optical
section), which can be digitally reconstructed into a 3-dimensional image. CLSM has provided qualitative information about the
crumb structure of bread (Bugusu and others 2002). CLSM has
also been used to investigate changes in starch granules in bread
during staling (Vodovotz and Chinachoti 1998). However, it has
been reported that there were no differences in confocal images
of fresh and 10-d old bread, suggesting that the changes that occur during staling are submicroscopic, that is, molecular only.
[Note: Since, as bread stales, starch molecules become more crystalline and more opaque, reflectance confocal laser scanning microscopy (R-CLSM) might provide more precise 3-D information
on the changes in the starch fraction during staling. R-CLSM offers
the highest resolution of CLSM modes (Hibbs 2000), but to our
knowledge has not been applied to investigations of bread staling.]
Electron microscopy. Electron microscopy has not been used
to study bread staling, but certainly has promise. Both transmission and scanning electron microscopy have been used to investigate doughs (Aranyi and Hawrylewicz 1968; Khoo and others
1975; Bechtel and others 1978; Evans and others 1981), bread
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Bread staling. . .
(Khoo and others 1975; Bechtel and others 1978), and pastes
(Fannon and BeMiller 1992 and references therein).
Sensory/organoleptic tests
Loss of flavor and aroma are among the most noticeable detrimental changes of bread upon staling. Reportedly, the decrease in
the acceptability of bread over 5 d of storage is correlated with a
reduction in aldehydes and an increase in ketones (Lorenz and
Maga 1972). The resulting flavor is one that has been described as
“bland” (Setser 1996). Changes in texture, of course, also accompany the bread staling phenomenon and can be measured by
both uniaxial compression methods (Section 8.1.1) and sensory
evaluations.
AACC Method 74-30 (AACC 2000) involves panel ratings of a
sum of factors affecting overall staling (“appearance” or “feel” of
the crumb/crust, “taste” and “mouthfeel”, “firmness”, “flavor”, and
“texture” change, or any other important factor noted by a panelist). A high correlation between measured crumb firmness and
staling as rated by panelists has been found (Cornford and others
1964; Axford and others 1968). Other examples of organoleptic
evaluations are discussed by Pomeranz and Shellenberger (1971)
and Setser (1996).
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MS 20010513 Submitted 9/18/01, Revised 8/12/02, Accepted 10/3/02, Received
10/5/02
Authors Gray and BeMiller are Ph.D. student and Professor, respectively,
with the Whistler Center for Carbohydrate Research, Dept. of Food Science, Purdue Univ., West Lafayette, Ind., U.S.A. Direct inquiries to author
BeMiller (E-mail: [email protected]).
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