in nonheated doughs. Cereal Chem. 69:1-6.
HIBBERD, G. E. 1970a. Dynamic viscoelastic behavior of wheat flour
doughs. Part II. Effects of water content in the linear region. Rheol.
Acta 9:497-500.
HIBBERD, G. E. 1970b. Dynamic viscoelastic behavior of wheat flour
doughs. Part III: The influence of the starch granules. Rheol. Acta
9:501-505.
HIBBERD, G. E. and PARKER, N. S. 1975. Measurement of the fundamental rheological properties of wheat-flour doughs. Cereal Chem.
52: 1r-23r.
HIBBERD, G. E., and WALLACE, W. J. 1966. Dynamic viscoelastic
behavior of wheat flour doughs. Part I: Linear aspects. Rheol. Acta
5:193-198.
HOSENEY, R. C., FINNEY, K. F., POMERANZ, Y., and SHOGREN,
M. D. 1971. Functional (breadmaking) and biochemical properties of
wheat flour components. VIII. Starch. Cereal Chem. 48:191-201.
LINDAHL, L., and ELIASSON, A-C. 1986. Effects of wheat proteins
on the viscoelastic properties of starch gels. J. Sci. Food Agric. 37:11251132.
MATSUMOTO, S. 1979. Rheological properties of synthetic flour
doughs. Pages 291-301 in: Food Texture and Rheology. P. Sherman,
ed. Academic Press: New York.
MEDCALF, D. G. 1968. Wheat starch properties and their effect on
bread baking quality. Baker's Dig. 42(4):48-50, 52, 65.
MENJIVAR, J. A. 1990. Fundamental aspects of dough rheology. Chap.
1 in: Dough Rheology and Baked Product Texture. H. Faridi, and
J. M. Faubion eds. AVI Publishing: New York.
MITA, T., and MATSUMOTO, S. 1984. Dynamic viscoelastic properties
of concentrated dispersions of gluten and gluten methyl ester:
Contributions of glutamine side chain. Cereal Chem. 61:169-173.
NAVICKIS, L. L. 1989. Rheological changes of fortified wheat and corn
flour doughs with mixing time. Cereal Chem. 66:321-324.
NAVICKIS, L. L., ANDERSON, R. A., BAGLEY, E. B., and JASBERG,
B. K. 1982. Viscoelastic properties of wheat flour doughs: Variation
of dynamic moduli with water and protein content. J. Texture Stud.
13:249-264.
SMITH, J. R., SMITH, T. L., and TSCHOEGL, N. W. 1970. Rheological
properties of wheat flour doughs III. Dynamic shear modulus and its
dependence on amplitude, frequency, and dough composition. Rheol.
Acta 9:239-252.
SZCZESNIAK, A. S., LOH, J., and MANNELL, W. R. 1983. Effect
of moisture transfer on dynamic viscoelastic parameters of wheat flour/
water systems. J. Rheol. 27:537-556.
WATSON, S. A. 1984. Corn and sorghum starches: Production. Pages
417-468 in: Starch: Chemistry and Technology, 2nd ed. R. L. Whistler,
J. N. BeMiller, E. F. Paschall, eds. Academic Press: New York.
WEIPERT, D. 1990. The benefits of basic rheometry in studying dough
rheology. Cereal Chem. 67:311-317.
[Received June 29, 1994. Accepted September 22, 1994.]
BREADMAKING
Effects of Certain Breadmaking Oxidants and Reducing Agents
on Dough Rheological Properties'
WEI DONG3 and R. C. HOSENEY 2
ABSTRACT
Cereal Chem. 72(l):58-64
A dynamic rheometer was used to characterize the effect of glutathione,
potassium bromate, and two ascorbic acid isomers on the rheological
properties of wheat flour doughs. During resting after mixing (dough
relaxation), G' decreased and the loss tangent increased. The major factor
causing those changes was suggested to be a sulfhydryl-disulfide interchange. Free radicals appeared to be involved in the sulfhydryl-disulfide
interchange. Addition of potassium bromate to dough resulted an increase
in G'and a decrease in loss tangent. L-threoascorbic acid was rheologically
more effective than D-erythroascorbic acid. This was explained by the
presence of an active glutathione dehydrogenase in wheat flour that is
specific for both glutathione and L-threoascorbic acid.
Sperling (1986) cataloged the causes of stress relaxation into
five categories: 1) a decrease in molecular weight caused by chain
scission as a result of oxidative degradation or hydrolysis; 2)
bond exchanges ongoing constantly in polymers, with or without
stress (in the presence of a stress, however, the statistical rearrangements tend to reform the chains, so the stresses are reduced);
3) viscous flow caused by linear chains slipping past one another;
4) thirion relaxation as a reversible relaxation of the physical
attracted the attention of many cereal chemists. The main premise
has been that these sulfhydryl groups are potentially capable of
undergoing a disulfide-sulfhydryl interchange that involves the
cleavage or reformation of disulfide bonds mediated by sulfhydryl
groups in flour or by relatively small amounts of added sulfhydryl
cross-links or trapped entanglements in elastomeric networks; 5)
molecular relaxation, especially near the glass transition temperature (Tg), that tends to relieve any stress of chains during the
experiment.
The role of the sulfhydryl groups in dough chemistry has
'Contribution 94-432-J. Kansas Agricultural Experiment Station, Manhattan.
Graduate research assistant and professor, respectively, Department of Grain
Science and Industry, Kansas State University, Manhattan.
3
Presently at Nabisco Brands, East Hanover, NJ.
2
a 1995 American Association of Cereal Chemists, Inc.
58
CEREAL CHEMISTRY
compounds.
Reduced glutathione (GSH) and oxidized glutathione (GSSG)
are both naturally occurring in wheat flour (Kuninori and
Matsumoto 1964, Hird et al 1968, Tkachuk 1969). Graveland
et al (1978) reported that flour contained 5-7 mmol of sulfhydryl
groups and 11-18 mmol of disulfide per kilogram. Kuninori and
Sullivan (1968) studied disulfide-sulfhydryl interchange in wheat
flour by adding radioactive glutathione. They reported that significant interchange took place in a flour-water dough, but not in
a flour suspension. They postulated that mixing promoted the
reaction of disulfide groups and GSH. Another possibility is that
a free radical (GS-) is formed during mixing and may be involved
in the disulfide-sulfhydryl interchange. Reaction with (GS-) can
cause session of protein disulfide forming a protein thiyl radical.
When interchain disulfide bonds are cleaved, the resulting
depolymerization of the gluten proteins decreases the molecular
weight and thereby reduces the elasticity and increases the
extensibility of dough. Evidence for the occurrence of the interchain disulfide bonds in gluten, particularly in the glutenin fraction, have been presented by Beckwith and Wall (1966) and
Yoshida et al (1980). Some of these interchain disulfide bonds
may be important in maintaining the gluten structure: a reduction
of 3-4% of the disulfide bond with mercaptoethanol caused a
depolymerization of 80% of the high molecular weight gluten
proteins (Jones et al 1974).
It is generally accepted that the rheological properties of dough
and its three-dimensional network are dependent on the arrangement and number of disulfide bonds and sulfhydryl groups of
the protein. The vital contribution of disulfide bonds to dough
stability has been shown in rheological studies by the addition
of either sulfhydryl compounds or sulfhydryl-blocking reagents.
A small amount of cysteine or reduced glutathione dramatically
increases the extensibility of dough. Bloksma (1972) showed that
both the viscous and elastic component of dough deformation
were increased by reduced glutathione.
Bloksma (1972) studied the relationship between the sulfhydryl
and disulfide contents of dough and its rheological properties.
He reported that only small fractions of the total sulfhydryl and
disulfide groups were rheologically effective, and these fractions
were much smaller than the chemically reactive ones. Jones et
al (1974) estimated, on the basis of farinograph measurements,
that -25-35% of sulfhydryl groups and 4-13% of the disulfide
bonds were rheologically effective. However, a small decrease
in crosslinking was sufficient to cause a considerable rheological
effect because of disappearance of rheologically effective groups
(Bloksma 1972). In most breadmaking processes, after mechanical
development of the gluten network, the structure must be stabilized by oxidants. Minute amounts of oxidizing reagents such
as potassium bromate or dehydroascorbic acid (DHAA) improve
the handling and baking characteristics of wheat flour. Loaf
volume increased, and bread had a better crumb grain (Jorgensen
1939). Most current theories on the improver action of oxidants
agree that sulphydryl groups are involved in the reaction mechanism. The bromate is assumed to oxidize low molecular SHpeptides (glutathione) and consequently hamper sulfhydryldisulfide interchange of gluten molecules (Bloksma 1972).
The sulfhydryl-disulfide interchange reaction also is used to
explain the action of ascorbic acid, but the number of steps
involved in its improver action are higher than with bromate.
The four stereo isomers of ascorbic acid and their dehydro-forms
display quite different activities as improvers. Walther and Grosch
(1987) reported that the specificity of the enzyme glutathione
dehydrogenase (dehydroascorbate reductase, EC 1.8.5.1) was
responsible for the difference in the improver action. L-threodehydroascorbic acid was the best and D-threodehydroascorbic acid
was the worst substrate of the four stereoisomers. This enzyme,
which was discovered in wheat by Kuninori and Matsumoto (1963,
1964), oxidizes glutathione to its corresponding disulfide with
DHAA as the oxidant. The enzyme appears to be specific for
both glutathione and L-threodehydroascorbic acid. Thus, the improver action of L-threodehydroascorbic acid is explained by the
oxidation of glutathione to the oxidized form. The decrease in
glutathione decreases the rate of sulfhydryl-disulfide interchange
in the dough. The order of substrate specificity of glutathione
dehydrogenase for the four stereoisomers corresponds well with
their improver action in dough (Mair and Grosch 1979).
The purpose of this study was to characterize the effect of
potassium bromate, glutathione, and ascorbic acid on the rheology
of flour-water doughs using a dynamic rheometer.
MATERIALS AND METHODS
Commercial bread flour, 12.1% protein (N X 5.7), 0.45 ash,
(14% mc) from Ross Industries (Cargill, Wichita, KS) was used.
The flour was fractionated into water-insoluble and soluble
fractions according to the procedure shown in Figure 1. One
part of flour was suspended in three parts of distilled water and
stirred continuously for 15 min. Then the suspension was centrifuged for 20 min at 1,000 X g. The insoluble residue, gluten
plus starch, was frozen and lyophilized. The water-soluble fraction
was boiled for 15 min and then frozen and lyophilized. Both
the water-insoluble and water-soluble fractions were ground in
12
10 F
8
Flour
Water
1 part
3 parts
6
I
Stir
0.7
I
I
I
H
15 min
Centrifuge
E
0.6 I
0
t
1000 x G - 20 min
z
Water soluble
Boil
1 15 min
Lyophilize
Fig. 1. Flour fractionation procedure.
Water insoluble
Lyophilize
0
0.5
0.4
x
0
. |~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
120
60
TIME (MIN)
Fig. 2. Dough rheological changes during resting time. Flour-water dough
(0) tested immediately after mixing or after resting in a bowl covered
with a plastic plate at ambient temperature (230 C) for various times.
Vol. 72, No. 1, 1995
59
a Ross Mill until they passed a I0XX sieve. The water solubles
and insolubles were blended and rehydrated by holding at 85%
rh until the flour reached about 14% moisture. Mixing time and
water absorption were determined with a mixograph.
All chemicals used in the study were reagent grade. Potassium
bromate was from Baker & Adamson; L-threoascorbic acid from
Fisher Scientific; and glutathione and D-erythroascorbic acid from
Eastman Kodak. Also obtained from Eastman Chemical Products
was Tenox-4, a food-grade antioxidant containing 20% butylated
hydroxyanisole (BHA), 20% butylated hydroxytoluene (BHT),
and 60% corn oil. Solutions of each chemical were prepared fresh
daily. For the interaction studies, the two solutions to be studied
were added to the water just before mixing.
The rheometer used was as described previously (Faubion et
al 1985, Dreese 1988a). Dynamic measurements were taken at
an oscillation frequency of 5 Hz and 1% strain. Dough was prepared with optimum mixing time and absorption except where
noted. The doughs were tested immediately after mixing or after
resting in a bowl covered with a plastic plate at ambient temperature (230C) for 60 min, 120 min, and 180 min. The results are
presented as plots of storage moduli G' and loss tangent versus
storage time. Data were evaluated using the Statistical Analysis
System (SAS Institute, Cary, NC).
RESULTS AND DISCUSSION
Effect of Resting Time
When a flour-water dough was rested at room temperature
for 180 min, it became softer. G' decreased, while the loss tangent
increased (Fig. 2). During resting, a number of factors may affect
dough rheology: relaxation of the stresses introduced during
mixing, continued hydration of flour components, and redistribution of water. It is also possible that an alteration of gluten and
starch may occur because of enzymatic action. Another possibility
is that the sulfhydryl-disulfide interchange continues during dough
resting and, as a result, the average molecular weight of the gluten
protein decreases.
Dough tested immediately after mixing had higher G' and
smaller loss tangent than did doughs that were allowed to rest
in a bowl for 15 min before testing (Fig. 2). Loading the dough
in the rheometer immediately after mixing (-1 or 2 min required),
did not allow sufficient time for the stresses to relax. Therefore,
placing the dough between the parallel plates of the rheometer
and allowing it to rest resulted in no change in the rheological
properties (Dreese et al 1988b).
However, dough allowed to rest in a bowl undergoes rearrangements that release the stress. As a result, the dough becomes
softer, G' becomes smaller, and the loss tangent becomes larger.
When doughs were rested in a bowl at room temperature for
30 and 45 min before testing, the rate of decrease of G' was
much slower than that for dough during the first 15 min of resting.
Glutathione, both reduced and oxidized, are naturally occurring
in wheat flour (Kuninori and Matsumoto 1964, Hird et al 1968,
Tkachuk 1969). During mixing, disulfide bonds are ruptured and
create thiyl radicals (MacRitchie 1975). This free radical can then
participate in and increase the rate of sulfhydryl-disulfide
interchange reactions. Because the viscoelastic properties of dough
are primarily related to its continuous protein phase, a decrease
in the average molecular weight of gluten protein would be
12
601
10
cL
b
10
8
o Control
* Tenox-4 1%
12 F
F
6
8
0.70
6
I
I
I
0.65
0.7 F
z
0.60
z
z
0.6
F
I-
0.55
z
H-
0.5 1
0.50
0.4
I --
0
I-
60
-
120
L-
TIME (MIN)
Fig. 3. Effect of antioxidant on dough rheology. Flour-water dough control
(0) and flour-water dough with 1% Tenox-4 (0) based on flour weight.
60
CEREAL CHEMISTRY
0
60
120
180
180
TIME (MIN)
Fig. 4. Effect of glutathione on dough rheology. Flour-water dough control
(0). Flour-water dough treated with 5 ppm (0), 25 ppm (A), or 100
ppm (A) glutathione based on flour weight.
expected to cause a reduction in G' and an increase in the loss
tangent.
To determine whether free radical-mediated sulfhydryl-disulfide
interchange reactions were major factors causing the dough to
become softer during rest, a free radical scavenger was added
to the dough. By terminating the radicals, the scavenger would
be expected to materially decrease the rate of interchange in the
dough. The mixing time increased significantly with the addition
of 1% of Tenox-4, as shown previously by Schroeder and Hoseney
(1978). Dough containing Tenox-4 did not decrease in G' after
the first 60 min (Fig. 3). The rapid decrease in G'-during the
first 60 min was considered to be a stress relaxation. The effect
of Tenox-4 can be explained by its action as a radical scavenger,
thus stopping the sulfhydryl-disulfide interchange during resting.
Increasing the Tenox-4 content to 2 and 3% only slightly increased
the G'. This experiment provides evidence to support the assumption that free radicals are involved in the sulfhydryl-disulfide
interchange during dough resting. These data indicate that strain
relaxation occurs rapidly (<60 min) and is followed by a slow
decay of G' brought about by free radical-mediated sulfhydryldisulfide interchange reactions.
Obviously, these functional groups may interact with charged
residues of proteins. However, the most reactive group in
glutathione is the sulfhydryl of the cysteine side chain. This group
can serve as a nucleophile, a reductant, and a scavenger of free
radicals. Reactions involving glutathione as a reductant usually
lead to formation of glutathione disulfide.
Addition of glutathione to a flour-water dough reduced G'
and increased the loss tangent (Fig. 4). The doughs become relatively less elastic as the glutathione content was increased from
5 ppm to 100 ppm, as expected. According to the continuous
network model of protein structure (Bloksma and Bushuk 1988),
the elasticity of dough is at least partly due to disulfide crosslinks
in the network of protein molecules. The storage modulus (G')
would be expected to increase with increased disulfide bonds in
gluten and the loss modulus (G") to increase with increased low
molecular sulfhydryl content. The presence of glutathione increases the rate of thiol-disulfide interchange reactions, which
decreases the size of large proteins and results in lower molecular
weight. The actual occurrence of such a reaction has been
demonstrated by chemical experiments (Kuninori and Matsumoto
1963, 1964).
Effect of Glutathione
Glutathione is a tripeptide, with two negatively charged carboxylate groups and one positively charged amino group.
l
O KBrO3 50 ppm
12
-
*
KBrO 3 50 ppm GSH 100 ppm
v
GSH 100 ppm
12
0
01-
10
C0
y
10
%_.
y
8
%_.
8
6
0.8
6
-I
_~~~~~
Y-
v-I
'-
0.7
IoI
f.
0, ,0
z
w
C,
z
z
w
C
z
,,
0.6
0.6
0.4
0.5
-
0
0
60
120
180
TIME (MIN)
Fig. 5. Effect of potassium bromate on dough rheology. Flour-water dough
control (0). Flour-water dough containing with 10 ppm (0) or 50 ppm
(A) potassium bromate.
60
120
180
TIME (min)
Fig. 6. Effect of potassium bromate and glutathione on dough rheology.
Flour-water dough control (0) (58% abs, pH = 4.9) containing 50 ppm
KBrO3 . Flour-water dough (58% abs, pH = 4.9) containing 50 ppm KBrO3
(0). Flour-water dough (58% abs, pH = 4.9) containing 100 ppm
glutathione (A).
Vol. 72, No. 1, 1995
61
During the first 60 min of resting, the decrease in G' of doughs
with different glutathione contents parallelled the decrease in G'
of the control dough (Fig. 4). However, the G' of doughs containing glutathione remained constant after 2 hr of resting, whereas
the G' of the control flour-water dough continued to decrease.
As shown above, sulfhydryl-disulfide interchange continues when
the dough rests for 180 min. Low molecular weight sulfhydryl
groups are more mobile than disulfide groups in protein. Therefore, added glutathione has more of a chance to react with low
molecular weight sulfhydryl groups to form a stable disulfide
bond (GS-SG). This can be viewed as free radical scavenging.
As a result, this reduces the chance of intermolecular cross-links
breaking and gives a constant G'.
Effect of Potassium Bromate
Potassium bromate has well-known effects on the rheological
properties of dough. A widely accepted explanation for its action
is that potassium bromate removes sulfhydryl groups by oxidizing
them to disulfide bonds (Bloksma and Bushuk 1988). Adding
potassium bromate to flour-water dough increased G' slightly
relative to the control during the first 60 min (Fig. 5). As stated
previously, sulfhydryl groups naturally occur in flour. The increase
of G' might be caused by a decrease of sulfhydryl groups in
the dough. Increasing the concentration of potassium bromate
from 10 ppm to 50 ppm did not change G' significantly. One
12
0
Control
L-AA 50 ppm
D-AA 50 ppm
a
reason could be that 10 ppm potassium bromate was sufficient
to give the maximum effect (Fig. 5).
When the dough was mixed with 50 ppm potassium bromate
and 100 ppm of glutathione, and the pH adjusted to 4.9, the
G' and loss tangent were about equal to those for dough mixed
with 50 ppm potassium bromate alone at pH 4.9 (Fig. 6). This
indicates that potassium bromate completely overcomes the effect
of glutathione, presumably by oxidizing it to its disulfide.
Effect of Ascorbic Acid and its Isomers
Immediately after mixing, the G' of doughs containing Lthreoascorbic acid or D-erythroascorbic acid was not changed
from that of the control. With increased rest time, G' of the
doughs containing the two isomers increased slowly. L-threoascorbic acid was more effective than D-erythroascorbic acid (Fig. 7).
The oxidizing effect of ascorbic acid can be explained by two
sets of reactions. First, the ascorbic acid was oxidized to its corresponding dehydroascorbic acid. This reaction was very fast, being
complete during mixing.
In the second reaction, L-threodehydroascorbic acid oxidized
the sulfhydryl groups to the corresponding disulfide. This reaction
was catalyzed by the enzyme glutathione dehydrogenase (EC
1.8.5.1).
Because sulfhydryl groups were oxidized to a relatively stable
disulfide, they were less active in thiol-disulfide interchange.
Consequently, G' increased slowly relative to the control. This
is in agreement with the improver action of ascorbic acid as shown
by Mair and Grosch (1979).
Dough mixed with 30 ppm glutathione and 50 ppm L-threoascorbic acid had a slightly lower G' than the dough containing
0
10
f
12
y
_E
*%
8
\
Control
0* L-AA 50ppm GSH 30ppm
v L-AA 50ppm
v GSH 30ppm
%f
10
_
ca
y
., %\
%_
81
6
I
-
I
I
I
_,I
6
I
0.7
0.65 _
z
0
z
zw 0.60 _
I--
0.6
C,
z
I--
I-5
0.5
'/
0.55
'
0.50
I I
I
0
I
I
60
120
TIME (min)
I
180
Fig. 7. Effects of ascorbic acid on dough rheology. Flour-water dough
control (0). Flour-water dough treated with 50 ppm D-erythroascorbic
acid (A) or 50 ppm L-threoascorbic acid (0).
62
CEREAL CHEMISTRY
0
I
I
a
60
120
TIME (MIN)
I
I
180
Fig. 8. Effects of L-threoascorbic acid and glutathione on dough rheology.
Flour-water dough control (0). Flour-water dough treated with 50 ppm
L-threoascorbic acid and 30 ppm glutathione based on the flour weight
(0). Flour-water dough treated with 50 ppm L-threoascorbic acid (A)
or 30 ppm glutathione (A).
12
7
10
0.
cL
b
by
6
8
5
6
,
_I
0.7
0.6
z
w
I-~~~~~~lv
0.5
z
0.5
0
60
120
TIME (MIN)
180
Fig. 9. Effects of D-erythreoascorbic acid and glutathione on dough
rheology. Flour-water dough control (0). Flour-water dough treated with
50 ppm D-erythreoascorbic acid and 30 ppm glutathione (0). Flour-water
dough treated with 50 ppm D-erythreoascorbic acid (A) or 30 ppm
glutathione (A).
only 50 ppm L-threoascorbic acid (Fig. 8). This indicated that
the L-threoascorbic acid eliminated the effect of glutathione.
Dough mixed with 30 ppm glutathione and 50 ppm D-erythroascorbic acid, had essentially the same G' as dough containing
only glutathione (Fig. 9). This indicated that D-erythroascorbic
acid was not used by glutathione dehydrogenase. In a similar
experiment, D-threodehydroascorbic acid did not oxidize glutathione (data not shown).
These Theological differences showed that L-threoascorbic acid
is a much stronger improver in a dough than D-threoascorbic
acid. Of course this was already known from baking experiments
(Walther and Grosch 1987). This specificity suggests that the
enzyme glutathione dehydrogenase catalyzes the oxidation of
glutathione to the corresponding disulfide with L-threodehydroascorbic acid as the oxidant, but not with D-threodehydroascorbic
acid as an oxidant.
If the above is true, removal of the enzyme glutathione dehydrogenase should result in L-threodehydroascorbic acid being less
active as an improver. Because the enzyme is assumed to be present
in the water-soluble part of flour, it would be inactivated after
that fraction was boiled for 15 min. Therefore, the reconstituted
flour should be enzyme-free.
When reconstituted flour dough was mixed with 50 ppm
glutathione and 100 ppm L-threoascorbic acid or mixed with 50
ppm glutathione and 100 ppm D-erythroascorbic acid, no significant differences occurred in G' or loss tangent between the two
treatments. The G' of these two treatments were essentially the
same as that for dough containing only glutathione (Fig. 10).
This shows that the reaction between sulfhydryl groups and
0.4
0
60
120
180
TIME (min)
Fig. 10. Effects of D-erythreoascorbic acid, L-threoascorbic acid, and
glutathione on reconstituted flour-water dough (0). Reconstituted flourwater dough treated with 30 ppm glutathione (0). Reconstituted flourwater dough treated with 50 ppm L-threoascorbic acid and 30 ppm
glutathione (A). Reconstituted flour-water dough treated with 50 ppm
D-erythroascorbic acid and 30 ppm glutathione (A).
dehydro-L-threoascorbic acid was catalyzed by an enzyme, presumably glutathione dehydrogenase.
LITERATURE CITED
BECKWITH, A. C., and WALL, J. S. 1966. Reduction and reoxidation
of wheat glutenin. Biochim. Biophys. Acta 130:155-162.
BLOKSMA, A. H. 1972. The relation between the thiol disulfide contents
of dough and its rheological properties. Cereal Chem. 49:104-117.
BLOKSMA, A. H., and BUSHUK, W. 1988. Rheology and chemistry
of dough. In: Wheat Chemistry and Technology. Vol. 2, ed. Y.
Pomeranz, ed. Am. Assoc. Cereal Chem.: St. Paul, MN.
DREESE, P. C., FAUBION, J. M., and HOSENEY, R. C. 1988a.
Dynamic Theological properties of flour, gluten, and gluten-starch
doughs. I. Temperature-dependent changes during heating. Cereal
Chem. 65:348-353.
DREESE, P. C., FAUBION, J. M., and HOSENEY, R. C. 1988b.
Dynamic Theological properties of flour, gluten, and gluten-starch
doughs. II. Effect of various processing and ingredient changes. Cereal
Chem. 65:354-359.
FAUBION, J. M., DREESE, P. C., and DIEHL, K. C. 1985. Dynamic
Theological testing of wheat flour doughs. In: Rheology of Wheat
Products. H. Faridi, ed Am. Assoc. Cereal Chem.: St Paul, MN.
GRAVELAND, A., BOSVELD, P., and MARSEILLE, J. P. 1978. Determination of thiol groups and disulfide bonds in wheat flour and dough.
J. Sci. Food Agric. 29:53-61.
HIRD, F. J. R., CROKER, I. W. D., and JONES, W. L. 1968. Low
molecular weight thiols and disulphides in flour. J. Sci. Food Agric.
Vol. 72, No. 1, 1995
63
19:602-604.
JONES, F. K., PHILLIPS, J. W., and HIRD, F. J. R. 1974. The estimation
of rheologically important thiol and disulfide groups in dough. J. Sci.
Food Agric. 25:1-10.
JORGENSEN, H. 1939. Further investigation into the nature of the action
of bromate and ascorbic acid on the baking strength of wheat flour.
Cereal Chem. 16:51-60.
KUNINORI, T., and MATSUMOTO, H. 1963. L-Ascorbic acid oxidizing
system in dough and dough improvement. Cereal Chem. 40:647-657.
KUNINORI, T., and MATSUMOTO, H. 1964. Glutathione in wheat
and wheat flour. Cereal Chem. 41:252-259.
KUNINORI, T., and SULLIVAN, B. 1968. Disulfide-sulfhydryl
interchange studies of wheat flour. II. Reaction of glutathione. Cereal
Chem. 45:486-495.
MacRITCHIE, F. 1975. Mechanical degradation of gluten proteins during
high-speed mixing of dough. J. Polymer Sci. Symp. 49:85-90.
MAIR, G., and GROSCH, W. 1979. Changes in glutathione content
(reduced and oxidized forms) and the effect of ascorbic acid and potas-
sium bromate on glutathione oxidation during dough mixing. J. Sci.
Food Agric. 30:914-920.
SCHROEDER, L. F., and HOSENEY, R. C. 1978. Mixograph studies.
II. Effect of activated double-bond compounds on dough-mixing
properties. Cereal Chem. 55:348-360.
SPERLING, L. H. 1986. Introduction to Physical Polymer Science. WileyInterscience: New York.
TKACHUK, R. 1969. Involvement of sulfhydryl peptidase in the improver
reaction. Cereal Chem. 46:203-205.
WALTHER, C., and GROSCH, W. 1987. Substrate specificity of the
glutathione dehydrogenase (dehydroascorbate reductase) from flour.
J. Cereal Sci. 5:299-305.
YOSHIDA, M., HAMAUZU, Z., and YONEZAMA, D. 1980. On reactivity of disulfide groups of gliadin and glutenin. Agric. Biol. Chem.
44:657-661.
WEAK, E. D., HOSENEY, R. C., SEIB, P. A., and BIAG, M. 1977.
Mixograph studies. I. Effect of certain compounds on mixing properties.
Cereal Chem. 54:794-802.
[Received April 26, 1994. Accepted October 14, 1994.]
MISCELLANEOUS
Dielectric Properties and Water Mobility for Heated Mixtures
of Starch, Milk Protein, and Water'
M. N. TSOUBELI, 2 E. A. DAVIS, 2 and J. GORDON 2
ABSTRACT
Cereal Chem. 72(l):64-69
The dielectric properties of hydrated whey protein isolate (WPI), Cacaseinate, and wheat starch, alone or in combination, were measured
at ambient temperature and during heating to 900C. At lower moisture
contents and ambient temperature, WPI exhibited higher dielectric properties than starch, whereas Ca-caseinate had a lower dielectric constant
than either WPI or starch. At higher moisture contents, the dielectric
properties were similar. At moisture contents of 30-80%, WPI showed
increasing microwave absorption properties with increasing temperature;
at higher moisture contents, microwave absorption by Ca-caseinate
decreased with temperature. Adding WPI affected the dielectric loss and
absorptivity of starch during heating but such an effect was not evident
with Ca-caseinate. The dielectric properties of these systems were compared with their hydration properties by electron spin resonance using
TEMPO, a water and oil-soluble noninteractive probe to correlate water
mobility results to dielectric relaxation phenomena. All systems were
equally effective in slowing the motion of TEMPO. However, their dielectric properties differed, indicating that the dielectric properties are
not only influenced by the water but by the macromolecules present as
well.
Milk protein ingredients are used in many applications by the
food industry because they provide a wide range of functional
properties and have high nutritional value. The influence of milk
proteins in various food systems has been extensively studied.
In one study, Pearce et al (1984) found that the adding nonfat
dry milk solids to cake batter influenced properties such as air
cell size, foam stability, and lipid emulsification. Other studies
have also addressed the functionality of milk proteins and their
incorporation into various products (Kinsella 1982). What is not
clear, however, is the relationship between the functional properties and the physicochemical interactions of the proteins with
other ingredients that cause functionality modification. In cerealbased products, such interactions include interactions between
milk proteins, starch, lipids, and water. With the advance of microwave-heated products, a better understanding of the dielectric
behavior of milk proteins, alone and in combination with starch,
is important to controlling cereal product quality.
'Paper 20,770 of the Minnesota Agricultural Experiment Station projects 18-027
and 18-063. Additional support from the Minnesota-South Dakota Dairy Foods
Research Center.
2
Department of Food Science and Nutrition, University of Minnesota, St. Paul.
© 1995
64
American Association of Cereal Chemists, Inc.
CEREAL CHEMISTRY
Dielectric properties, the interaction of a material with electro-
magnetic radiation, can be expressed as: dielectric constant (k'),
a measure of the ability to store the electromagnetic radiation,
which is related to polarity; dielectric loss (k"), a measure of
the ability of a material to transform or dissipate the electric
energy into heat; and absorptivity (/ Ri), a measure of how the
waves will be affected when they enter the material, defined as
the inverse of the penetration depth. The penetration depth provides information on how far a wave will penetrate into a material
before it is reduced to 1/ e of its original intensity (Mudgett 1986).
In heating by electromagnetic energy, water is the primary component of a food system that interacts with electromagnetic radiation at 2,450 MHz. Water mobility has been assessed by many
methods for various systems. Electron spin resonance (ESR) using
the noninteractive probe TEMPO has been used to assess water
mobility in gluten and water systems (Pearce et al 1988) and
whey protein concentrate and water systems (Schanen et al 1990).
In both studies, a splitting in the high field line of a typical threeline TEMPO spectrum indicated a partitioning of the probe into
two water environments with differing mobility. Water self-diffusion coefficients (D) and dielectric properties have been also obtained (Umbach et al 1992) for conventional and microwave-