Journal of Environmental Polymer Degradation, Vol. 2, No. 2, 1994
Storage Stability of Injection-Molded Starch-Zein Plastics
Under Dry and Humid Conditions 1'*
S. L i m 2 a n d J. J a n e z'3
Corn starch and zein mixtures (4 : 1 dry weight) were extruded and injection-molded in the presence of plasticizers (glycerol and water). Tensile strength and percentage elongation of the molded
plastics were measured before and alter 1 week of storage under a dry or humid condition (11 or
93% RH). With 10-12% glycerol and 6-8% water, injection-molded plastics had relatively good
tensile properties (20- to 25-MPa tensile strength and 3,5-4.7% elongation). But while exposed
to dry conditions (11% RH), the molded plastics lost weight (0.5-1.5% in 7 days) and became
very brittle, with significant decreases in tensile strength and elongation. Partial replacement
(5-10%) of starch with a maltodextrin (average DE 5) reduced the glass transition and melting
temperatures of the starch-zein mixture as well as the dry storage stability. Using potato starch
instead of corn starch significantly improved the dry storage stability of the injection-molded
starch-zein plastics (18- vs I I-MPa tensile strength). Anionic corn starches with a maleate or
succinate group (DS < 0.01) produced injection-molded plastics with improved tensile properties
and storage stability. Plastics prepared from the starch maleate and zein mixture retained the
strength during 1 week of dry storage without a significant change (26-MPa tensile strength and
3.7% elongation after 1 week of storage).
KEY WORDS: Starch-zein plastic; storage stability; injection-molding; anionic starch.
have been made to improve the water resistance o f the
starch-based plastics. These include blending starch with
water-insoluble synthetic thermoplastic polymers (4),
using aliphatic polyester coatings (6), incorporating organosilane (2, 3), and cross-linking with water-insoluble cereal protein (9).
Starch melting is highly dependent on plasticizer
types and contents. Starch does not melt in the absence
of plasticizer but decomposes at an elevated temperature
(e.g., 200°C) (4, 10). Water is a natural and common
plasticizer for starch melting. For starch injection molding, 5-30% water content has been suggested (1). But
the temperature for extrusion or molding of the starchwater mixture was usually higher than the boiling point
of water (100°C). High-temperature and pressurechanges during processing may force water evaporation
or migration, causing inconsistent product properties.
Zein, a water-insoluble corn protein, has been reported to have a plasticizing effect on thermal process-
INTRODUCTION
Among the degradable biopolymers, starch is the
most naturally abundant and economically feasible for
commercialization. Starch has been used as a raw material for various molded plastics (1-9). But the hydrophilic nature of starch causes the starch-based plastics
to be sensitive to the atmospheric moisture (4). The mechanical strength and original shape are often lost by
exposing the plastics to the atmosphere. Various efforts
*Paper presented at the Bio/Environmentally Degradable Polymer
Society--Second National Meeting, August 19-21. 1993, Chicago,
Illinois.
~Joumal paper No. J-15561 of the Iowa Agriculture and Home Economics Experiment Station. Ames, Iowa. Project No. 2863.
-'Center for Crops Utilization Research and Department of Food Science and Human Nutrition, Iowa State University, Ames, Iowa
50011.
3To whom correspondence should be addressed.
111
I 0f~.-7564/94/0400-01 I I $07.00/0 i~' 1994 Plenum Publishing Corporation
112
ing of starch and to improve the water resistance of the
starch-based plastics (9).
Glycerol has been used for amylose film preparations (I 1-13). Because it has a high boiling point (290°C
at the atmosphere), using glycerol as a plasticizer may
reduce plasticizer evaporation and migration. Glycerol
may also improve storage stability of the plastics, especially under dry conditions, because of its hygroscopic nature.
The objectives of this study were to optimize the
plasticizer contents (glycerol and water) and injectionmolding temperature lbr a starch-zein mixture (4 : I ) and
to improve tensile properties and storage stability of the
molded plastics. Plastics were stored for 1 week under
dry and humid conditions [11 and 93% relative humidity (RH), respectively]. The effect of maltodextrin incorporation on the tensile properties and storage stability was examined. Different types of starch, such as
potato starch and anionic corn starches, were also examined.
M A T E R I A L S AND M E T H O D S
Materials
Zein and potato starch were purchased from Freeman Industries (Tuckahoe, NY) and Sigma Chemical
Company (St. Louis, MO), respectively. Corn starch,
aluminum complex of corn starch octenyl succinate
(Dry-Flo, DS 0.05), and maltodextrin (Maltrin 40, average DE 5) were donated, respectively, by American
Maize Products Company (Hammond, IN), National
Starch and Chemical Company (Bridgewater, N J), and
Grain Processing Corporation (Muscatine, IA). Lecithin
was donated by American Lecithin Company (Danbury,
CT).
Plastic Preparation
Three starch-zein formulations (Table I) were prepared to examine the effects of glycerol and moisture
contents on tensile properties of the molded plastics. The
dry-weight ratio of starch and zein in the formulations
was 4: 1. Glycerol content was varied from 10 to 12%
by weight, whereas water and lecithin contents were
constant (10 and 1%, respectively). The mixtures were
compounded and extruded as 3.2-mm-diameter strands
by using a Brabender counter-rotating twin-screw mixer
(C. W. Brabender Instruments, Hackensack, N J). The
extruder barrel was thermocontrolled at three consecutive heating zones (110, 123, and 113°C in the direction
Lim and Jane
Table !. Formulations (% by Weight) of Starch-Zcin Plastic Raw
Materials
Glycerol
Water
Starch
Zein
Lecithin
10
11
12
10
10
10
63.2
62.4
61.6
15.8
15.6
15.4
1
1
1
toward the die), and the die was heated to 95°C. The
screw rpm for extrusion was 15. The strands were pelletized with a Brabender pelletizer and dried in an oven
(50°C) for approximately 10-20 h until the moisture
content reached 6 to 9%. The final glycerol and moisture contents in the pellets were estimated from the
weight loss by drying. We assumed that only the moisture was evaporated during drying, and the glycerol remained in the pellets. The pellets were injection-molded
as ASTM tensile specimens (Type I) by using Boy-22S
Dipronic injection-molding machine (Boy Machines
Inc., Exton, PA). Molding temperatures were 150°C at
the transition section and 160°C at the metering section.
It was assumed that the glycerol and moisture contents
in the material did not change during extrusion and injection-molding.
Anionic Starch Preparation
Corn starch maleate and succinate were prepared
by following the method of Caldwell (14). Corn starch
was dispersed in water (1:2 w/w), then adjusted pH to
11 with a 1 N NaOH solution. Succinic or maleic anhydride (0.01 molar ratio of anhydroglucose unit of
starch) was slowly added to the starch dispersion, while
the pH was maintained at I I with 1 N NaOH by using
a pH controller (Chemcadet, Cole-Palmer Corporation,
Chicago, IL). The reaction was continued at room temperature and pH 11 for 2 h and was stopped by adjusting
the pH to 6.0 with a 1 N HCI solution. The modified
starch was vacuum-filtered, washed three times with
water, and dried at 50°C overnight. To confirm the
modification reaction, the pasting consistency of both
starches (7 % in water, w/w) was tested at pH 6 by using
a Brabender Amylograph (C. W. Brabender Instruments, Hackensack, N J).
Effect of Molding Temperature
A corn starch and zein mixture (4:1 dry weight)
containing 11% glycerol and 10% moisture was extruded and dried to 11.5% glycerol and 6.6% moisture.
The dried pellets were injection-molded at different tern-
Storage Stability of Starch-Zein Plastics
peratures. The metering-zone temperature of the molding machine was varied front 150 to 165°C at a 5°C
interval. The transition-zone temperature was adjusted
10°C lower than the metering zone temperature.
Storage and Analysis
Injection-molded tensile specimens were stored for
7 days in a closed chamber under dry (11% RH equilibrated with a saturated aqueous lithium chloride solution) or humid (93% RH equilibrated with a saturated
aqueous potassium nitrate solution) conditions. Tensile
strength and percentage elongation of the injectionmolded specimens were measured before and after 1
week of storage following ASTM D 638-86 method
(15). For each treatment, 5 - I 0 specimens were tested.
Weight change of the molded specimens during the storage was also measured. A differential scanning calorimeter (DCS-7, Perkin-Elmer Corporation, Norwalk, CT)
was used to measure the glass transition temperature (T~)
and melting temperature (Tin) as the onset temperature
from a thermogram. The glass transition temperature of
the starch-zein mixture was measured on the rescanned
thermogram.
RESULTS AND DISCUSSION
Starch Melting by Extrusion and Injection-Molding
Corn starch in a mixture with zein (4: 1), glycerol
(11%), and water (10%) exhibited its melting peak on
DSC thermogram in a broad temperature range of 165220°C (Fig. la). Also, the starch melting was appeared
as a multiphase transition. This multiple broad transition
may result front the inhomogeneity of the starch-zein
mixture. Zein itself (6% moisture content) did not show
any melting peak on a separate DSC thermogram indicating its amorphous structure (data not shown). A DSC
thermogram of the extruded pellet, in which starch and
zein existed in a homogeneous phase, showed that more
than 85 % (calculated from enthalpy) of the starch crystal in the original materials had disappeared during extrusion (Fig. lb). Considering the significant temperature difference between the extrusion (95-125°C) and
the DSC onset (185°C) temperature for starch melting,
the mechanical energy, such as shear force and pressure
during extrusion, was most attributable to starch melting.
For injection-molding, which requires greater melt
flowability than extrusion, an elevated temperature may
be necessary. Furthermore, the melt fiowability of the
113
extruded pellets was decreased by reducing the moisture
content to 6-9%. For injection-molding the dried pellets, a molding temperature of at least 150°C was necessary, The DSC thermogram showed that starch melting was almost completed by the injection-molding at
160°C (Fig. lc).
Effect of Molding Temperature
The injection-molded corn starch-zein plastics at
11.5 % glycerol and 6.6 % water contents exhibited good
tensile properties (22-25 MPa and 4.5-5.3% elongation) in a molding temperature range of 150 to 160°C
(Fig. 2). Average tensile strength and percentage elongation gradually increased as the temperature increased.
But when the temperature reached 165°C, both values
sharply dropped. Toughness (energy to break the specimen) also exhibited a trend similar to that of tensile
strength (data not shown). The molded specimen became darker and more brittle at 165°C, indicating thermal degradation of starch and/or protein at the temperature. At a temperature below 150°C, however, the
starch-zein melt had a significantly low melt flowability
so that injection was not possible even at the machine's
maximum injection pressure. As the molding temperature increased to 160°C, starch melting was almostcomplete (Fig. 1) and melt flowability increased. Also,
there might be more intermolecular interaction between
starch and zein at elevated temperatures; as a result, the
strength of the molded plastics increased.
Effects of Glycerol and Moisture Contents
Glycerol was added to the starch-zein mixture in a
range of 10 to 12% based on the total weight of the
material (Table I). With 13% or more glycerol content,
the molded plastics were darker, and the strength of the
products decreased. This indicated that a high glycerol
content increased the likelihood of thermal degradation
or browning reaction of starch and protein. Below 10%
glycerol, however, extrusion was difficult with the
moisture content (10%) in the given temperature range
(95-125°C).
The extruded pellets containing 10-12% glycerol
and 10% moisture were dried in an oven (50°C) to reduce the moisture content to 6-9% for injection-molding. At a moisture content above 9%, the molded products became soft and physically weak. A moisture
content below 6%, however, made the injection process
difficult in the temperature range (150-160°C) because
of decreased melt flow.
114
Limand Jane
lO
7.5
el
f
(7
0
,-3
""
<e
fa.1
2.5
"---
).oo
i:JO.oo do:Oo
i,lo.oo
do.oo
idO.oO......................
~.-)biO0
do.oo--~o.oo
2do.oo- 2to.oo
Temperature(C)
Fig. 1. DSC thermograms of raw material (a), extruded pellet (b), and injection-molded product (c) of a corn starch-zein mixture (4: 1. w/w).
The raw material and extruded pellet contained I 1% glycerol and 10% moisture, whereas the injection-molded product contained 11.5% glycerol
and 6.6% moisture. Scanning rate: 10.0°C/min. Sample Wt: 16 mg.
Moisture migration (weight loss) of the specimen
(6-9% moisture) was most significant in the first 2 days
of storage, accounting for more than half of the total
weight change for 1 week. The migration rate gradually
decreased after 2 days, dropping to less than 0.2% per
day after 7 days. The fast moisture loss in the early stage
of dry storage indicated that a large portion of the moisture in the plastics was loosely embedded in the polymer
matrix without strong interactions with macromolecules. The glycerol content, in the range from 10.3 to
12.5%, did not show a significant effect on the moisture
migration (Fig. 3). At moisture contents between 6 and
8%, the total weight change of the molded plastics during 7 days of storage at 11 or 93% RH was less than
1.5% (Fig. 3). The plastics containing 9% moisture,
however, showed a significant weight loss of 3-4% at
11% RH. Also, the plastics with high moisture contents
(8 and/or 9%) developed cracks (marked CR in Fig. 3)
during dry storage because of the severe moisture loss
and disruption of internal structure. The overall weight
change of the plastics during storage at 11 and 93 % RH
was minimal (less than 1.5%) at the lowest moisture
content of 6% (Fig. 3).
The tensile strength and percentage elongation of
the injection-molded starch-zein plastics varied with the
plasticizer contents (Figs. 4 and 5). With 10.3-12.5%
glycerol and 6-8% moisture contents, the tensile
strength and percentage elongation of the plastics ranged
approximately from 20 to 25 MPa and from 3.5 to 4.7%,
respectively (Figs. 4 and 5). Moisture content in the
molded specimen had a more significant effect on the
tensile properties than glycerol content. Both humid and
Storage Stability of Starch-Zein Plastics
30
I
I
1
11.5% Glycerol
D..
115
93~RH
~
11%RH
1 0 . 3 - 1 0 . 6 % Glycerol
CR
i
6.6% Moisture
27
g~
0
E
E
(D
"F
Cn
24
03
-2
E
I.--
-4
6.2
21
145
I
I
I
i
150
155
160
165
l
I
I
I
7.0
8.1
9.1
170
v
®
0
E
O
t-
t
tO
k,j-O
~5
O
r~
.T:
-2
1 1 . 1 - 1 1 . 5 % Glycerol
-4
O
145
t
i
t
150
155
160
Molding
CR
6.2
7.2
7.7
!
I
l
8.6
!
.......z.
165
70
Temperature (C)
Fig. 2. Tensile strength and percentage elongation of corn starchzein plastics injection-molded at different temperatures. Plastics contained 11.4% glycerol and 6.6% moisture.
o~
c
o
r-
-2
CR
1 2 . 1 - 1 2.5% Glycerol
CR
-4
dry storage weakened the plastics. At I 1% RH, the tensile strength of the plastics substantially dropped to less
than half of the original value because of the moisture
loss and structural disruption. The percentage elongation of the plastics also dropped significantly to below
2%, by the dry storage (Fig. 5). Absorbing moisture
during humid storage may have loosened the internal
structure of the plastics, resulting in the decreased values of tensile strength and percentage elongation (Figs.
4 and 5). When glycerol and water contents were compared, the plastics with 11.1-11.5% glycerol and 6-7%
moisture displayed the best stability under dry conditions, showing 1 I- to 12-MPa tensile strength and approximately 2 % elongation after 1 week of storage at
11% Rh (Figs. 4 and 5). When stored at 93 % RH, the
tensile strength and percentage elongation of the plastics
became 18-19 MPa and 3-4%, respectively.
During the dry or humid storage, DSC analysis of
the plastics revealed that starch in the plastics did not
retrograde (recrystallize) (data not shown). The plasti-
5.9
7.0
7,9
Moisture Content (%)
9.3
Fig. 3. Percentage weight change of corn starch-zein plastics prepared with various glycerol and moisture contents by 1 week of storage at 11 or 93 % RH. "'CR'" indicates cracks on the plastics. Glycerol
and moisture contents were the values before storage.
cizer contents in the plastics were not sufficient for starch
molecules in the plastics to have enough mobility for
recrystallization.
Maitodextrin
Maltodextrin (average DE 5) was incorporated into
a cam starch-zein mixture (4: 1) by replacing 5 or 10%
of the starch. The presence of maltodextrin did not
change the initial tensile strength, elongation, and storage stability under the humid condition. Stability under
the dry condition, however, decreased as the maltodextrin content increased (Fig. 6). During the dry storage,
the plastics containing maltodextrin became more brittle
116
Lim and Jane
Odginol
~
93~RH ~
11~RH
30
10.3-10.6% G l y c e r o l
11.1-11.5~; Glycerol
Ii'
12.1-12.5~; Glycerol
"
o
~ Ii
8.2
I~,'~
7.0
8.1
9.1
6.2
Moisture Content (~)
7.2
7.7
8,6
Molsture Content (%)
5.9
7.0
7.9
9.3
Moisture Content (~)
Fig. 4. Tensile strength of corn starch-zein plastics prepared with various glycerol and moisture contents
before and after I week of storage at 11 or 93 % RH. Glycerol and moisture contents were the values before
storage,
than those without maltodextrin. Moisture loss from the
specimens by the dry storage was not affected by the
maltodextrin contents. Possibly the short dextrin chains,
which had more freedom of molecular movement, aggregated more readily than the large starch molecules as
the moisture content decreased. Those aggregated small
Original
~
10.3-10,6% Glycerol
4
g3%RH
chains may have resulted in the brittleness of the plastics.
Incorporating a miscible polymer of a lower glass
transition temperature decreases the glass transition
temperature of the mixture (16, 17). The blend of starch,
zein, and maltodextrin displayed a single glass transi-
1 1%RH
.5% Glycerol
-
I
12.1 - 1 2 . 5 %
Glycerol
T
8
0
~n
2
1
0
6.2
7.0
8.1
9.1
Moisture Content (%)
7.7
8.6
6.2 7.2
Moisture Content (~)
7.0 7.9 g.3
5.9
Moisture Content (%)
Fig. 5. Percentage elongation of corn starch-zein plastics prepared with various glycerol and moisture contents
before and alter 1 week of storage at 11 or 93% RH. Glycerol and moisture contents were the values before
storage.
Storage Stability of Starch-Zein Plastics
11.5% Glycerol
117
l 1% Glycerol
6 . 1 - 6 . 3 % Moisture
30
I originol ~
I
25
L
D-
10~ Moisture
100
9.39;RH
11%RH
_T...
80
20
Jc
60
cn
15
l
c
~
,%1
5
Gn
I--
40
20
0
0
5
0
0
5
10
5
I0
200
_f_
1-.
s.
190
f,
¢.J
o
c
o
"'
E
I--
2
180
170
160
150
0
0
0
5
10
Maltodextrin Content (~)
Maltodextrin Content (%)
Fig. 6. Effect of mahodextrin on the storage stability of corn starchzein plastics. Maltodextrin (average DE 5) was added by replacing 5
or 10% of starch.
Fig. 7. Effect of maltodextrin on the glass transition and melting temperatures of a corn starch-zein mixture. The mixture contained 11%
glycerol and 10% moisture,
tion on the DSC thermogram, indicating compatibility
of the polymers. By incorporating maltodextrin, the
glass transition and melting temperatures of the mixture
were decreased (Fig. 7). This implies that the extrusion
and molding process can be performed with less thermal
energy by including maltodextrin.
plastics had 11 MPa and 2.0%. After the storage at 93%
RH, corn starch plastics had slightly higher values for
tensile strength and percentage elongation than did potato starch plastics.
Potato starch amylose has a higher average molecular weight than corn starch amylose (18, 19), which
may have caused the greater rigidity of the potato starch
plastics. Also, potato starch naturally possesses organic
phosphate monoesters linked to starch molecules [0.06%
(20, 21)] that could form chemical linkages and change
interactions with protein during the thermal processing.
These cross-linkages and charge interactions between
starch and zein may improve the strength and storage
stability of the plastics.
Potato Starch
Using potato starch improved the storage stability
of the molded plastics under the dry condition (Fig. 8).
The weight change of the potato starch plastics during
the dry or humid storage was approximately the same as
that of the corn starch plastics. The original strength of
the potato starch plastics was slightly higher than that
of corn starch plastics, whereas the percentage elongation of potato starch was lower. However, after 1 week
of storage at 11% RH, the tensile strength and percentage elongation of the potato starch plastics were 18 MPa
and 3.5 %, respectively, whereas those of the corn starch
Anionic Starches
Corn starch maleate and succinate (DS less than
0.01) were tested using a Brabender Amylograph to
compare their pasting consistency with unmodified corn
118
Lim and Jane
1 1 . 5 - 1 1 . 6 % Glycerol
6 . 0 - 6 . 2 % Moisture
30
%"
original
93%RH
11%RH
25
35
1,
%"
n
0,_
"-"
3O
I original
93%RH
11%RH
1 1 . 4 - 1 1 . 5 % Glycerol
6 . 2 - 6 . 4 % Moisture
z
T
25
Z"
I"
20
N
20
15
~
15
10
"~ 10
x~
tin
x~
II
F--
¢.
5
5
x~
Native
Corn St.
Maleate
Succinate
O.Succ.
Potato St.
L
m
4
to
tO
~3
0
~n
tO
I
I
0
bJ
2
Native
Corn St.
Potato St.
Fig. 8. Storage stability of potato starch-zein plastics.
starch. As shown in Table II, both starch maleate and
succinate had a greater consistency and lower pasting
temperature than native starch, which are c o m m o n results for anionic esterification.
Both starch maleate and succinate significantly increased the stability o f the molded plastics as shown in
Fig. 9. The initial tensile strength (26-28 MPa) o f the
Table II. Pasting Temperature and Consistency (95°C) of Native.
Malated. and Succinated Corn Starches on Brabender Amylographs"
Starch
Pasting temp. (°C)
Consistency (BU)~'
Native
Maleated
Succinated
78
72
73
220
430
260
"Starch (7%} was dispersed in water to make 400 g total weight, and
then the pH of the dispersion was adjusted to 6.0 with 0.1 N NaOH.
The molar ratio of maleic and succinic anhydride to starch for reaction was 0.01.
hBrabender Units.
Maleate
Succinate
O.Succ.
Fig. 9. Storage stability of anionic corn starch-zein plastics. The degree of substitutions of maleic and succinic acids was less than 0.01,
whereas that of octenyl succinic acid was 0.05.
plastics was greater than that o f unmodified c a m starch
plastics (23 MPa). The weight change o f the anionic
starch plastics during storage was similar to that of corn
starch. After 1 week o f storage at 11% RH, the starch
maleate plastics retained their strength (26 MPa and
3.7%), whereas the unmodified corn starch plastics were
substantially weakened (Figs. 4 and 5). Also, under humid conditions, the starch maleate showed a greater
storage stability than unmodified corn starch.
The carboxylic acid groups on starch molecules
might react with the amino groups o f protein during
thermal processing, forming amide cross-linkages.
Charge interactions between the unreacted carboxylic
and amino groups could also enhance the strength o f the
plastic matrix. These cross-linkages a n d / o r charge interactions improved the strength and stability o f the
plastics. The plastics made from starch maleate exhibited a greater stability during the dry storage than those
from starch succinate. The unsaturated bonds in maleate
Storage Stability of Starch-Zein Plastics
11~ Glyeerol t 0~ Moisture
119
molecules greater hydrophilicity and plasticizing effects, resulting in reduced thermal transition temperatures,
100
80
S"
t,.-
60
CONCLUSIONS
40
Corn starch and zein mixtures ( 4 : i dry weight)
were optimally injection-molded in the presence o f 1012% glycerol and 6 - 8 % water with good tensile properties (20- to 25-MPa tensile strength and 3 . 5 - 4 . 7 %
elongation). But the molded plastics lost weight (0.51.5% in 7 days) and became very brittle while exposed
to dry conditions (11% RH). Plastics made from a potato starch and zein mixture had a greater storage stability under dry conditions than did the corn starch and
zein mixture. By using corn starch maleate or succinate
(DS < 0.01) instead o f unmodified starch, the mechanical strength and storage stability under dry conditions
o f the injection-molded plastics were significantly improved,
20
0
Native
Maleate
Succinate
200
190
180
E
I--
170
160
150
m
~m
Maleate
Succinate
Native
Fig. 10. Effect of anionic substitution of starch on glass transition
and melting temperatures of a starch-zein mixture. The mixture contained 11% glycerol and 10% moisture.
groups possibly reacted with other polymer molecules
during processing.
Corn starch octenyl succinate aluminum complex
(DS 0.05) did not give a significant increase in tensile
strength compared with native corn starch, whereas percentage elongation increased significantly. Unlike starch
maleate, the free carboxylic acid groups in starch octenyl succinate formed a complex with aluminum ion
(AI3+), possibly restraining the cross-linkage formation
and charge interaction. The bulky moiety o f octenyl
groups, however, might sterically enhance freedom for
the molecular movement of starch, increasing the softness and percentage elongation of the plastic products.
DSC thermograms revealed that anionic starch-zein
mixtures displayed significantly reduced glass transition
and melting temperatures compared to native corn
starch-zein mixtures (Fig. 10). Corn starch maleate had
50 and 165°C glass transition and melting temperatures,
respectively, whereas the values for native corn starch
were 88 and 190°C. Those ionic groups provided starch
ACKNOWLEDGMENTS
The authors thank the Iowa Corn Promotion Board
and the Iowa Department of Economic Development for
financial support; the National Starch and Chemical
Company, American Maize Products Company, Grain
Processing Corporation, and American Lecithin Company for providing starches, maltodextrin, and lecithin;
and Mr. Daniel Burden for editorial assistance.
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1.
2.
3.
4.
5,
6.
7.
8.
9.
F. Wittwer and 1. Tomka (1987) U.S. Patent 4673438.
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E. T. Cole and R. Daumesnil (1988) GB 2214920A.
G. Lay, J. Rehm, R. F. Stepto, and M. Thoma (1989) European
Patent Application 0327505 A2,
C, Bastioli, V. Bellot, L. Del Giudice, G. Del Tredici, R. Lombi,
and A. Rallis (1990) PCT WO 90/10671.
L Tomka and S. Schmidlin (1990) PCT WO 90/01043.
G. Lay, J. Rehm, R. F. Stepto, M. Thoma, J.-P. Sachetto, D.
Lentz, and J. Silbiger (1992) U.S. Patent 5095054.
R. Nakatsuka, S. Suzuki, S. Tanimoto, and E. Funatsu (1978)
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