Understanding the Mechanism of Cross-Linking Agents (POCl3, STMP,

and EPI) Through Swelling Behavior and Pasting Properties

of Cross-Linked Waxy Maize Starches1

Julie B. Hirsch2,3 and Jozef L. Kokini2,4

ABSTRACT Cereal Chem. 79(1):102–107

The effects of cross-linking waxy maize starch with phosphorous a sign of flocculation. The magnitude of BU for all of the treatments after
oxychloride (POCl3), sodium trimetaphosphate (STMP), or epichloro- 41 min, plotted versus calculated molar concentration of cross-linking
hydrin (EPI) on degree of swell and pasting properties were studied. As agent, showed a similar trend for all three reagents, indicating that type of
expected, increased concentration of cross-linking agent resulted in de- reagent plays little effect on the overall pasting behavior of cross-linked
creased granule swelling potential, Q (mL/g). The slower acting reagents, waxy maize. However, when BU was plotted versus Q, starches treated
STMP (4-hr reaction time) and EPI (17-hr reaction time), showed a with POCl3 again separated themselves with much higher viscosities than
similar relation between Q value and molar concentration of agent, which the collectively grouped EPI- and STMP-treated starches. The combi-
was different from the faster-acting POCl3 (30-min reaction time). nation of the reduced swell and higher viscosity indicates that POCl3-
Brabender viscoamylograph results show decreased peak viscosity with treated granules have a more rigid external surface area, with hard crust
increasing amounts of cross-link agent due to increased inhibition to formed on the outer layers of the granule. This information shows that the
swelling. Brabender viscosities (BU) continued to increase after the time mechanism of action of the individual reagents plays a major role in the
interval in which an uncross-linked sample would dissolve, which may be physicochemical behavior of the starches.

Cross-linked starch plays a major role in the manufacture of In general, for preparation of cross-linked starches, unswollen,
foods to thicken, stabilize, and provide texture. The invention of native granules are mixed in an aqueous system with reagents ca-
cross-linked starch stemmed from the need for starch granules pable of reacting with at least two of the hydroxyl groups of
that are tough enough to resist disintegration on cooking with neighboring molecules (Wurzburg and Szymanski 1970; Rutenberg
water. To avoid a thick, pasty mass, Felton and Schopmeyer (1943) and Solarek 1984). The type of reagent used and cross-linking
designed an inexpensive process to chemically treat native starch conditions determine the ratio of mono and di-type bonds (esters
with acid chlorides including phosphorous oxychloride (POCl3) in with phosphorous based agents and glycerols with epichlorohydrin)
water. Other researchers followed suit with novel chemical ap- due to cross-linking reaction mechanism and available starch hy-
proaches to cross-linking starch using other reagents such as epi- droxyls (Koch et al 1982).
chlorohydrin (EPI) or sodium trimetaphosphate (STMP) (Konigsburg When phosphorous oxychloride (phosphoryl chloride, POCl3;
1950; Hofreiter et al 1960; Lloyd 1970). MW 153.5) is added to a starch slurry under alkaline conditions
Waxy starches, which are made up almost entirely of amylo- (pH 8–12), the hydrophilic phosphorous group immediately reacts
pectin, are often used as the base for cross-linked starches because with the starch hydroxyls, forming a distarch phosphate. In
amylose retrogrades on cooling and forms an irreversible gel addition to phosphodiester linkages in POCl3-treated corn starch,
(Katzback 1972). Cross-linking agents bind neighboring anhydro- byproducts, including monophosphate derivatives and other types
glucose units (AGU) in the amorphous regions of the waxy maize of phosphate esters, have been found in the insoluble starch
amylopectin. Cross-links prevent the granules from fully swelling fractions using phosphorous-31 nuclear magnetic resonance (NMR)
and ultimately disintegrating. The covalent cross-link network also spectroscopy (Kasemsuwan and Jane 1994). STMP (Na3P3O9;
makes the granules tolerant to pH extremes and high shear processes MW = 305.9) has a ring structure that necessitates a bimolecular
common to food manufacturing. reaction that ultimately results in the formation of starch tripolyphos-
The extent of the effects of cross-linking on swelling and vis- phate. The mechanism of the EPI (1-chloro-2, 3-epoxypropane;
cosity depends both on the treatment conditions of raw starch and MW 92.5) reaction with starch occurs over a series of steps. With
on how the starch is prepared in the final application. Factors EPI, a multifunctional reaction can proceed whereby either one or
important in the cross-linking reaction include chemical composition two molecules of EPI are consumed to form a single cross-link.
of reagent, reagent concentration, pH, reaction time, and temperature Regardless of cross-linking agent, diesters and diglycerols represent
(Rutenberg 1980; Lim and Seib 1993). Because the degree of cross- the cross-linked starch molecules (Kerr and Cleveland 1957).
linking for food starches is very low, the extent of reaction and It is possible to control starch thickening properties through
yield of cross-linked starch are difficult to measure chemically; changing the degree of cross-linking and manipulating the extent
hence the need for physical property measurement. Maximum extent of swelling. Researchers have shown a relationship between rheo-
of cross-linking reaction for EPI with corn starch, assuming that logical properties and swelling capacity of starch granules (Evan
the percentage of reacted EPI that results in cross-links is constant, and Haisman 1979; Bagley and Christianson 1982). However, the
was reported at 90% (42 hr at 25°C) (Hamerstrand et al 1960). No relative effects of different cross-linking agents are not well under-
such quantitative values are reported for POCl3 or STMP. stood. The flow behavior and textural properties of cross-linked
starch are very complex due to the effects of starch concentration,
heating rate, heating temperature, and amount of shear, as well as
1 Publication No. D10544-1-98 of the New Jersey Agricultural Experiment Station competition with other dissolved solutes and polymers (Doublier
supported by state funds and the Center for Advanced Food Technology (CAFT).
2 Department of Food Science, Center for Advanced Food Technology, Rutgers et al 1987; Steeneken 1989; Gluck-Hirsch and Kokini 1997).
University, 65 Dudley Road New Brunswick, NJ 08901. Relative effects that the different cross-linking agents have on
3 Current address: Gorton’s, 303 Main Street, Gloucester MA 01930. E-mail: physical properties have been studied (Evans and Haisman 1979;
julie.hirsch@ gortons.com Eliasson 1986; Steeneken 1989; Evans and Lips 1992). However,
4 Corresponding author. E-mail: kokini@aesop.rutgers.edu
uncertainty remains as to how cross-linking is achieved at the macro-
Publication no. C-2001-1205-01R. molecular level of the three-dimensional granule structure. There-
© 2002 American Association of Cereal Chemists, Inc. fore, microstructural differences have not been used to explain the
102 CEREAL CHEMISTRY
physical properties nor have physical measurements been used as Preparation of Cross-Linked Starches
a tool to help determine cross-linked granule architecture. Concentrations of cross-linking agents (% w/w) added to starch
The objectives of this research are to contribute to the under- and the respective molar concentrations are shown in Tables I–III.
standing of the mechanism of action of POCl3, EPI, and STMP by A total of 12 starch treatments were analyzed (three cross-linking
comparing and quantifying the degree of swelling that different agents, each at four concentrations). For each of the 12 treatments,
concentrations of POCl3, STMP, and EPI independently have on Amioca (1,000 g wb, 12% moisture) was suspended in 1,500 g of
waxy maize starch, and to relate the effects of swelling on pasting tap water under constant mixing (Boehm stirrer with three-blade
behavior. Such quantitative information is not available in the propeller; Fisher Scientific Co., Fair Lawn, NJ). Before adding
literature. The relative effects of the different agents are expected to the reagent, 0.5 g of NaCl (0.5% wt of starch) was added, and the
give new insight as to how cross-linking is achieved within the slurry was adjusted to pH 12 by adding a 3% NaOH solution. For
starch granules. the STMP reactions, in addition to the NaCl, 3 g of CaCl2 (0.1%
wt of starch) also was added.
MATERIALS AND METHODS The POCl3 solution and STMP powder were added directly
while the slurries were being stirred, covered with foil, and left to
Materials react. The starch with POCl3 was reacted for 35 min at 25°C, and
Native waxy maize starch (Amioca), was donated by National the starch with STMP was reacted for 5 hr at 30°C. The 30°C
Starch and Chemical Co. (Bridgewater, NJ). Two different lots of temperature for the STMP reactions was maintained by keeping
waxy maize starch were used as the base starches for the cross- the slurry beakers submerged in a 30°C water bath. For the EPI-
linking reactions. One lot was used for the POCl3 and EPI-treated treated starch, each of the slurries was transferred to a 1-gallon
starches, and a different lot was used for the samples cross-linked plastic container and the EPI was added using a syringe due to the
with STMP. low concentrations. The containers were immediately capped and
POCl3 (99%) and EPI (99%) solutions were purchased from the inverted, and placed in a heated tumbler oven programmed to con-
Aldrich Chemical Co. (St. Louis, MO), and STMP (powder) from tinuously rotate the slurry-filled containers end-over-end for the
Monsanto (St. Louis, MO). Blue dextran was purchased from Sigma. designated reaction time (17 hr) and temperature (40°C) (Table IV).
Other chemicals were reagent-grade. After the designated cross-linking time had elapsed, the starch
slurries were brought to pH 5.25 ± 0.25 using a 3% HCl solution
(Fisher Scientific) and then filtered using a Buchner funnel and filter
paper (Reeve Angel grade 226 24-cm). The starch was then washed
TABLE I
Swell Factor (Q) for Waxy Maize Treated three times, air-dried overnight (50% rh at 30°C), and ground.
with Phosphorous Oxychloride (POCl3)a
Moisture Determination
Conc. (%) Molecules/g of Starch (×107) Q (mL/g) POCl3 The weight of starch in the suspensions was confirmed using
0.0050% 3.70 23.0 ± 0.67 Approved Method 44-15A (AACC 2000). Weighed aliquots of the
0.010% 7.40 19.6 ± 1.0 wet gelatinized starch samples, taken from the bottles after tem-
0.015% 11.1 17.5 ± 0.82 pering, were put in aluminum moisture dishes, covered, cooled in a
0.020% 14.8 14.9 ± 1.3
dessicator containing Hammond Drierite anhydrous calcium sulfate
a Determined by dye exclusion of a 2% starch suspension. (≈1 hr), weighed, and dried overnight in an oven (14–16 hr) at
102°C. Dried samples were then cooled in the dessicator (≈1 hr)
TABLE II
and weighed. Concentration of starch (1% moisture) was then cal-
Swell Factor (Q) for Waxy Maize Treated culated.
with Sodium Trimetaphosphate (STMP)a
Determination of Maximum Granule Swelling
Conc. (%) Molecules/g of Starch (×107) Q (mL/g) STMP
Using Dye Exclusion
0.050% 18.6 19.4 ± 0.90 Maximum granule swelling potential (Q) of each of the starch
0.10% 37.1 19.3 ± 1.7 treatments and the starch base was determined using a blue dye
0.15% 55.7 16.8 ± 1.4
0.20% 74.3 16.3 ± 1.8
exclusion procedure described by Tester and Morrison (1990).
a Determined
Blue dextran, a HMW polysaccharide, (MW = ≈2,000,000) does
by dye exclusion of a 2% starch suspension. not penetrate the swollen starch granules. As a result, this method
enables the determination of the amount of intragranular water by
TABLE III measuring the concentration of the dye excluded from the starch
Swell Factor (Q) for Waxy Maize Treated granules into the supernatant.
with Epichlorohydrin (EPI)a Maximum granule swell was achieved by heating the starch
Conc. (%) Molecules/g of Starch (×107) Q (mL/g) EPI
through gelatinization. For each of the 12 starch treatments and
the base starch, heating and mixing of the starch slurries were
0.005% 6.14 24.3 ± 0.76 accomplished simultaneously using a spinning rotary evaporator
0.010% 12.3 21.3 ± 2.1
(Bush Rotavapor RE121, Switzerland). Starch (1 g, wb) and 50 mL
0.015% 18.4 20.2 ± 0.31
0.020% 24.6 18.2 ± 1.1 of distilled water were added to a 100-mL freestanding Pyrex
a Determined
round flask. The flask was immersed in a 110°C Buchi 471 oil
by dye exclusion of a 2% starch suspension.

TABLE IV
Comparison of General Requirements for POCl3, STMP, and EPI Cross-Linking Reactionsa
Cross-Link Agent Reaction Time Reaction Temperature Final pH Special Requirements
POCl3 35 min 25°C 5.25 ± 0.25 NaCl (0.5% wt of starch)
STMP 5 hr 30°C 4.75 ± 0.25 NaCl (0.5% wt of starch)
… Water bath … CaCl2 (0.1% wt of starch)
EPI 17 hr 40°C 5.50 ± 0.50 Tumbler oven
a POCl3 = phosphorous oxychloride, STMP = sodium trimetaphosphate, and EPI = epichlorohydrin.

Vol. 79, No. 1, 2002 103

bath and spun at 150 rpm. The benefit of using this apparatus is before the granules started to dissolve, was once the uncross-
the absence of direct shear, whereby the flask spins, as opposed to linked starch temperature reached 87°C (≈45 sec after transfer to
a blade spinning in the slurry. test tubes). Once cooled to 20°C, 5 mL of blue dextran (Pharmacia,
For all of the cross-linked treatments, the spinning flasks were 5 mg/mL) was added to each test tube, and the test tubes were
heated for 30 min. Uncross-linked starch granules dissolve when gently inverted to mix the starch slurry and blue dextran. With the
heated to >85°C, therefore a less severe heat treatment needed to overall concentration of blue dextran <0.05%, the osmotic pres-
be used for the base starch. Instead of heating for a given length sure effects (deswelling) were assumed to be negligible (Bastide
of time, the unmodified starch was heated to the point where the et al 1981).
suspension instantaneously changed from opaque to clear (approxi- The tubes were then centrifuged twice (Sigma Laborzentri-
mate gelatinization point) at ≈70 ± 10 sec. fugen GmbH 2-15) at 1,500 × g for 10 min. After the first
Each of the heated starch suspensions was then transferred to a centrifugation, 10 mL of the clear blue supernatant was pipetted
100-mL Nalgene test tube (Fisher Scientific). The tubes containing out and added to a 5-mL centrifuge tube, then centrifuged again.
cross-linked starch were immediately cooled from 100 to 20°C by Tester and Morrison (1990) centrifuged the samples once and then
submersion into an ice bath. Before cooling on ice, the temper- measured the absorption of the excluded supernatant. In this
ature of the uncross-linked starch slurries continued to rise, indicating research, we centrifuged the samples twice to ensure no contam-
that swelling continued to occur, even after they were transferred ination from floating granules, as was confirmed by lack of
to test tubes. Optimal time to halt the heating and swelling process, turbidity in the supernatant.
The absorbance of the second and final supernatant (As) was
measured spectrophotometrically at 620 nm (HP8452A diode array
spectrophotometer). The excluded dye-water mixture was very
clear (no turbidity), indicating that little or no low molecular weight
fractions leached into the supernatant. Each experiment was per-
formed in duplicate with errors <10%. Reference tubes absorbances
(Ar) with 5 mL of blue dextran and 50 mLof water also were
measured at 620 nm.
Total swelling of the granules, swell factor Q (mL/g), was cal-
culated as the ratio of the volume of swollen starch (V2) per gram
of dry starch (W). V2 is calculated as the sum of the initial volume
of starch (V0) plus the absorbed, intragranular water (V1): V2 = V0
+ V1. To determine V2, V0 is calculated from a dry starch weight
basis (W) in grams, using 1.49 g/mL as dry weight starch density,
with V0 (mL) = W/1.49. To determine V1, the free or interstitial
plus supernatant water (FW), is first determined from the spectro-
metry results, where FW = 55 (Ar/As) – 5. (Note: 55 is the total
volume, and 5 is the volume of the blue dextran solution). Then,
the absorbed, intragranular water, V1, can be calculated as: V1 =
50 – FW. Finally, Q (mL/g) can be determined as: Q = V2/W.

Viscoamylograph Pasting Procedures

The effects of heating on starch behavior for all the starch treat-
ments were compared using a viscoamylograph (C. W. Brabender
Fig. 1. Swelling potential (Q) versus cross-linking agent molar concen- Instruments, Inc., South Hackensack, NJ) with a 700-gm-cm trans-
tration; effect of cross-linking agent and cross-link agent concentration ducer. Starch slurries (5.5%, wb) were each prepared by weighing
on granule swell 25.3 g (wb) of starch and 434.7 g of distilled water into a 600-mL
beaker (pH ≈ 7.2). Each slurry was mixed with a glass stirrer, to
suspend the starch and transferred to the amylograph sample cup.
Slurries were heated at the rate of 1.5°C/min through gelatinization
(and pasting). Once the slurry temperature reached 95°C, it was
maintained for a minimum of 30 min. All 12 cross-linked samples
and the waxy maize base were run in duplicate.

RESULTS

Effect of Cross-Linking Agent and Agent Concentration

on Granule Swell
The granule swell factor, Q (mLl/g) for the 12 cross-linked
samples are shown in Tables I–III. The Q values were 14.9–23.9
mL/g for the POCl3-treated waxy maize, 16.3–19.4 mL/g for
STMP, and 18.2–24.3 mL/g for EPI. The Q value for the uncross-
linked base starch was 32.2 mL/g. As expected, a higher concen-
tration of cross-linking agent results in a lower degree of swell.
The degree of Q as a function of calculated cross-linking agent
(molar) concentration for the three reagents at each concentration
is plotted in Fig. 1. Theoretically, maximum granule swell can only
be as high as would be achieved by the uncross-linked starch
control. With all three curves originating from the same uncross-
Fig. 2. Comparison of viscosity profiles for 5.5% starch slurries (wb) of linked (or zero) cross-link agent concentration value, POCl3
phosphorous oxychloride-treated (POCl3) waxy maize follows one decreasing swelling regime, and STMP and EPI seem

104 CEREAL CHEMISTRY

to follow another. The fast-acting POCl3 results in a greater reduc- ever, the relatively high degree of swell enables a strong interaction
tion in Q with increased molar concentration of reagent as compared between granules, resulting in a larger peak viscosity than with an
with the slower acting STMP and EPI starches. Considering that uncross-linked starch. The slight decrease in viscosity of the
the data are plotted as Q versus molar concentration of cross-link 0.005% POCl3 with time indicates that some granules are not
agent, accounts for the stoichiometry of the reactions. Therefore, sufficiently cross-linked and are most likely dissolving (Rutenberg
the differences between the reagents may very well be due to a and Solarek 1984).
mechanistic difference in the cross-link reaction. This shows that Although the applied rate of heating is equal for all the samples
the effect of cross-linking on granule structure of the very fast acting (1.5°C/min), the time interval of the exponential increase in viscos-
POCl3 is different than the slower acting agents STMP and EPI. ity seems to be a little longer for the less cross-linked samples. The
The dependence of Q on starch treatment best fit with a log- gelatinization temperature does not change (Rutenberg 1980) but
arithmic model of the data because r2 = 0.983 for POCl3 and the with a higher density of cross-links the larger granules appear to
combined fit of the STMP and EPI treated starches is r2 = 0.929. require more time to swell. This is also expected because the in-
There is remarkable superposition of the EPI and STMP data in creasing number of cross-links could be posing a diffusional resis-
the range where the data are superimposable. tance to water penetration. This is most apparent in the EPI-treated
The swelling factor represents an important physical parameter starches (Fig. 4), whereby the start of the continual rise begins at
because it defines the maximum swelling potential of a starch 37, 38, 39, and 41 min for the 0.005, 0.010, 0.015, and 0.020%
granule heated through gelatinization. Decreases in Q with increasing treatments, respectively.
concentration of cross-linking agent directly show the constraints
imposed by cross-links on swelling behavior. There is a much
stronger negative correlation of degree of swell with cross-linking
agent concentration for POCl3-treated starches compared with
STMP- and EPI-treated starches. The cross-links appear to be much
more effective in preventing the swelling action of POCl3-cross-
linked starches compared with STMP- and EPI-treated starches.
One explanation for this is that there is a large concentration of
POCl3 cross-links at the surface of the granule that cause a hard
crust on the outer layer (Gluck-Hirsch 1997; Huber and BeMiller
2001). The slower acting cross-linking agents, STMP and EPI,
penetrate further into the interior of the granule, and the cross-links
have a much more diffuse impact because they are essentially evenly
distributed throughout the granule volume.

Effect of Degree of Cross-Linking on Viscosity

A comparison of four concentration levels of POCl3, EPI, and
STMP and their untreated waxy maize counterparts is shown in
Figs. 2–4. For all of the samples, there is a bimodal response to
heating consisting of a sharp initial rise in viscosity at 30 min,
and then either a sharp decline in viscosity for the uncross-linked
samples or a continual rise in viscosity for all the cross-linked sam-
ples. The deviation between the uncross-linked and cross-linked
samples becomes apparent at 30–35 min. The exponential viscosity
decline of the uncross-linked starch at 36.5 min is the result of
Fig. 3. Comparison of viscosity profiles for 5.5% starch slurries (wb) of
starch granules breaking down. sodium trimetaphosphate-treated (STMP) waxy maize
Conversely, there is a gradual rise in the viscosity of the cross-
linked samples. The strength of the additional covalent bonds of
the cross-linked starch enables the granules to withstand the 95°C
temperature and not dissolve at >30 min.
With increasing concentration of cross-linking agent, the peak
viscosity decreases for all three cross-linking agents (Figs. 2–4).
The lower peak viscosity of the more highly cross-linked samples
can be attributed to the higher density of cross-links, and is con-
sistent with the decreasing swelling behavior with increasing cross-
linking agent concentration (Fig. 1). The greater the degree of
cross-linking, the smaller the granule volume, which precludes the
more highly cross-linked starches from coming into contact with
each other as much as the granules with a higher volume (i.e., more
lightly cross-linked starches with a greater Q). As a result, the
maximum viscosity observed for the highly cross-linked starches
is lower than the lesser cross-linked starches at this starch concen-
tration.
Note that the peak viscosity of the POCl3 cross-linked samples
at 0.005% concentration is considerably larger than the uncross-
linked starch, and the peak of the 0.005% EPI-treated starches is
approximately equal to its uncross-linked counterpart. The rate of
swelling of the starch granules and their dissolution into a dispersed,
polymeric phase may determine the peak viscosity (Kokini et al
1992). It is therefore possible that very small amounts of cross-
Fig. 4. Comparison of viscosity profiles for 5.5% starch slurries (wb) of
linking agent such as 0.005% POCl3 may inhibit dissolution. How- epichlorohydrin-treated (EPI) waxy maize

Vol. 79, No. 1, 2002 105

 By 41 min, all of the treated samples are well into the secondary these three different reagents and combined to show that they are
regime, with viscosity increasing with time. A plot of viscosity all actually very similar.
values at 41 min versus calculated molar cross-linking agent concen- A comparison of viscosity at 41 min versus degree of swelling
tration (Fig. 5) shows excellent correlation (r2 = 0.943) for all three (Fig. 6) shows that suspension viscosity for each agent increases
cross-linking treatments combined. This relationship of viscosity with increased degree of swelling (Q). Increasing cross-linking agent
and degree of cross-linking shows that these very different chemical decreases the degree of granule swell, thereby reducing granule
reagents enact similar macromolecular functionalities using this type interaction at low starch concentrations. At low concentrations, the
of measurement comparison. Wolfe (1987), who found that cross- smaller the granules, the further away they will be from each other
linked latex particles with different degrees of cross-linking fall in a suspension. Therefore, decreasing cross-linking agent is compar-
along the same curve, attributed such a relationship to volume frac- able to increasing starch concentration because, in both cases, the
tion effects, with swelling as the primary influence on dispersion granules are closer together. With lightly cross-linked starch, as with
viscosity for such microgels. Certainly, in this case, it is possible to a more concentrated suspension, granules are in close proximity
do separate regression analyses on the effect of amount of cross- to each other and are sterically confined, requiring a greater force
linking of each individual agent on suspension viscosity, with to deform enough to move past each other (Ketz et al 1988).
very good individual correlations (POCl3 R2 = 0.944; STMP R2 = Individually, the treated starches all have decreased swell and
0.967; EPI R2 = 0.991). Thus, one can contend that each agent decreased viscosity as cross-linking increases (Figs. 1 and 6).
generates a unique relationship between viscosity and cross-linking The information in Fig. 6 (viscosity vs. calculated molar cross-
concentration. However, a more interesting picture develops for link concentration) is consistent with that presented in Fig. 1 (Q vs.
the physical behaviors, expressed as degree of cross-linking of calculated molar cross-link concentration). The slower reacting
agents STMP and EPI effectively regressed as one, showing similar
dependence of viscosity on Q, whereas POCl3 follows a different
trend. Each of the cross-linking agents help maintain the granular
integrity, but how the cross-links occur in or on the granules affects
both the swelling and suspension viscosity.
All of the POCl3 cross-linked starches have higher viscosities
than the STMP and EPI treatments (Fig. 6). However, surprisingly,
the POCl3-treated granules generate a higher viscosity with swelling
values that are lower than the others. The EPI- and STMP-treated
granules occupy a larger volume than the POCl3-treated starches
but have lower viscosity. Although the POCl3 cross-linked granules
are smaller than the granules treated with STMP and EPI, they
have a higher viscosity.
Due to the high reactivity of POCl3, cross-links predominate on
the outer layer of the cross-linked granules, forming a hard shell
(Gluck-Hirsch 1997; Huber and BeMiller 2001). The ability of
the POCl3-treated granules to generate a higher viscosity than both
EPI- and STMP-cross-linked starches at a given swelling ratio
suggests that the POCl3 granules are more rigid than the other
treated granules. Maximum packing fraction (φm) is a measure of
how well a particle compacts and has an inverse relationship with
viscosity (Metzner 1985; Willett 2001). In general, hard spheres
(φm = 0.64) generate higher viscosities than deformable microgels
Fig. 5. Comparison of the effects of cross-link agent and cross-link agent (φm ≥ 0.64), due to their inability to compact (Wolfe 1987). The
concentration on viscosity surface crust of the POCl3-treated granules may contribute to inhi-
bition of swelling of the individual granules, but it enables a greater
overall suspension viscosity than the softer, more deformable STMP
or EPI at the same molar concentration of cross-linking agent.
The surface crust that develops from POCl3 cross-linking causes
the granules to swell less but, because they are more rigid, the
granules compress and compact less and occupy a greater total vol-
ume (lower degree of maximum packing) than the other treated
starches. In addition, consistent with more deformable granules
having lower viscosities, the STMP- or EPI-treated granules may
be softer due to a loss of crystallinity from the long reaction time
of the starch at pH 12 and 30–40°C.
Another factor that may explain the contradictory behavior of
POCl3 suspensions is surface friction. As a result of deformation
during shear, the granules may exude water, which acts as a lubri-
cant and enables the granules to slide more easily past each other
(Ketz et al 1988; Willett 2001). Viscosity is the integral of all fric-
tional resistances to flow and deformation. Therefore, the more
rigid POCl3-treated granules may exude less fluid than the more
deformable STMP- and EPI-cross-linked granules, thereby genera-
ting less lubricity and a greater frictional resistance, leading to a
greater viscosity.
As a final explanation, it may be possible that the POCl3-treated
granules flocculate during mixing, perhaps due to surface entangling
Fig. 6. Effect of degree of swell on viscosity; comparison of three cross-
linking agents
or attractive forces, whereas the STMP- and EPI-treated starches

106 CEREAL CHEMISTRY

either do not, or not to the same extent. A higher POCl3 system Evans, I. D., and Haisman, D. R. 1979. Rheology of gelatinized starch
viscosity may be due a greater effective volume of the aggregates. suspensions. J. Texture Stud. 10:347-370.
The viscosity of the suspensions continue to rise over time, although Evans, I. D., and Lips, A. 1992. Viscoelasticity of gelatinized starch
very slightly, which is consistent with the physical properties of dispersions. J. Texture Stud. 23:69-86.
Fannon, J. E., Hauber, R. J., and BeMiller, J. N. 1992. Surface pores of
colloidal gels (Figs. 2–4). The surfaces of unmodified starch gran-
starch granules. Cereal Chem. 69:284-288.
ules are very complex, containing graininess, holes, pores, and deep Fannon, J. E., Shull, J. M., and BeMiller, J. N. 1993. Interior channels of
depressions (Hall and Sayre 1970; Fannon et al 1992, 1993; Huber starch granules. Cereal Chem. 70:611-613.
and BeMiller 2001). Therefore, cross-linking derivitization sites on Felton, G. E., and Schopmeyer, H. H. 1943. Thick bodied starch and
the surface may enhance intergranular friction and granule bridging. method of making. U.S. patent 2,328,537.
Gluck-Hirsch, J. 1997. Quantification of cross-link induced physio-
CONCLUSIONS chemical changes in waxy maize starch using rheological and chemical
methods. PhD thesis. Rutgers University: New Brunswick, NJ.
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[Received February 28, 2001. Accepted September 4, 2001.]

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