adhesion between the dispersed phas

adhesion between the dispersed phase (the starch granules) and the continuous
phase can also be affected when another macromolecule is introduced.
10.6.1.5.1 Proteins
The influence of proteins on the gelatinization of starch has been studied with
the DSC, but in most systems the interpretation is difficult because the protein
denaturation endotherm and the starch gelatinization endotherm overlap
[265,266]; however, gluten is one protein for which it is possible to investigate
the interaction [267]. Gluten by itself shows no, or at least very minor, transitions [268,269]. When gluten was added to wheat starch, an increase in To
with increasing amounts of gluten was measured [267]. The thermal transitions
in a mixture of starch and fish protein were found to proceed independently
of each other [265], and in a surmi system the Tofor starch was shifted to a
higher temperature compared with the starch–water system [270].
The influence of gluten on the rheological properties of starch gels has
been investigated, and it was found that the effect of adding gluten depends
on the type of starch to which it is added [119]. For mixtures of gluten and
wheat starch, a weakening of the starch paste or gel in the presence of gluten
was observed [270a]. The rheological behavior of starch–caseinate mixtures
was studied in steady shear [271]. The mixed gel showed a shear-thinning
behavior, as did the starch. A synergistic effect was found, as the starch–caseinate mixed gel showed higher apparent viscosities than did the single component. For mixtures of pea starch and egg white heated together, the modulus
at small deformations increased for the mixtures compared with any of the
components alone [272]. This was also the case for mixtures of amylose and
egg white. Microscopic examination revealed a phase-separated structure in
both systems. For mixed protein–starch gels (potato starch, annealed potato
starch, pregelatinized potato starch, or cassava starch and bovine serum albumin [BSA] or gelatin) it was found that both the transition temperature and
the rates of gelation of the components were critical for the behavior of the
complex system [266]. When the starch gel was formed before the protein gel
(e.g., BSA and cassava), G′ and G″of the complex system could be predicted
by the simple addition of the moduli of the components at corresponding
concentrations. When the gelation occurred in the reverse order (e.g., BSA
and annealed potato starch), the gels were considerably stronger than predicted
by simple addition. When starch is present in surmi, the rigidity of the mixture
at increasing temperature is higher for the surmi without starch [270].
Another aspect of the gel formation of starch and starch components is
the influence on diffusion of molecules in the gels. This effect was studied for
the diffusion of BSA in amylose and amylopectin gels [273]. The diffusion
coefficient of BSA in amylose and amylopectin gels was found to decrease
with increasing polysaccharide concentration. No difference between amylose
and amylopectin gels was observed.
© 2006 by Taylor & Francis Group, LLC
444 Carbohydrates in Food
Because starch is present as particles in most products, a starch interface
exists and the starch can be regarded as a solid phase. The adsorption of
proteins on starch has been studied, and it was found that BSA is adsorbed to
a very low extent and wheat storage proteins are adsorbed to a much higher
extent [78]. Differences between starches were also observed; for example,
the adsorption of wheat storage proteins was much higher on potato starch
than on wheat or maize starch.
It has been suggested that the interaction between starch and protein
determines the endosperm hardness in cereals [39,274]. The phenomenon has
even been attributed to a single protein, friabilin [69], which has been suggested to influence the desorption of protein from starch during the starch
preparation procedure [275]. Interactions between amylose or amylopectin and
the protein oryzin were found to decrease during storage and were related to
the stickiness of cooked rice [276]. The binding of oryzin to the starch components had a positive influence on the stickiness of cooked rice.
The presence of glass transition temperatures when heating 1:1 mixtures
of amylopectin and gluten or amylopectin and casein was used to study the
miscibility of these polymers [277]. It was evident from the presence of two
separate Tgvalues that amylopectin and gluten are immiscible, whereas the
results were not as conclusive in the case of amylopectin and casein due to
their similar Tg
values.
The molecular interaction between amylose or amylopectin and protein in
solution has not been investigated to a great extent, except for some studies
that have used the iodine-binding property of starch for detecting interactions
[278,279]. It was concluded that the association between wheat starch and
wheat proteins occurs at neutral and acidic pH values. When starch, proteins,
and lipids are all present, many interactions are possible. Three-component
interactions were demonstrated in sorghum starch, whey proteins, and free
fatty acids [279a]. This was observed as a cooling stage viscosity when all
three components were present in the RVA. Certain proteins are lipid binding
(e.g., whey proteins). The expected interactions between the starch and the
lipids might then be cancelled because the lipids are bound by the protein and
thus not available for complexation with amylose or amylopectin [279b,c].