Influence of other food components

The presence of other major and minor food components, besides starch, has also been shown to influence the glycemic response of carbohydrate foods. Various factors at both macroscopic and microscopic levels of foods can affect such digestion variables as gastric emptying rate, creation of matrix barriers to access starch during digestion, and interactions between digestive enzymes and other food components.

9.5.1 Protein-starch interaction

Extruded pasta products represent an excellent example of the effect of proteins in slowing starch digestion rate. Several studies have demonstrated that digestion of pasta in both healthy and diabetic subjects is characterized by low glycemic and insulinemic responses (Parillo et al., 1985; Wolever et al., 1986; Granfeldt and Bjorck, 1991). In vitro digestion studies on pasta (Colonna et al., 1990; Fardet et al., 1998, 1999) have shown that restricted swelling and the entrapment or encapsulation of starch in structured protein network-associated dense food results in decreased accessibility and, hence, lowered digestion by amylase. Furthermore, pre-incubation of pasta with pepsin (a proteolytic enzyme) enhanced starch digestibility, thereby providing support for the importance of protein components in slowing starch digestion. Even in gelatinized flour pastes in cereals such as sorghum, entrapment of starch in protein web-like structures has been seen to impede starch digestibility (Bugusu, 2003). Figure 9.4 shows such protein-starch interrelated structures in a maize flour paste where the trapped gelatinized starch is digested after free, unencumbered gelatinized starch (M. Venkatachalam and B. R. Hamaker, unpublished observations, 2006).

Fig. 9.4 Light micrograph of cooked maize flour (1 : 10 w/v flour-to-water, 20 min at 100 °C) stained for starch with periodic acid-Schiff reagent and for proteins with a Coumassie Brilliant Blue-based stain. Web-like areas represent protein networks (dark-stained regions) with entrapped gelatinized starch (light-stained regions) (M. Venkatachalam and B. R. Hamaker, unpublished observations, 2005).

Fig. 9.4 Light micrograph of cooked maize flour (1 : 10 w/v flour-to-water, 20 min at 100 °C) stained for starch with periodic acid-Schiff reagent and for proteins with a Coumassie Brilliant Blue-based stain. Web-like areas represent protein networks (dark-stained regions) with entrapped gelatinized starch (light-stained regions) (M. Venkatachalam and B. R. Hamaker, unpublished observations, 2005).

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