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( Hamaker & Bugusu, 2003 ) . Cooking or processing may sometimes reduce
the starch digestibility as the conformational changes in proteins may occur
that could facilitate the formation of disulfide-linked polymers ( Oria et al.,
1995 ). The presence of a protein barrier surrounding the starch granule has
been confirmed by the addition of pronase enzyme to hydrolyze the protein
matrix and a significant enhancement of in vitro starch digestibility was
observed afterward due to the clearance of passage for amylase and
amyloglucosidase ( Rooney & Pflugfelder, 1986 ). Wong et al. (2009) have
reported that a greater abundance of disulfide-bonded proteins and the pres-
ence of nonwaxy starch and the granule bound starch synthase enzyme may
affect the digestibility of both starch and protein in sorghum grain endo-
sperm. Another study by Choi, Woo, Ko, and Moon (2008) reported an
increase in the in vitro starch digestibility when sodium sulphite was added
during cooking of waxy sorghum flour. The reducing agents such as sodium
sulphite or bisulphite may prevent the formation of enzyme-resistant
disulphide-linked plant polymers, which facilitates an easy access of amylo-
lytic enzymes to the starch granule. Effects of protein matrix on in vitro starch
digestibility of processed starch products such as pasta have also been
reported ( Kim et al., 2008 ) . Jenkins et al. (1987) studied the effect of
starch-protein interaction in wheat and its effect on starch digestibility.
Their reports suggested that the occurrence of a starch-protein interaction
in white flour may account for the decreased glycemic response and reduced
rate of digestion. It has already been reported ( Liener, 1980 ) that 10-20% of
the starch in white wheat flour may be malabsorbed as judged by breath-
hydrogen production. Subsequent studies verified that significant amounts
of starch may indeed enter the colon ( Wolever et al., 1986 ). It has been
observed that removing gluten from the wheat flour eliminated the starch
malabsorption but this effect was not reversed by subsequently adding back
the gluten to the gluten-free flour. This raised the question of whether the
natural starch-protein interaction is responsible for the reduced digestibility
of starch ( Jenkins et al., 1987 ) . In vitro digestion studies showed that the con-
centration of total starch-digestion products was significantly lower for
white bread than for gluten-free bread. In terms of possible mechanism,
all-purpose wheat flour is composed of granules with a starch core sur-
rounded by a protein network. The protein network may inhibit the rate
of hydrolysis in the lumen of the small intestine ( Jenkins et al., 1987 ) .
The addition of toppings or fillings based on cottage cheese, baked beans,
and tuna has been reported to reduce the GI of potato-, pasta-, and toast-
based meals ( Henry, Lightowler, Kendall, & Storey, 2006 ).
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