Food Microstructure and Starch Digestion - Advances in Food and Nutrition Research - page 168

5.2. Influence of food processing on starch digestion
A wide range of techniques is being used by the industry for processing var-
ious food materials. Processing leads to an alteration in the food structure and
also influence the nutritional characteristics of the food including starch
digestibility. The outcome of some recent studies related with different
processing techniques and their effect on starch digestibility is presented
in Table 4.1 . When starch molecules are heated in excess water, the crystal-
line structure is disrupted and water molecules become linked by hydrogen
bonding to the exposed hydroxyl groups of amylose and amylopectin, which
causes an increase in granule swelling and solubility. Therefore, the water
activity or the availability of water is an important factor that determines
the extent of starch digestibility through enzymatic hydrolysis. Processing
has been observed to result in an increase in the degree of starch hydrolysis,
with reaching values higher than 90% (at the end of incubation with
pancreatine) for wheat, barley, and oats ( Anguita, Gasa, Mart ´ n-Or ´ e, &
P´rez, 2006 ). Starches in tubers and legumes are particularly well protected
from the polar environment of luminal fluids, and even cereals such as wheat
may not have access to a -amylase in the intestinal lumen unless they have

been physically altered.
The principal process facilitating starch availability for water penetration
and consequent -amylase action is physical processing and cooking by

heating to 100 C for several minutes. Cooking increases the rate of hydrolysis
by gelatinizing the starch and making it more easily available for enzymatic
attack. Bravo, Siddhuraju, and Saura-Calixto (1998) studied the effect of
various processing methods such as direct cooking, with and without soaking
and germination on the in vitro starch digestibility and RS content of Indian
pulses. Sprouting and direct cooking resulted in lowest RS in freshly cooked
and stored legumes, respectively, while soaking increased the in vitro starch
digestibility to a considerable extent. Processed legumes contain significant
amounts of RS in comparison to other food products such as cereals and pota-
toes, irrespective of the processing treatment. The starch-digestion rate and
the release of glucose in the blood stream are very low after the ingestion
of processed legumes ( Bravo et al., 1998 ).
Extrusion cooking significantly increases the in vitro digestibility of
starches ( Alonso et al., 2000; Altan, McCarthy, & Maskan, 2009 ). The
increase in digestibility of starch may be explained on the basis that the
starch granules lose their structural integrity due to increased shearing action
and kneading in the extruder barrel, which ultimately increase their

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