Food Microstructure and Starch Digestion - Advances in Food and Nutrition Research - page 138

1. INTRODUCTION

Starch, a major storage carbohydrate in plants, consists of two types of
molecules: amylose (linear polymer of

a - D -glucose units linked by a -1,4

glycosidic linkages) and amylopectin (branched polymer of a - D -glucose

units linked by -1,6 glycosidic linkages). Starches can be classi-

fied according to their digestibility, which is generally characterized by the
rate and the duration of the glycemic response. Most starches contain a por-
tion that digests rapidly (rapidly digesting starch, RDS), a portion that digests
slowly (slowly digesting starch, SDS), and a portion that is not hydrolyzed by
the enzymes in the small intestine and passes to the large intestine, therefore,
considered as resistant to digestion (resistant starch, RS; Englyst, Englyst,
Hudson, Cole, & Cummings, 1999 ) . One of the most widely used methods
to classify the starches was suggested by Englyst, Kingman, and Cummings
(1992) and is based on the kinetics of in vitro starch digestion by simulating
stomach and small intestinal conditions and measuring glucose release at dif-
ferent times. The diets containing higher quantities of RDS raise the glucose
levels in the blood quickly than those containing SDS and RDS
( Lehmann & Robin, 2007 ) .
The microstructure of a food is highly dependent on the composition,
processing, and post-processing storage and plays a vital role in determining
the rate of starch digestibility in different foods ( Bj ¨ rck, 1996; Singh,
Dartois, & Kaur, 2010; Tester, Karkalas, & Qi, 2004 ). The microstructure
of natural foods and the properties of cell wall polymers (parenchyma cell size,
nature of pectic materials, etc.) are two important factors which can influence
their processing and starch digestibility characteristics ( Singh et al., 2010;
Waldron, Smith, Parr, Ng, & Parker, 1997 ). The physical texture of the food
also affects starch digestion and the absorption of its hydrolysis products. Food
matrix viscosity has been reported to be one of the major factors affecting
enzymatic digestibility of starch and glycemic response ( Dartois, Singh,
Kaur, & Singh, 2010; Singh et al., 2010 ). Some food components such as
polysaccharide-based gums increase the viscosity of foodmatrix significantly,
even at a very low polymer concentration and therefore alter the viscosity of
digesta. This may decrease the postprandial carbohydrate absorption after
ingestion of the starchy food. Additionally, a high viscosity food matrix
may influence the water availability, which is an important requirement
for the enzymatic substrate reaction. Lipids and proteins are sometimes pre-
sent naturally in starch granule or may be present in the foodmatrix as a part of

a -1,4 and a