Digestion

Essentially, only cooked starch can be digested effectively by humans. Amylases are the enzymes that catalyze the hydrolysis of the glycosidic bonds of the polysaccharides of starch (10). a-Amylases are endo-enzymes, that is, enzymes that catalyze the hydrolysis of internal bonds of starch polysaccharides. Although saliva contains an a-

Table 5. Important Properties of Specific Food Starch Products

Property

Product Examples

Adhesion

Anticaking (flowing aid)

Antistaling

Binding

Clouding

Crispiness

Dusting

Foam strengthening Gelling

Glazing

Hydration

Moisture retention

Molding

Oil absorption

Pulpiness

Shaping

Stabilizing emulsions and suspensions Thickening

Breaded products, pan-coated nuts and candies Baking powder, powdered sugar, salt Baked goods

Extruded foods, meat products Beverages, cream fillings Breaded products Bread, chewing gum, rolls Confectioneries

Gum candies, processed cheeses, puddings, spoonable dressings Nuts

Dry mixes Breadings Gumdrops Peanut butter

Fruit drinks, tomato products Meat products, pet foods Beverages, salad dressings

Baby foods, cream-style corn, gravies, pie fillings, sauces, soups, yogurt amylase, very little starch hydrolysis occurs in the mouth because of the short dwell time.

Almost all starch digestion and absorption takes place in the small intestine. The pancreatic juice secreted into the small intestine contains another a-amylase. This enzyme effects a rapid reduction in molecular weight of the starch polysaccharides, producing starch oligosaccharides (maltooligosaccharides), primarily of six and seven a-D-glucopyranosyl units (Fig. 4). The a-amylase then acts more slowly on these oligosaccharides to reduce them to smaller fragments (maltose and maltotriose). The enzyme acts even more slowly on maltotriose; it does not catalyze the hydrolysis of the glycosidic bond of the disaccharide

Rapidly

Slowly

D-Glucose Maltose

Maltotriose

Very slowly

Figure 4. Action of a-amylase on amylose. Glc = d-glucose or a-d-glucosyl unit.

maltose. a-Amylases catalyze the hydrolysis of a-d-(l-ยป4) linkages only, never a-D-(l->6) linkages. Therefore, the products of the action of pancreatic a-amylase on amylose and amylopectin are both linear and branched maltooli-gosaccharides.

Other enzymes are then needed to catalyze the hydrolysis of the maltooligosaccharides. Complete hydrolysis to d-glucose is required because only monosaccharides can be absorbed. Disaccharidases are located on the surface of cells lining the inner surface of the small intestine. Maltase catalyzes the hydrolysis of maltose, 4-0-(a-d-glucopyranosyl)-d-glucose (see the article Carbohydrates: classification, chemistry, labeling), to d-glucose. Isomaltase catalyzes the hydrolysis of isomaltose, 6-0-(a-d-glucopyranosyl)-d-glucose, to d-glucose. Both enzymes act on higher oligosaccharides but more slowly than they do on the disaccharides.

Starch that resists and escapes digestion by endogenous enzymes of the upper gastrointestinal tract and is thus available to the large intestine as a fermentable substrate is called resistant starch. Resistant starch has physiological benefits, primary among which may be a decreased incidence of colorectal cancer and ulcerative colitis. The resistant starch content of foods can be manipulated by choice of the botanical source of starch, processing, cooking, and storage conditions.

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