Food Chemistry and Fish in Nutrition ( 2+1)

6.1.2.Digestion

This process begins in the mouth during chewing, through the action of amylase secreted by the salivary glands.  Salivary amylase hydrolyzes many of the α(1à4) glycosidic linkages of starch and glycogen to yield a mixture of maltose, glucose and oligosaccharides. The digestion of digestible polysaccharides to yield D-glucose is continued and completed in the small intestine, largely by the action of pancreatic amylase, made by the pancreas and secreted via the pancreatic duct into the upper protein of the small intestine. This segment of the small intestine in which most of its digestive activity occurs, is called the duodenum.

         Disaccharides are hydrolyzed by enzymes located in the outer border of the epithelial cells lining the small intestine. Sucrose is hydrolyzed to D-glucose and D-fructose by sucrose, also called invertase: lactose is hydrolyzed to D-glucose and D-galactose by lactose β-galactosidase is hydrolyzed by maltose, yielding two molecules of D-glucose. The larva stores the glucose as glycogen and releases glucose as and when needed to maintain blood glucose level.

         Cellulose cannot be enzymatically digested and used by most mammals for lack of enzymes capable of hydrolyzing the β (1à4) linkages between successive D-glucose residues of cellulose. Nevertheless undigested cellulose residues of plant foods provide bulk or fiber (also called “roughage”) in the diet and are desirable for proper motility of the intestine. Cellulose can be digested by ruminant animals, but only indirectly. The rumen bacteria hydrolyze cellulose to yield D-glucose, which they ferment to yield lactate, acetate, and propionate, absorbed into the blood. Lactate and propionate are converted by the liver into blood sugar in ruminants.