Human digestive system
Chewing, in which food is mixed with saliva, begins the mechanical process of digestion. This produces a bolus which can be swallowed down the esophagus to enter the stomach. Here it is mixed with gastric acid until it passes into the duodenum where it is mixed with a number of enzymes produced by the pancreas. Saliva also contains a catalytic enzyme called amylase which starts to act on food in the mouth. Another digestive enzyme called lingual lipase is secreted by some of the lingual papillae on the tongue and also from serous glands in the main salivary glands. Digestion is helped by the chewing of food carried out by the muscles of mastication, by the teeth, and also by the contractions of peristalsis, and segmentation. Gastric acid, and the production of mucus in the stomach, are essential for the continuation of digestion.
Carbohydrate digestion
Digestion of starch begins with the action of salivary alpha-amylase/ptyalin, although its activity is slight in comparison with that of pancreatic amylase in the small intestine.The action of the salivary amylase is inhibited by the acidic pH of the stomach. The acidic content of the stomach is then neutralized by the bicarbonate  of the pancreatic juice of the pancreas and the pancreatic alpha-Amylase continues the process of starch digestion. Amylase hydrolyzes starch to alpha-dextrin, which are then digested by gluco-amylase (alpha-dextrinases) to maltose and maltotriose. The products of digestion of alpha-amylase and alpha-dextrinase, along with dietary disaccharides are hydrolyzed to their corresponding monosaccharides by enzymes (maltase, isomaltase, sucrase and lactase) present in the brush border of the small intestine. In the typical Western diet, digestion and absorption of carbohydrates is fast and takes place usually in the upper small intestine. However, when the diet contains carbohydrates not easily digestible, digestion and absorption take place mainly in the ileal portion of the intestine.