There are no starch-digesting enzymes in the stomach. In the small intestine, a pancreatic enzyme is secreted which completes the work that saliva began by breaking starch down into simple sugars.
The stomach has thousands of glands releasing gastric juices during digestion, with 6 different sets of glands in its wall.
At work, the stomach contracts about 3 times per minute, churning food and mixing it with gastric juice which comprises water, hydrochloric acid, the enzyme pepsin, and mucin, the main component of mucus and saliva.
Hydrochloric acid creates the acidic environment which pepsin needs to begin breaking down proteins. The acid also kills microorganisms that may have been ingested. Mucus forms a coating that protects the stomach from the effects of the acid and pepsin.
Eating protein is digestively intense. The sight and taste of chewing a concentrated protein, such as nuts or cheese, signals the stomach to secret hydrochloric acid and pepsin, so digestion can immediately begin.
Protein digestion in the stomach can take several hours. The result – chyme – is partly digested protein which is passed to the small intestine to break down into amino acids through enzymatic action.
By contrast, eating a starchy food, such as a potato, does not stimulate secretion of a high concentration of acid. Starch digestion in the stomach occurs in a more alkaline environment. The salivary enzyme ptyalin acts, and 2/3rds of digestion occurs in the stomach itself. The first part of the small intestine, the duodenum, receives short starch chains and sugars, and digestion is easily completed.
The order in which food is eaten can be significant to digestive processing, though digestion under different scenarios is not well understood. Succeeding portions of a meal are arranged in corresponding layers in the stomach and digested in order.
The stomach-churning during digestion does not disrupt the layering of food. Liquids that are drunk slip by, around digesting food, and enter the duodenum.
Thicker fluids require more digestion. Next to clear fluid, fruits are most quickly digested, within an hour or 2 from the time eaten.
Protein foods are trumped in the digestive work required only by fats. Oils and fats delay emptying the stomach more than any other food type.
Stimulants, such as caffeine (tea or coffee), and spices stimulate gastrointestinal churning, and so hasten emptying the stomach after a meal. Stimulants and food additives such as salt can interfere with gastric digestion by irritating the stomach walls. The Japanese, who historically have low incidence of vascular disease, tend to stomach cancer because of high salt intake.
Large chunks of poorly chewed food are kept longer in the stomach. If a chunk heads to the juncture of the stomach and duodenum it is squirted back for further stomach digestion.
The lining of the stomach comprises protein cells protected from gastric juices by mucus, which is constantly flowing around the stomach lining during digestion. Even so, much of the mucus, along with some cells from the stomach lining, are digested along with protein and lipoprotein food.
Slightly more of the protein digested from a meal comes from the stomach itself rather than from protein-intense food. This is why heavy protein diets accelerate aging: by forcing the body to expend excessive energy maintaining the digestive tract.