The hydrochloric acid and the pepsin cause the principal chemical changes in the food while in the stomach. They act only on the proteins. The hydrochloric acid must be present before the pepsin can act, as only in an acid medium can pepsin dissolve the proteins. It is also of an antiseptic nature and hinders or prevents the decomposition of food.
The rennin ferment precipitates the casein.
The only digestion of starches in the stomach is that continued by the saliva.
Gastric juice begins to flow into the stomach soon after eating, but normally it is not secreted in sufficient quantity to supersede salivary digestion for from twenty to forty-five minutes.
The result of gastric digestion of proteins is their conversion, first, into albumin, then into proteoses, and, lastly, into peptone, which is protein in a more simple, soluble, and diffusible form. In the form of peptone, the proteins are in condition to be absorbed.
If the food has been properly cooked and masticated, gastric digestion will be completed in from one and one-half to three hours. If not properly cooked and masticated, the stomach digestion may continue from one to two hours longer. It should, however, be completed in three hours.
It will be seen that the evening meal is ordinarily digested before sleep, as one does not retire for from three to five hours after eating.
If, through imperfect mastication, or a disordered condition of the stomach, the digestion is not completed in about three hours, the food is likely to be retained in the stomach and by its weight may cause prolapsus of that organ if the supporting tissues are weak. Fermentation may ensue and give rise to gases which may cause acute distress.
Animal foods, which are readily digested, remain in the stomach for a shorter time. Meat, as a rule, is easily digested, because the digestive juices of the animal have converted the starches and sugars. The white meat of chicken is digested in a shorter time than the red or the dark meat.
Corn, beets, peas, beans, etc., take about three and a half hours to digest; baked potatoes about two and a half hours.