This section is from the book "Elements Of The Theory And Practice Of Cookery", by Mary E. Williams. Also available from Amazon: Elements Of The Theory And Practice Of Cookery; A Textbook Of Domestic Science For Use In Schools.
In the mouth amylase begins to digest starches, by splitting them into dextrins. If the food stays in the mouth long enough, a little of the dextrin may be split into sugar (maltose) and dissolved. Saliva also dissolves any salt and sugar not in solution when eaten.
When starch enters the stomach, its digestion is under way. Proteins and fats enter the stomach unchanged. Pepsin is a protein-splitting 2b enzym. It changes (hydrolyzes) proteins into compounds simpler than proteins. Lipase splits fats, but the lipase in the stomach seems to act only on emulsified fats and not very strongly on these. Rennin curdles milk. Its action is an exception to the general rule that digestion liquefies. Pepsin, however, soon liquefies the curd formed by rennin. Amylase continues to digest starch in the stomach until that part of the stomach contents upon which it is working becomes acid from mingling with hydrochloric acid.
Gradually the stomach contents becomes a grayish pulp. This pulp, called chyme, escapes little by little into the small intestine. After a meal which has included all the foodstuffs, chyme contains dextrins and sugars; perhaps some undigested starch; proteins and the first products of protein-digestion; fat, and perhaps a little fatty acid and glycerin formed by the splitting of fat. Gastric digestion is preparatory to intestinal digestion.
Intestinal digestion is very complex. Pancreatic juice, intestinal juice, and bile mingle and act together on the chyme. The half-digested proteins and fats are further digested. Starch digestion continues. All sugars that require digestion are digested here. Grape-sugar and fruit-sugar undergo no change.
All the sugars present appear as two or three of the simplest, most soluble kinds. The proteins, having passed through many changes, are reduced largely to amino-acids. The fats have been split into glycerin and fatty acids. A part, at least, of these fatty acids is converted into soap before being absorbed. All these products of digestion are mixed together into a creamy fluid termed chyle.
A little food may be absorbed from the stomach, and a very little from the large intestine, but the bulk of it is absorbed from the small intestine. While digestion is going on, the products formed are being sucked up through tiny thread-like cells called villi, which project into the stream of chyle. The details of absorption and assimilation are wonderfully interesting, but as they are not directly affected, as digestion is, by the way food is prepared, we do not need to consider them in connection with cooking.
For further development of topics treated in this section see: -
Ritchie : Primer of physiology. Ch. 8, Digestive organs; 9, Digestion, absorption, and oxidation; 10, Dietetics.
Sherman : Chemistry of food and nutrition. Ch. 2, 3, and 4.
Stiles: Nutritional physiology. Ch. 6-11, Digestion; 22 and 23, Hygiene of nutrition; 25, Internal secretion.'
Buchanan : Household bacteriology. Ch. 22, Enzyms of microorganisms.
Fowler : Bacteriological and enzym chemistry. Ch. 5, 6, and 7.