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.
All foodstuffs except water, mineral salts, and two of the sugars have to undergo a process of change called digestion before they can be built into the body. Digestion means taking apart. Foodstuffs, like all other substances, are considered by chemists to be composed of minute particles, which are called molecules (little masses). Whenever the molecules of any substance are divided, the substance is changed into something else. This is called a chemical change. (See physical and chemical changes, p. 55.) Protein molecules are larger than the molecules of most other substances, but even they are far too small to be seen, even with a microscope. Digestion splits up the molecules of proteins, fats, starches, and some sugars, forming smaller molecules of new substances. Usually repeated splittings occur. Fat molecules have to be split only once, but protein and starch molecules are split many times. At last molecules are formed which are small enough to enter the cells of the body. As a rule, at each splitting, the new substances formed unite chemically with a certain amount of water. This chemical union with water is called hydrolysis. Repeating to yourself, "Split and take up water; split again and take up water," will help you to remember the most prominent feature of digestion.
People often say this or that food is "indigestible," when they mean that some people feel discomfort after eating it. This is a wrong use of the term. An indigestible food would not be a food at all. Some foods are more quickly digested than others, but the quickly digested food may not be so completely digested as one which takes a longer time. Cheese is more completely digestible than rice. It is best to avoid the word indigestible except when referring to cellulose, grape-seeds, or things of that sort, and to use the word digestible only in the sense of completeness of digestion. Very large percentages of ordinary foods are digestible.
The process of digestion and the digestibility of foods are affected by many things. Among these are the quantity of food eaten, its taste, the way it is cooked, and the state of mind and body of the person who eats it. Unhappy feelings interfere with digestion. Good cheer at the table promotes good health.
When food is swallowed, it goes down a soft muscular tube, the oesophagus, to the stomach. The stomach is a pouch with muscular walls and a soft, smooth lining. After being partly digested in the stomach, the food passes through a narrow opening into another muscular tube, the small intestine. Here it is further digested and most of the products of digestion are absorbed. The material left passes into the large intestine, from which there is an exit for waste. Mouth, oesophagus, stomach, small intestine, and large intestine are spoken of together as the alimentary canal.
The only stage of digestion of which a healthy person is conscious is that which takes place in the mouth. Here teeth, tongue, and face-muscles work together to divide the food and mix it with saliva. The saliva softens and partially dissolves it. The muscles of the stomach and intestines continue working the food about and squeezing it along. At the same time the digestive juices flow into the stomach and intestines and mingle with the food, so that it becomes constantly more finely divided, liquefied, and dissolved. These mechanical changes aid digestion, but it is the chemical changes which really digest the food.
These chemical changes are brought about by the action of enzyms. We are already familiar with the work of a few enzyms. (See Enzyms, p. 131.) It is hard to say exactly what enzyms are, because not enough of one can be obtained for a satisfactory examination. They seem to be substances secreted by living cells, which can work chemical changes in other substances without themselves undergoing change. Each digestive enzym works on one class of foodstuffs only, in some cases, on a single foodstuff. The enzym, amylase, acts on all starches, but there is a different enzym for each sugar.
Each digestive enzym is a constituent of some digestive juice. Some of these juices come from the walls of the alimentary canal, some from organs lying near the canal. Saliva is secreted chiefly by three pairs of glands near the mouth. It furnishes amylase, also alkaline salts which favor the action of amylase. Gastric juice is secreted by glands in the stomach wall. It furnishes three enzyms, pepsin, rennin, and lipase. Gastric juice also contains hydrochloric acid, without which pepsin will not act. Pancreatic juice is so called because it is secreted by the pancreas, a large gland back of the stomach. It flows into the small intestine through a duct. Pancreatic juice contains amylase and lipase, and a third substance, which soon becomes the enzym trypsin. Glands in the intestinal wall secrete a mixture of fluids known as the intestinal juice. These contribute at least five enzyms. Another fluid, bile, enters the small intestine through a duct from the liver. Bile contains no enzyms. It contains other substances, however, which aid the digestion and the absorption of food, particularly of fats.