It is important that the housekeeper not only understand the nature and composition of foods, but she should also know something of their digestive properties, since food, to be serviceable, must be not only nutritious, but easily digested. Digestion is the process by which food rendered soluble, and capable of being absorbed for use in carrying on the various vital processes.
The digestive apparatus consists of a long and tortuous tube called the alimentary canal, varying in length from twenty-five to thirty feet, along which are arranged the various digestive organs,—the mouth, the stomach, the liver, and the pancreas,—each of which, together with the intestines, has an important function to perform. In these various organs nature manufactures five wonderful fluids for changing and dissolving the several food elements. The mouth supplies the saliva; in the walls of the stomach are little glands which produce the gastric juice; the pancreatic juice is made by the pancreas; the liver secretes bile; while scattered along the small intestines are minute glands which make the intestinal juice. Each of these fluids has a particular work to do in transforming some part of the food into suitable material for use in the body. The saliva acts upon the starch of the food, changing it into sugar; the gastric juice digests albumen and other nitrogenous elements; the bile digests fat, and aids in the absorption of other food elements after they are digested; the pancreatic juice is not confined in its action to a single element, but digests starch, fats, and the albuminous elements after they have been acted upon by the gastric juice; the intestinal juice is capable of acting upon all digestible food elements.
The Digestion of a Mouthful of Bread—A mouthful of bread represents all, or nearly all, the elements of nutrition. Taking a mouthful of bread as a representative of food in general, it may be said that its digestion begins the moment that it enters the mouth, and continues the entire length of the alimentary canal, or until the digestible portion of the food has been completely digested and absorbed. We quote the following brief description of the digestive process from Dr. J.H. Kellogg's Second Book in Physiology[A]:—
[A] Good Health Pub. Co., Battle Creek, Mich.
"Mastication.—The first act of the digestive process is mastication, or chewing the food, the purpose of which is to crush the food and divide it into small particles, so that the various digestive fluids may easily and promptly come into contact with every part of it.
"Salivary Digestion.—During the mastication of the food, the salivary glands are actively pouring out the saliva, which mingles with the food, and by softening it, aids in its division and prepares it for the action of the other digestive fluids. It also acts upon the starch, converting a portion of it into grape-sugar.
"Stomach Digestion.—After receiving the food, the stomach soon begins to pour out the gastric juices, which first makes its appearance in little drops, like beads of sweat upon the face when the perspiration starts. As the quantity increases, the drops run together, trickle down the side of the stomach, and mingle with the food. The muscular walls of the stomach contract upon the food, moving it about with a sort of crushing action, thoroughly mixing the gastric juice with the food. During this process both the openings of the stomach are closed tightly. The gastric juice softens the food, digests albumen, and coagulates milk. The saliva continues its action upon starch for sometime after the food reaches the stomach.
"After the food has remained in the stomach from one to three hours, or even longer, if the digestion is slow, or indigestible foods have been eaten, the contractions of the stomach become so vigorous that the more fluid portions of the food are squeezed out through the pylorus, the lower orifice of the stomach, thus escaping into the intestine. The pylorus does not exercise any sort of intelligence in the selection of food, as was once supposed. The increasing acidity of the contents of the stomach causes its muscular walls to contract with increasing vigor, until finally those portions of the food which may be less perfectly broken up, but which the stomach has been unable to digest, are forced through the pylorus.
"Intestinal Digestion.—As it leaves the stomach, the partially digested mass of food is intensely acid, from the large quantity of gastric juices which it contains. Intestinal digestion cannot begin until the food becomes alkaline. The alkaline bile neutralizes the gastric juice, and renders the digesting mass slightly alkaline. The bile also acts upon the fatty elements of the food, converting them into an emulsion. The pancreatic juice converts the starch into grape-sugar, even acting upon raw starch. It also digest fats and albumen. The intestinal juice continues the work begun by the other digestive fluids, and, in addition, digests cane-sugar, converting it into grape-sugar.
"Other Uses of the Digestive Fluids.—In addition to the uses which we have already stated, several of the digestive fluids possess other interesting properties. The saliva aids the stomach by stimulating its glands to make gastric juice. The gastric juice and the bile are excellent antiseptics, by which the food is preserved from fermentation while undergoing digestion. The bile also stimulates the movements of the intestines by which the food is moved along, and aids absorption. It is remarkable and interesting that a fluid so useful as the bile should be at the same time composed of waste matters which are being removed from the body. This is an illustration of the wonderful economy shown by nature in her operations.
"The food is moved along the alimentary canal, from the stomach downward, by successive contractions of the muscular walls of the intestines, known as peristaltic movements, which occur with great regularity during digestion.
"Absorption.—The absorption of the food begins as soon as any portion has been digested. Even in the mouth and the esophagus a small amount is absorbed. The entire mucous membrane lining the digestive canal is furnished with a rich supply of blood-vessels, by which the greater part of the digestive food is absorbed.
"Liver Digestion.—The liver as well as the stomach is a digestive organ, and in a double sense. It not only secretes a digestive fluid, the bile, but it acts upon the food brought to it by the portal vein, and regulates the supply of digested food to the general system. It converts a large share of the grape-sugar and partially digested starch brought to it into a kind of liver starch, termed glycogen, which it stores up in its tissues. During the interval between the meals, the liver gradually redigests the glycogen, reconverting it into sugar, and thus supplying it to the blood in small quantities, instead of allowing the entire amount formed in digestion to enter the circulation at once. If too large an amount of sugar entered the system at once, it would be unable to use it all, and would be compelled to get rid of a considerable portion through the kidneys. The liver also completes the digestion of albumen and other food elements."
Time Required for Digestion.—The length of time required for stomach digestion varies with different food substances. The following table shows the time necessary for the stomach digestion of some of the more commonly used foods:—
hrs min Rice 1 00 Sago 1 45 Tapioca 2 00 Barley 2 00 Beans, pod, boiled 2 30 Bread, wheaten 3 30 Bread, corn 3 15 Apples, sour and raw 2 00 Apples, sweet and raw 1 30 Parsnips, boiled 2 30 Beets, boiled 3 45 Potatoes, Irish, boiled 3 30 Potatoes, Irish, baked 2 30 Cabbage, raw 2 30 Cabbage, boiled 4 30 Milk, boiled 2 00 Milk, raw 2 15 Eggs, hard boiled 3 30 Eggs, soft boiled 3 00 Eggs, fried 3 30 Eggs, raw 2 00 Eggs, whipped 1 30 Salmon, salted, boiled 4 00 Oysters, raw 2 55 Oysters, stewed 3 30 Beef, lean rare roasted 3 00 Beefsteak, boiled 3 00 Beef, lean, fried 4 00 Beef, salted, boiled 4 15 Pork, roasted 5 15 Pork, salted, fried 4 15 Mutton, roasted 3 15 Mutton, broiled 3 00 Veal, broiled 4 00 Veal, fried 4 30 Fowls, boiled 4 00 Duck, roasted 4 30 Butter, melted 3 30 Cheese 3 30 Soup, marrowbone 4 15 Soup, bean 3 00 Soup, mutton 3 30 Chicken, boiled 3 00
The time required for the digestion of food also depends upon the condition under which the food is eaten. Healthy stomach digestion requires at least five hours for its completion, and the stomach should have an hour for rest before another meal. If fresh food is taken before that which preceded it is digested, the portion of food remaining in the stomach is likely to undergo fermentation, thus rendering the whole mass of food unfit for the nutrition of the body, besides fostering various disturbances of digestion. It has been shown by recent observations that the length of time required for food to pass through the entire digestive process to which it is subjected in the mouth, stomach, and small intestines, is from twelve to fourteen hours.
Hygiene of Digestion.—With the stomach and other digestive organs in a state of perfect health, one is entirely unconscious of their existence, save when of feeling of hunger calls attention to the fact that food is required, or satiety warns us that a sufficient amount or too much has been eaten. Perfect digestion can only be maintained by careful observance of the rules of health in regard to habits of eating.
On the subject of Hygiene of Digestion, we again quote a few paragraphs from Dr. Kellogg's work on Physiology, in which is given a concise summary of the more important points relating to this:—
"The hygiene of digestion has to do with the quality and quantity of food eaten, in the manner of eating it."Hasty Eating.—If the food is eaten too rapidly, it will not be properly divided, and when swallowed in coarse lumps, the digestive fluids cannot readily act upon it. On account of the insufficient mastication, the saliva will be deficient in quantity, and, as a consequence, the starch will not be well digested, and the stomach will not secrete a sufficient amount of gastric juice. It is not well to eat only soft or liquid food, as we are likely to swallow it without proper chewing. A considerable proportion of hard food, which requires thorough mastication, should be eaten at every meal.
"Drinking Freely at Meals is harmful, as it not only encourages hasty eating, but dilutes the gastric juice, and thus lessens its activity. The food should be chewed until sufficiently moistened by saliva to allow it to be swallowed. When large quantities of fluid are taken into the stomach, digestion does not begin until a considerable portion of the fluid has been absorbed. If cold foods or drinks are taken with the meal, such as ice-cream, ice-water, iced milk or tea, the stomach is chilled, and a long delay in the digestive process is occasioned.
"The Indians of Brazil carefully abstain from drinking when eating, and the same custom prevails among many other savage tribes.
"Eating between Meals.—The habit of eating apples, nuts, fruits, confectionery, etc., between meals is exceedingly harmful, and certain to produce loss of appetite and indigestion. The stomach as well as the muscles and other organs of the body requires rest. The frequency with which meals should be taken depends somewhat upon the age and occupation of an individual. Infants take their food at short intervals, and owing to its simple character, are able to digest it very quickly. Adults should not take food oftener than three times a day; and persons whose employment is sedentary say, in many cases at least, adopt with advantage the plan of the ancient Greeks, who ate but twice a day. The latter custom is quite general among the higher classes in France and Spain, and in several South American countries.
"Simplicity in Diet.—Taking too many kinds of food at a meal is a common fault which is often a cause of disease of the digestive-organs. Those nations are the most hardy and enduring whose dietary is most simple. The Scotch peasantry live chiefly upon oatmeal, the Irish upon potatoes, milk, and oatmeal, the Italian upon peas, beans, macaroni, and chestnuts; yet all these are noted for remarkable health and endurance. The natives of the Canary Islands, an exceedingly well-developed and vigorous race, subsist almost chiefly upon a food which they call gofio, consisting of parched grain, coarsely ground in a mortar and mixed with water.
"Eating when Tired.—It is not well to eat when exhausted by violent exercise, as the system is not prepared to do the work of digestion well. Sleeping immediately after eating is also a harmful practice. The process of digestion cannot well be performed during sleep, and sleep is disturbed by the ineffective efforts of the digestive organs. Hence the well-known evil effects of late suppers.
"Eating too Much.—Hasty eating is the greatest cause of over-eating. When one eats too rapidly, the food is crowded into the stomach so fast that nature has no time to cry, 'Enough,' by taking away the appetite before too much has been eaten. When an excess of food is taken, it is likely to ferment or sour before it can be digested. One who eats too much usually feels dull after eating.
"How Much Food is Enough?—The proper quantity for each person to take is what he is able to digest and utilize. This amount of various with each individual, at different times. The amount needed will vary with the amount of work done, mental or muscular; with the weather or the season of the year, more food being required in cold than in warm weather: with the age of an individual, very old and very young persons requiring less food than those of middle age. An unperverted appetite, not artificially stimulated, is a safe guide. Drowsiness, dullness, and heaviness at the stomach are indications of an excess of eating, and naturally suggest a lessening of the quantity of food, unless the symptoms are known to arise from some other cause.
"Excess of Certain Food Elements.—When sugar is too freely used, either with food or in the form of sweetmeats or candies, indigestion, and even more serious disease, is likely to result. Fats, when freely used, give rise to indigestion and 'biliousness.' An excess of albumen from the too free use of meat is harmful. Only a limited amount of this element can be used; an excess is treated as waste matter, and must be removed from the system by the liver and the kidneys. The majority of persons would enjoy better health by using meat more moderately than is customary in this country.
"Deficiency of Certain Food Elements.—A diet deficient in any important food element is even more detrimental to health than a diet in which certain elements are in excess.
"The popular notion that beef-tea and meat extracts contain the nourishing elements of meat in a concentrated form, is a dangerous error. Undoubtedly many sick persons have been starved by being fed exclusively upon these articles, which are almost wholly composed of waste substances. Prof. Paule Bernard, of Paris, found that dogs fed upon meat extracts died sooner than those which received only water."
Food combinations.—Some persons, especially those of weak digestive powers, often experience inconvenience in the use of certain foods, owing to their improper combinations with other articles. Many foods which are digested easily when partaken of alone or in harmonious combinations, create much disturbance when eaten at the same meal with several different articles of food, or with some particular article with which they are especially incompatible. The following food combinations are among the best, the relative excellence of each being indicated by the order in which they are named: Milk and grains; grains and eggs; grains and vegetables or meats; grains and fruits.
Persons with sound stomachs and vigorous digestion will seldom experience inconvenience in making use of other and more varied combinations, but dyspeptics and persons troubled with slow digestion will find it to their advantage to select from the bill of fare such articles as best accord with each other, and to avoid such combinations as fruits and vegetables, milk and vegetables, milk and meats, sugar and milk, meat or vegetables, fats with fruits, meats, or vegetables, or cooked with grains.