[3]An illustration of vapor rising may be given by breathing upon a mirror.
[3]An illustration of vapor rising may be given by breathing upon a mirror.
The following are familiar examples of compounds of each of the four principal classes of nutrients:
Protein:
Proteids.
Albuminoids,e.g., albumen of eggs; myosin, the basis of muscle (lean meat); the albuminoids which make up the gluten of wheat, etc.
Gelatinoids, constituents of connective tissue which yield gelatin and allied substances,e.g., collagen of tendon; ossein of bone.
"Nitrogenous extractives" of flesh,i.e., of meats and fish. These include kreatin and allied compounds, and are the chief ingredients of beef tea and most meat extracts. Amids: this term is frequently applied to the nitrogenous non-albuminoid compounds of vegetable foods and feeding stuffs, among which are amido acids, such as aspartic acid and asparagin. Some of them are more or less allied in chemical constitution to the nitrogenous extractives of flesh.
Fats.
Fat of meat: fat of milk; oil of corn, wheat, etc. The ingredients of the "ether extract" of animal and vegetable foods and feeding stuffs, which it is customary to group together roughly as fats, include, with the true fats, various other substances, as lecithians, and chlorophylls.
Carbohydrates, sugars, starches, celluloses, gums, woody fibre, etc.
Mineral matter.
Potassium, sodium, calcium and magnesium chlorids, sulphates and phosphates. (Atwater).
The terms (a) "nitrogenous" and (b) "carbonaceous" are frequently used to designate the two distinct classes of food, viz.: (a) the tissue builders and flesh formers; (b) fuel and force producers.
Each of these classes contains food material derived from both the animal and vegetable kingdom, although the majority of the animal substances belong to the nitrogenous, and the majority of the vegetable substances to the carbonaceous group.
Therefore, for practical purposes, we will confine ourselves to the more general terms used in Atwater's table.
First, food is used to form the materials of the body and repair its waste; second, to yield energy in the form of (1) heat to keep the body warm, (2) to provide muscular and other power for the work it has to do. In forming the tissues and fluids of the body the food serves for building and repair. In yielding energy, it serves as fuel for heat and power. The principal tissue formers are the albuminoids; these form the frame-work of the body. They build and repair the nitrogenous materials, as those of muscle, tendon and bone, and supply the albuminoids of blood, milk and other fluids. The chief fuel ingredients of food are the carbohydrates and fats. These are either consumed in the body or are stored as fat to be used as occasion demands.
By referring to a preceding chapter we find that water composes three-fifths of the entire body. The elasticity of muscles, cartilage, tendons, and even of bones is due in great part to the water which these tissues contain. The amount of water required by a healthy man in twenty-four hours (children in proportion) is on theaverage between 50 and 60 ounces, beside about 25 ounces taken as an ingredient of solid food, thus making a total of from 75 to 85 ounces. One of the most universal dietetic failings is neglect to take enough water into the system. Dr. Gilman Thompson gives the following uses of water in the body:—
(1) It enters into the chemical composition of the tissues; (2) it forms the chief ingredient of all the fluids of the body and maintains their proper degree of dilution; (3) by moistening various surfaces of the body, such as the mucous and serous membranes, it prevents friction and the uncomfortable symptoms which might result from drying; (4) it furnishes in the blood and lymph a fluid medium by which food may be taken to remote parts of the body and the waste matter removed, thus promoting rapid tissue changes; (5) it serves as a distributer of body heat; (6) it regulates the body temperature by the physical processes of absorption and evaporation.
(1) To regulate the specific gravity of the blood and other fluids of the body; (2) to preserve the tissues from disorganization and putrefaction; (3) to enter into the composition of the teeth and bones. These are only a few of the uses of salts in the body, but are sufficient for our purpose. Fruits and nuts contain the least quantity of salts, meat ranks next, then vegetables and pulses, cereals contain most of all (Chambers). Sodium chloride (common salt) is the most important and valuable salt. It must not however be used in excess. Potassium saltsrank next in importance.[4]Calcium, phosphorus, sulphur and iron are included in this class.
The quantity of salts or mineral matter contained in some important articles of vegetable and animal food is shown in this table (Church):
Mineral Matter in 1,000 lbs. of 14 Vegetable Products.
Lbs.Apples4Rice5Wheaten flour7Turnips8Potatoes10Barley11Cabbage12Bread12Watercress13Maize20Oatmeal21Peas30Cocoa nibs36Wheaten bran60
Mineral Matter in 1,000 lbs. of 8 Animal Products.
Lbs.Fat Pork5Cow's milk7Eggs (without shells)13Lean of mutton17Flesh of common fowl16Bacon44Gloucester cheese49Salted herrings158
"In most seeds and fruits there is much phosphate in the mineral matter, and in most green vegetables much potash. One important kind of mineral matter alone is deficient in vegetable food, and that is common salt."
FOOTNOTE:
[4]See Vegetables, Chap. VII.
[4]See Vegetables, Chap. VII.
It is not within the scope of this book to deal with the science of nutrition; but a few general principles may be given which concern the effect upon the system of the different classes of food. Animal food requires a considerable quantity of oxygen for its complete combustion. Meat in general has a more stimulating effect upon the system and is more strengthening than vegetable food. There is, however, a tendency to eat too much meat, and when its effects are not counter-balanced by free outdoor exercise, it causes biliousness and sometimes gout and other troubles. Albuminous foods can be eaten longer alone without exciting loathing than can fats, sugars or starches. A carbonaceous diet taxes the excretory organs less than animal food. Meat is not necessary to life. Nitrogenous food man must have, but it need not be in the form of meat. The estimate commonly given is, that meat should occupy one-fourth and vegetable food three-fourths of a mixed diet, but in many cases the meat eaten is much in excess of this allowance. The proper association of different foods always keeps healthy people in better condition; there are times, however, when it may be necessary to abstain from certain articles of diet. It may be well to bear in mind, that the protein compounds can do the work of the carbohydrates and fats in being consumed for fuel, but the carbohydrates and fats cannot do the work of protein in building andrepairing the tissues of the body. As already stated, a mixed diet is the only rational one for man. An exclusively vegetable diet, while it may maintain a condition of health for a time, eventually results in a loss of strength and power to resist disease. Therefore it is necessary to understand the approximate value of each class of food in arranging the daily dietary.
It has been stated that "a quart of milk, three-quarters of a pound of moderately lean beef, and five ounces of wheat flour contain about the same amount of nutritive material;" but we pay different prices for them, and they have different values for nutriment. The milk comes nearest to being a perfect food. It contains all the different kinds of nutritive materials that the body requires. Bread made from wheat flour will support life. It contains all the necessary ingredients for nourishment, but not in the proportion best adapted for ordinary use. A man might live on beef alone, but it would be a very one-sided and imperfect diet. Meat and bread together make the essentials of a healthful diet. In order to give a general idea of food economy, it will be necessary to deal briefly with the functions of the various food principles. As our bodies contain a great deal of muscle, the waste of which is repaired by protein found in such food as lean meat, eggs, cheese, beans, peas, oatmeal, fish, etc., a supply of these articles must be considered in purchasing the daily supply. Fatty tissue (not muscle) serves as fuel, therefore the value of such foods as butter, cream, oils, etc., is apparent. Carbohydrates form fat and serve as fuel and force producers; these come in the form of starches, sugars,—vegetables and grains being the most important. In being themselves burned to yield energy, the nutrients protect each other from being consumed. The protein and fats of body tissue are used like thoseof foods. An important use of the carbohydrates and fats is to protect protein (muscle, etc.) from consumption. "The most healthful food is that which is best fitted to the wants of the user: the cheapest food is that which furnishes the largest amount of nutriment at the least cost: the best food is that which is both healthful and cheap." By referring to the various charts a fair estimate of food values may be obtained.
As will be noticed, the animal foods contain the most protein and fats, while the vegetable foods are rich in carbohydrates. A pound of cheese may have 0.28 pound of protein, as much as a man at ordinary work needs for a day's sustenance, while a pound of milk would have only 0.04, and a pound of potatoes 0.02 pound of protein. The materials which have the most fats and carbohydrates have the highest fuel value. The fuel value of a pound of fat pork may reach 2.995 calories, while that of a pound of salt codfish would be only .315 calories. On the other hand, the nutritive material of the codfish would consist almost entirely of protein, while the pork contains very little. Among the vegetable foods, peas and beans have a high proportion of protein. Oatmeal contains a large proportion also. Potatoes are low in fuel value as well as in protein, because they are three-fourths water. For the same reason milk, which is seven-eights water, ranks low in respect to both protein and fuel value, hence the reason why it is not so valuable as food for an adult as many of the other food materials.
These few illustrations will help to show the need of an intelligent idea of food values before attempting to purchase the supplies for family use. As one-half alaboring man's income goes towards providing food, it must follow that such knowledge will help the housewife very materially in securing the best results from the amount expended.
Theaverage daily dietof an adult should contain (Church):—
Nutrients.In 100 Parts.Each 24 Hours.lb.oz.gr.Water81.558.320Albuminoids3.904.178Fat3.003.337Common salt3.700.325Phosphates, potash, salts, etc.0.3000.170
The quantity of food required to maintain the body in a vigorous condition depends upon the following conditions:—(1) Climate and season, (2) clothing, (3) occupation, (4) age and sex. In civilized countries more food is eaten, as a rule, than is necessary to maintain health and strength. Climate and seasons influence the quantity of food eaten. A cold, bracing atmosphere stimulates the appetite, tempts one to exercise, while a hot climate has the contrary effect; hence the need for more or less food. Abundant clothing in cold weather conserves the body heat; less food is therefore required to maintain life. Exercise and muscular work cause greater oxidation in the tissues and greater waste of the muscles; this must be replaced by proper food. Outdoor work requires more food than indoor, and physical labor more thanmental. It has been estimated "that a child of ten years requires half as much food as a grown woman, and one of fourteen an equal amount. The rapidly growing active boy often eats as much as a man, and the middle-aged man requires more than the aged. A man of seventy years may preserve health on a quantity which would soon starve his grandson."
Just what ingredients of the food serve for nourishment of the brain and nerves, and how they do that service, are mysteries which have not yet been solved. Brain and nerve contain the elements nitrogen and phosphorus, which are found in protein compounds but not in the true fats, sugars, and starches, which contain only carbon, hydrogen and oxygen. We naturally infer that the protein compounds must be especially concerned in building up brain and nerve, and keeping them in repair. Just how much food the brain worker needs is a question which has not yet been decided. In general it appears that a man or a woman whose occupation is what we call sedentary, who is without vigorous exercise and does but little hard muscular work, needs much less than the man at hard manual labor, and that the brain worker needs comparatively little of carbohydrates or fats. Many physicians, physiologists and students of hygiene have become convinced that well-to-do people, whose work is mental rather than physical, eat too much; that the diet of people of this class as a whole is one-sided as well as excessive, and that the principal evil is the use of too much fat, starch and sugar. It is well to remember that it is the quantity of food digested which builds the body, and more injury is likely to result from over-eating than from a restricted diet, hence the valueof having food cooked so as to aid digestion. The following dietary standards may be interesting to the more advanced pupils, housewives, etc.:—
Standards for Daily Diet of Laboring Man at Moderate Muscular Work.
Nutrients in Daily Food.Author.Protein.Fats.Carbohydrates.Fuel Value.lb.lb.lb.Calories.Playfair, England.26.111.173.140Moleschotte, Italy.29.091.213.160Wolff, Germany.28.081.193.030Voit, Germany.26.121.103.055Atwater, United States.2817.3388.1.213.500
It is a great mistake to think that the best is the cheapest in regard to the food question, that the higher priced meats, fish, butter, etc., contain special virtues lacking in the cheaper articles.Poor cooking is the chief cause of this error in judgment.No doubt a well broiled steak is more appetizing and delicate in flavor than some of the cheaper cuts, but in proportion to the cost is not equal in nutritive value; careful cooking and judicious flavoring render the cheaper pieces of beef equally palatable. That expensive food is not necessary to maintain life has been clearly demonstrated by the traditional diet of the Scotch people with their oatmeal and herring; the Irish, potatoes and buttermilk; New England, codfish and potatoes, and pork and beans; the Chinese, rice, etc. Monotony of diet, however, is not recommended, for reasonsgiven in a previous chapter, and in the countries where a special diet prevails owing to the climate, nature of soil and markets, the results have not warranted us in believing that it is as good as a mixed diet. From this necessarily brief outline of the food question we have learned (1) that a knowledge of the requirements of the body are absolutely necessary in regulating a proper diet; (2) to furnish the food principles in a cheap rather than a dear form; (3) to understand the art of cookery so as to secure the full nutritive value and at the same time stimulate the appetite; (4) the value of economy in regard to food principles. When the housekeeper has acquired this knowledge she will have covered the field of food economy. Prof. Atwater says: "When we know what are the kinds and amount of nutritive substances our bodies need and our food materials contain, then and not till then shall we be able to adjust our diet to the demands of health and purse."
It is sometimes asked, why do we cook our food? As many opportunities will occur during this course of instruction for a comparison of the customs and diet of the various countries, and the advance of civilization in this direction, we will confine ourselves to the definition of the term as it concerns ourselves.
Mr. Atkinson says, that "Cooking is the right application of heat for the conversion of food material."
As much of our food requires cooking, how we shall cook it so as to render it more palatable, more digestible, and with the greatest economy of time, fuel and money,is an object deserving the most careful attention. The art of cooking lies in the power to develop certain flavors which are agreeable to the palate, or in other words, which "make the mouth water," without interfering with the nutritive qualities of the food prepared, to understand by what method certain foods may be rendered more digestible, and to provide variety. Monotony of diet and of flavor lessens the appetite and fails to stimulate the digestive organs.
The chemical changes, produced by cooking food properly, aid digestion, beside destroying any germs which may be contained in the food. Nearly all foods—except fruit—require cooking. The digestibility of starch depends almost entirely upon the manner in which it is cooked, especially the cereal class. Gastric troubles are sure to follow the use of improperly cooked grains or starches. (See Chap. VII.)
The following are the usual methods observed in cooking, viz.: (1) boiling, (2) stewing, (3) roasting, (4) broiling, (5) frying, (6) braising, (7) baking, (8) steaming.
BOILING.
Water boils at a temperature of 212° F. Simmering should be at a temperature of from 175° F. to 180° F. When water has reached the boiling point, its temperature cannot be raised, but will be converted into steam; hence the folly of adding fuel to the fire when water has already reached the boiling point.
STEWING.
Stewing allows the juices of the meat to become dissolved in water heated to the simmeringpoint. The juices thus dissolved are eaten with the meat. If not injured by the addition of rich sauces or fats, this is usually a very digestible method of preparing certain kinds of meat.
BROILING.
Broiling is cooking directly over the hot coals. A coating of coagulated albumen is formed upon the outer surface. This coating prevents the evaporation of the juices, which with the extractive materials are retained and improve the flavor. Meat cooked in this way has a decided advantage, in both flavor and nutritive value, over that which has been boiled or stewed. There are, however, only certain kinds of meat that are suitable for broiling.
FRYING.
Frying is cooking in hot fat. The boiling point of fat is far above that of water. Fat should not be heated above 400° F., as it will then turn dark and emit a disagreeable odor. Fried food, unless very carefully prepared, is considered unwholesome. The only proper method for frying is to immerse the food completely in a bath of hot fat.
BRAISING.
Braising is cooking meat in a covered vessel surrounded by a solution of vegetable and animal juices in a strong but not boiling temperature. Tough meat may be rendered very palatable and nutritious by cooking in this way. The cover of the pan or kettle must fit closely enough to prevent evaporation. It requires long, steady cooking. The flavor is improved by browning themeat in either hot fat or in a very hot oven before braising.
BAKING.
Baking is cooking in confined heat. Meat properly cooked in an oven is considered by many authorities as quite equal in delicacy of flavor to that roasted before a fire, and is equally digestible.
STEAMING.
Steaming is cooking food over condensed steam, and is an excellent method for preparing food which requires long, slow cooking. Puddings, cereals, and other glutinous mixtures are often cooked in this way. It is an economical method, and has the advantage of developing flavor without loss of substance.
Food is preserved by the following processes: (1) drying, (2) smoking, (3) salting, (4) freezing, (5) refrigerating, (6) sealing, (7) addition of antiseptic and preservative substances.
DRYING.
Drying in the sun and before a fire is the usual method employed by housekeepers. Fruits and vegetables, meat and fish may be preserved by drying, the latter with the addition of salt.
SMOKING.
Smoking is chiefly applied to beef, tongue, bacon, ham, and fish, which are hung in a confined chamber, saturated with wood smoke for a long time until they absorb a certain percentage of antiseptic material, which prevents the fat from becoming rancid, and the albumen from putrefying. Well smoked bacon cut thin andproperly cooked is a digestible form of fatty food, especially for tubercular patients. Smoking improves the digestibility of ham.
SALTING.
Salting is one of the oldest methods of preserving food. The addition of a little saltpetre helps to preserve the color of the meat. Brine is frequently used to temporarily preserve meat and other substances. Corned beef is a popular form of salt preservation. All salted meats require long, slow cooking. They should always be placed in cold water and heated gradually in order to extract the salt. Salt meats are less digestible and not quite so nutritious as fresh meats.
FREEZING.
Food may be kept in a frozen condition almost indefinitely, but will decompose very quickly when thawed, hence the necessity for cooking immediately. Frozen meat loses 10 per cent. of its nutritive value in cooking.
REFRIGERATING.
This process does not involve actual freezing, but implies preservation in chambers at a temperature maintained a few degrees above freezing point. This method does not affect the flavor or nutritive value of food so much as freezing.
SEALING.
Sealing is accomplished not only in the process of canning but by covering with substances which are impermeable. Beef has been preserved for considerable time by immersing in hot fat in which it was allowed to remain after cooling.
CHEMICALS.
Chemicals are sometimes used in the preservation of food, but the other methods are safer.
Animal foods contain nutritive matter in a concentrated form, and being chemically similar to the composition of the body is doubtless the reason why they assimilate more readily than vegetable foods, although the latter are richer in mineral matter. The most valuable animal foods in common use are meat, eggs, milk, fish, gelatin and fats.
MEAT.
Meat is composed of muscular tissue, connective tissue or gristle, fatty tissue, blood-vessels, nerves, bone, etc. The value of meat as food is due chiefly to the nitrogenous compound it contains, the most valuable being the albuminoids: the gelatinoid of meat is easily changed into gelatin by the action of hot water. Gelatin when combined with the albuminoids and extractives has considerable nutritive value. Extractives are meat bases, or rather meat which has been dissolved by water, such as soup stock and beef tea. The object in cooking meat is to soften and loosen the tissue, which renders it more easily digested. Another object is to sterilize or kill any germs which may exist and to make it more palatable. The digestibility of meat is influenced by the age of the animal killed and the feeding. The following table is given as an average of the digestibility of animal foods:—
Table of Comparative Digestibility.Commencing with the most digestible and ending with the leastdigestible of meats and other animal foods.(Thompson.)
Cooking affects the digestibility of meat, which is evident from the figures given in the following table (Church):—Time of Digestion.
Hours.Beef, raw2Beef, half boiled21/2Beef, well boiled23/4 to 3Beef, half roasted23/4 to 3Beef, well roasted21/4 to 4Mutton, raw2Mutton, boiled3Mutton, roasted31/4Veal, raw21/2Pork, raw3Pork, roasted51/4Fowl, boiled4Turkey, boiled21/2Venison, broiled11/2
It may be well to add here that animal food is more digestible when cooked between 160° and 180° F. than at a higher temperature.
(For more general information, see Recipes.)
In boiling meat two principles must be considered, the softening of the fibre and preservingof the juices. If the meat alone is to be used it should be placed in sufficient boiling water to completely cover, and kept at boiling point (212° F.) for at least ten minutes, so as to harden the albumen and prevent the escape of the juices. The temperature should then be allowed to fall to simmering point (175° F.). If the water is kept boiling it will render the meat tough and dry. If the juice is to be extracted and the broth used, the meat should be placed in cold water; if bones are added they should be cut or broken into small pieces in order that the gelatin may be dissolved. If the water is heated gradually the soluble materials are more easily dissolved. The albumen will rise as a scum to the top, but should not be skimmed off, as it contains the most nutriment and will settle to the bottom as sediment.
STEWING.
If both meat and broth are to be used the process of cooking should be quite different. In stewing, the meat should be cut into small pieces, put into cold water in order that the juices, flavoring material and fibre may be dissolved. The temperature should be gradually raised to simmering point and remain at that heat for at least three or four hours, the vessel being kept closely covered. Cooked in this way the broth will be rich, and the meat tender and juicy. Any suitable flavoring may be added. This is a good method for cooking meat containing gristle.
ROASTINGandBROILING.
When the meat alone is to be eaten, either roasting, broiling or frying in deep fat is a moreeconomical method, as the juices are saved. The shrinkage in a roast of meat during cooking is chiefly due to a loss of water. A small roast will require a hotter fire than a larger one, in order to harden the exterior and prevent the juices from escaping. Meat is a poor conductor of heat, consequently a large roast exposed to this intense heat would become burned before the interior could be heated. The large roast should be exposed to intense heat for a few minutes, but the temperature should then be reduced, and long steady cooking allowed.
Broiling (see broiling in previous chapter, p. 19.)
BEEF TONGUE.
Beef tongue is a tender form of meat, but contains too much fat to agree well with people of delicate digestion.
VEAL.
Veal, when obtained from animals killed too young, is apt to be tough, pale and indigestible, but good veal is considered fairly nutritious. It contains more gelatin than beef, and in broth is considered valuable, especially for the sick.
MUTTON.
Mutton is considered to be more digestible than beef, that is well fed mutton from sheep at least three years old; but as it is more difficult to obtain tender mutton than beef, the latter is more generally preferred. Mutton broth is wholesome and valuable in sickness.
LAMB.
Lamb, when tender and of the right age, is quite as digestible as beef or mutton, but the flesh contains too large a proportion of fat.
Fig. 1.—Diagram of cuts of beef.Fig. 1.—Diagram of cuts of beef.
Fig. 2.—Diagram of cuts of veal.Fig. 2.—Diagram of cuts of veal.
Fig. 3.—Diagram of cuts of pork.Fig. 3.—Diagram of cuts of pork.
Fig. 4.—Diagram of cuts of mutton.Fig. 4.—Diagram of cuts of mutton.
VENISON.
Venison is a tender meat with short fibres, which is very digestible when obtained from young deer, but is considered to be rather too stimulating. Its chemical composition is similar to lean beef.
PORK.
Pork is a tender-fibred meat, but is very indigestible owing to the high percentage of fat, which is considerably more than the nitrogenous material it contains. Pork ribs may have as much as 42 per cent. of fat.
HAM AND BACON.
Ham is more digestible when well boiled and eaten cold. Bacon is more easily digested than either ham or pork; when cut thin and cooked quickly—until transparent and crisp—it can often be eaten by dyspeptics, and forms an excellent food for consumptives.
FOWL.
Chicken is one of the most digestible of meats, contains considerable phosphorus and is particularly valuable as food for invalids. Turkey is somewhat less digestible than chicken. Ducks and geese are difficult of digestion, unless quite young, on account of the fat they contain.
GAME.
Game, if well cooked, is fairly digestible.
SWEETBREAD.
Sweetbread, which is thymus gland of the calf, is a delicate and agreeable article of diet, particularly for invalids. Tripe, heart, liver and kidneys are other forms of animal viscera used as food—valuable chiefly as affording variety.
FISH.
The chief difference in fish is the coarseness of fibre and the quantity of fat present. Fish which are highly flavored and fat, while theymay be nutritious, are much less easy of digestion than flounder, sole, whitefish, and the lighter varieties. The following fish contain the largest percentage of albuminoids:—Red snapper, whitefish, brook trout, salmon, bluefish, shad, eels, mackerel, halibut, haddock, lake trout, bass, cod and flounder. The old theory that fish constituted "brain food," on account of the phosphorus it contained, has proved to be entirely without foundation, as in reality many fish contain less of this element than meat. The tribes which live largely on fish are not noted for intellectuality. Fish having white meat when broiled or boiled—not fried—are excellent food for invalids or people of weak digestion. Fish should be well cooked.
OYSTERS.
Oysters are a nutritious food, and may be eaten either raw or cooked. Lobsters, crabs and shrimps are called "sea scavengers," and unless absolutely fresh are not a desirable food.
MILK.
Milk contains all the elements which are necessary to maintain life; and constitutes a complete diet for infants. It will sustain life in an adult for several months. Although milk furnishes a useful food, it is not essential to a diet required for active bodily exercise. It is seldom given to athletes while in active training. Adults who are able to eat any kind of food are kept in better health by abstaining from milk, except as used for cooking purposes. An occasional glass of hot milk taken as a stimulant for tired brain and nerves is sometimes beneficial.Milk is composed of water, salts, fat, milk sugar or lactose, albumen and casein. Average milk has from 8 to 10 per cent. of cream. Good milk should form a layer of cream about 2-1/2 in. thick as it stands in a quart bottle. Lactose (milk sugar) is an important ingredient in milk. It is less liable to ferment in the stomach than cane sugar. In the presence of fermenting nitrogenous material it is converted into lactic acid, making the milk sour. Casein is present in milk chiefly in its alkaline form, and in conjunction with calcium phosphate. Milk absorbs germs from the air and from unclean vessels very readily. Good, clean, uncontaminated milk ought to keep fresh, exposed in a clean room at a temperature of 68° F., for 48 hours without souring. If the milk is tainted in any way it will sour in a few hours. Boiled milk will keep fresh half as long again as fresh milk. Milk absorbs odors very quickly, therefore should never be left in a refrigerator with stale cheese, ham, vegetables, etc., unless in an air-tight jar. It should never be left exposed in a sick room or near waste pipes. Absolute cleanliness is necessary for the preservation of milk; vessels in which it is to be kept must be thoroughly scalded with boiling water, not merely washed out with warm water.
Methods of Preserving Milk.