MILK.
Limitation of Name.
—By the term “milk,” unless qualified in some way, is meant a lacteal secretion of the healthy cow, free of colostrum and of standard quality. If the milk of other mammals is meant the name of the class of animal is used in connection with the term, such as ewe’s milk, goat’s milk, etc. Milk is one of the most important articles of commerce and, by reason of its composition, high nutritive character, and easy digestibility, it is not only the natural food of infants but a most important food for children and adults. It is also an indispensable food in many, if not most, cases of disease where nutrition is impaired. In some cases life may often be sustained over a critical period by the use of milk as a food where other forms of food would fail of digestion and prove injurious instead of beneficial.
The United States standard for [milk] is found in [Appendix A].
Average Composition of Milk.
—Perhaps there is no food substance which has been subjected to so many and such severe analytical tests as milk. Hundreds of thousands of analyses have been made in all civilized countries, not only of the milk of the individual cow but of herds of greater or less size.
There is a great variation in the composition of milk in different breeds of cattle and also in different individuals of the same breed. For instance, the Holstein breed of cattle affords a milk with a very low content of fat, sometimes as low as 3.25 percent, and in individual cases lower. On the other hand the Jersey breed of cattle affords a milk of a very high content of fat, sometimes reaching as high as 6 percent, and in individual cases very much higher. The content of the nitrogenous element in milk is more stable than that of fat and the common content of casein in milk ranges from 21⁄2 to 31⁄4 percent. The sugar in the milk is usually the complementary substance with the fat, diminishing in relative proportions as the fat increases and vice versa. The average content of sugar in cow’s milk is approximately 4 percent. The content of mineral substances in milk is also quite constant, being about 0.70. The ash contains the phosphoric acid which is one of the essential food components of milk. A milk of fair average quality contains 12 percent of solids and 88 percent of water. This is an expression for milk during the various seasons of the year and from all breeds and kinds of cows. The influence of season has much to do with the quantity of milk produced. It is always greater in the spring and summer months, when the cows are turned out to pasture and the growth on which they feed is unusually succulent. The increase in volume is not attended with a proportionate increase of solids, and thus the percentage of solids in spring and summer milk is less than that in the winter milk unless the cows are particularly well fed during the winter on a generous diet, including large quantities of roots.
Fig. 13.—Cow Stables, Mapletown Farm, Sumner, Washington.
The character of the milk is greatly influenced by the environment in which the cow lives. The stable in which the cow is kept should be clean, well ventilated, and protected against extreme changes in temperature, thus being cooler in the summer than the hot air on the outside and much warmer in the winter. An excellent arrangement of the stables to secure cleanliness and good ventilation is shown in [Fig. 13]. Cows should be supplied with an abundant quantity of pure water and should not be allowed access to stagnant pools when pasturing in the summer. Every animal giving milk should be examined from time to time by a competent veterinarian to determine, by the injection of serum or otherwise, whether or not the animal is afflicted with tuberculosis. Every animal infected with tuberculosis should be separated from the herd and destroyed. Tuberculosis is an infectious disease and may spread from a single cow to every one in the herd. It is still by some authorities claimed that there is no authentic case of transmission of bovine tuberculosis to the human system. Other authorities hold that such transmission is possible, even if it has not been proven in a particular case. Since experts disagree on this point the same rule is applicable here as in other cases of the same kind, namely, where experts disagree on a point relating to the public health the benefit of the doubt, if any, should be given to the public, and the advice of those experts followed which is the most radical respecting the protection of health from infection of any kind. It would be difficult to prove, for example, in any case of tuberculosis in man that it had been contracted from the sputa of tuberculosed patients, yet because it is possible, in the opinion of many experts, that such infection and transmission of disease can take place, it is the part of wisdom to guard against it.
It is, I think, a statement which will be accepted by all that it is possible in this country to secure and keep a sufficient number of healthy cows to give the milk supply of the nation. Therefore, it is the duty of the state, either by municipal, state, or federal inspection, to eliminate, as far as possible, and, if necessary, at the expense of the state, every diseased animal from the dairy herd. The farmer whose herd becomes infected through no fault of his can justly claim a compensation for the destruction of his animals for the common good. There is, perhaps, no more important point connected with the keeping of sanitary conditions than the proper inspection of the dairy, not only furnishing milk for family use, but especially for sale. It is the plain duty of every municipality and state to prohibit the sale of milk to its citizens from dairies which are not periodically and frequently subjected to the most rigid expert inspection. Such inspection would not only secure the health of the animals but tend directly toward the cleanliness of the dairy. Only by the exercise of unusual care is it possible to keep milk from becoming contaminated.
Preparation of Milk.
—Every part of the animal, especially the udders, should be kept scrupulously clean by proper currying and washing. The milk should be collected in vessels with as small an orifice as possible. As soon as drawn the milk should be strained and artificially cooled to a temperature of at least 50 degrees F., if not lower. A convenient apparatus for cooling the milk is shown in [Fig. 14]. In this condition, without being exposed to infection and being protected from every point by closed vessels, stoppered when necessary by sterilized cotton, the milk is conducted into sterilized bottles and again stoppered with a sterilized cork of some description. The milk is kept cold until delivered to the consumer and by the consumer should be kept cold until used. By following these precautions it is possible to deliver a pure, wholesome, unpasteurized milk in a condition which keeps practically unchanged for even a longer period than twenty-four hours.
Fig. 14.—Apparatus for Cooling Milk.
Certified Milk.
—Dairies which are inspected either by operation of the law or, voluntarily, by a competent body of medical and scientific experts duly authorized to make such inspection furnish to the market what is known as certified milk. Each bottle of this milk bears the stamp of certification and this stamp may be used from the time of one inspection until a certain date specified on the stamp when the next inspection takes place. The duty of the inspectors is to see that diseased animals are at once removed from the dairy, that the sanitary conditions of the stable are perfect, that the food is abundant and wholesome, that the milking process is conducted according to the principles above outlined, and that the proper precautions are taken to prevent infection during the preparation of the milk for the market. The milk should be examined chemically and bacteriologically at each inspection, or oftener, to see that it is of a standard quality, both in respect of the number and character of the organisms which it contains and of its chemical constituents. Certified milk is, of course, more expensive than non-certified, inasmuch as the dairy is necessarily called upon to bear the expense of inspection. However, the superior quality of such milk and its certain freedom from infection more than offsets the increased price, and makes certified milk the ideal food of a milk character, not only in the family, but especially in the hospitals, orphan asylums and other public institutions. It seems quite certain that in the near future practically all the milk that is sold upon the market of the country will be of a certified quality.
Pasteurized Milk.
—When milk is heated to a temperature of about 140 to 160 degrees the greater part of the living organisms contained therein are destroyed. At the same time the temperature is not high enough to give to the milk that peculiar taste which it acquires when boiled. Such pasteurized milk, placed in sterilized bottles, stoppered with sterilized stoppers and kept in a cool place, will keep many days and even weeks without apparent deterioration. Physicians and hygienists are quite agreed that pasteurized milk is not so wholesome, especially for children, as certified milk which has not been subjected to a heat sufficiently high to kill the organisms contained therein. The natural ferments of the milk, namely, the enzymes which produce the lactic fermentations, promote rather than interfere with the digestion of the product. The killing of the beneficial organisms of the milk is only justified when there is danger of pathological germs being present. Hence the pasteurization of milk must in this sense be regarded as a substitute for inspection and certification.
There may arise cases where pasteurizing even of certified milk may be desirable, namely, when from necessity it must be kept for a considerable period before use, as on shipboard, and other places inaccessible to a daily supply of fresh milk. Pasteurizing is also justifiable in miscellaneous milk supplies, the origin of which is unknown. It is safer, by far in this case, to pasteurize than take the chance of consuming pathological germs.
Pasteurizing of Milk.
—A convenient method of pasteurizing milk is recommended by the Dairy Division of the Department of Agriculture, which is as follows:
Directions for the Pasteurization of Milk.
[20]—The pasteurization of milk for children, now quite extensively practiced in order to destroy the injurious germs which it may contain, can be satisfactorily accomplished with very simple apparatus. The vessel containing the milk, which may be the bottle from which it is to be used or any other suitable vessel, is placed inside of a larger vessel of metal, which contains water. If a bottle, it is plugged with absorbent cotton, if this is at hand, or in its absence other clean cotton will answer. A small fruit jar loosely covered may be used instead of a bottle. The requirements are simply that the interior vessel shall be raised about half an inch above the bottom of the other, and that the water shall reach nearly or quite as high as the milk. The apparatus is then heated on a range or stove until the water reaches a temperature of 155 degrees Fahrenheit, when it is removed from the heat and kept tightly covered for half an hour. The milk is rapidly cooled without removing it from its containers and kept in a cool place. It may be used any time within twenty-four hours. A temperature of 150 degrees maintained for half an hour is sufficient to destroy any germs likely to be present in the milk, in cold weather, or when it is known that the milk reaches the consumer soon after milking, and it is generally safe to adopt this limit. It is found in practice that raising the temperature to 155 degrees and then allowing the milk to stand in the heated water for half an hour insures the proper temperature for the required time. If the temperature is raised above 155 degrees the taste and quality of the milk will be affected.
[20] By Dr. De Schweinitz.
Inasmuch as the milk furnished to consumers in large cities in summer contains at the time of delivery an immense number of miscellaneous bacteria, this procedure may not fully meet the requirements during hot weather, not only because such milk will not remain sweet for twenty-four hours unless kept in a good refrigerator, but also because the bacteria not destroyed by the heating may at times produce digestive disturbances in the very young. Under such circumstances it is best to keep the bottles in the water until it boils or to use one of the many steamers now on the market. After the bottles have been kept at the boiling point for three to five minutes (or longer if they are large) they should be cooled as promptly as possible and kept in a refrigerator until used.
The simplest plan is to take a tin pail and invert a perforated tin pie-plate in the bottom, or have made for it a removable false bottom perforated with holes and having legs half an inch high to allow circulation of the water. The milk-bottle is set on this false bottom, and sufficient water is put into the pail to reach the level of the surface of the milk in the bottle. A hole may be punched in the cover of the pail, a cork inserted, and a chemical thermometer put through the cork, so that the bulb dips into the water. The temperature can thus be watched without removing the cover. If preferred, an ordinary dairy thermometer[21] may be used and the temperature read from time to time by removing the lid. This is very easily arranged, and is just as satisfactory as the patented apparatus sold for the same purpose. Any other simple method of procedure will give the same result.
[21] Before using the dairy thermometer it is best to have it tested, as it may be unreliable in the upper parts of the scale.
Average Content of Fat in American Milk.
—From the thousands of analyses of American milks that have been made it appears that the average content of fat therein is about 3.90 percent. Of the different breeds of cows the Holsteins produce milk with the least content of fat and the Jerseys with the greatest. It is not unusual to find in the milk of a Jersey cow a content of 6 or 7 percent of fat.
Comparison of Cow’s Milk with Other Varieties.
—Human milk differs from milk chiefly in having a much lower content of casein and a higher content of milk sugar. Goat’s milk has a higher content of casein than milk, somewhat higher content of fat, and slightly less sugar. Ewe’s milk is very rich both in protein and fat. Mare’s has a low casein and fat content and is exceptionally rich in sugar. Ass’s milk has less casein and protein than milk but more sugar.
Cream.
—When milk is allowed to stand for some hours in a cool place or when it is mechanically treated in a separator the fat particles, being of a less specific gravity, are separated, and when they reach a certain degree of consistence they form a product known as cream. The quantity of fat in cream varies according to the method of separation. On standing for a period of about twelve hours in a cool place the separated cream may be removed by skimming and should contain at least 18 percent of milk fat. Under the action of the separator, cream of a much greater content of fat is usually produced, often reaching as much as 30 percent or more. The separation of cream mechanically in a separator is preferable to the method of time separation by gravity alone. The cream secured by the separator is very much fresher, as it can be removed as soon as the milk is drawn and cooled. Its content of butter fat can also be regulated to the desired amount and, in the third place, a more complete separation is secured than by gravity. By the proper manipulation of the separator almost all of the fat in milk is readily removed. Cream should be kept under the same conditions as has been described for sanitary milk. When placed in sterilized containers, properly stoppered and kept cool, fresh cream will keep sweet as long as milk under similar circumstances.
In large dairy industries the separator is practically the only method now employed for securing cream while for farm use the gravity method of standing in a cool place for twelve or twenty-four hours is the commonly practiced method.
Cream is used on the table with fruit and cereal foods and especially in beverages such as tea and coffee. It is also prescribed by physicians for certain diseases and derangement of the digestive organs where the nitrogen content of milk produces irritation and fails of digestion. Cream is not a complete food in the sense that milk is inasmuch as the other constituents of milk are less in proportion as the percentage of fat is increased, yet cream contains at least a part of all the food elements in milk, as, for example, nitrogenous constituent, principally, casein, milk, sugar, and mineral matters.
It must be remembered in this case that the fat is the variable element and as that is increased the proportion of other ingredients, necessarily, is diminished.
The most important use of cream is in the manufacture of butter.
Standards of Cream.
—The composition of cream varies with almost every sample. The standards for cream vary in different states and cities. The national standard requires 18 percent of fat.
Skimmed Milk.
—The residue which is left from the removal of cream is known as skimmed milk. Skimmed milk contains the principle part of the nitrogenous constituents of milk, the greater quantity of its sugar and a very large quantity of its mineral matter. It is still a very valuable food product, lacking only the element of fat. When eaten with nuts or other oily food skimmed milk would complete the ration and make a well balanced food. The chief prejudice against skimmed milk is that it has been so often sold for whole milk. When sold and consumed under its own name it is not a fraudulent body and is deserving of a higher place in the dietary than has been ascribed to it. In the large creameries of the country the skimmed milk is usually fed to animals. It is one of the most highly esteemed foods for pigs and poultry, and is largely used for those purposes.
Composition of Skimmed Milk.
—Naturally the composition of skimmed milk would be that of milk corrected for the abstraction of fat. It contains some little fat when prepared by the gravity method and only a very small portion when separated mechanically. The abstraction of the fat increases the relative proportions of sugar and casein.
Curd Test for Purity of Milk.
—The Wisconsin curd test is conducted as follows: 1. Sterilize milk containers so as to destroy all bacteria in vessels. This step is very important, and can be done by heating cans in boiling water or steam for not less than one-half hour.
2. Place about one pint of milk in covered jar and heat to about 98 degrees F. ([Figs. 15] and [16]).
3. Add ten drops of standard rennet extract and mix thoroughly with the milk to quickly coagulate.
4. After coagulation, cut curd fine with case knife to facilitate separation of whey; leave curd in whey one-half hour to an hour; then drain off whey at frequent intervals until curd is well matted.
5. Incubate curd mass at 98 to 102 degrees F. by immersing jar in warm water. Keep jars covered to retain odors.
6. After 6 to 9 hours incubation, open jar and observe odor; examine curds by cutting the same with sharp knife and observe texture as to presence of pin holes or gas holes. Observe odor.
7. Very bad milks will betray presence of gas-producing bacteria by the spongy texture of the curd and will have an off flavor.
8. If more than one sample is tested at the same time, dip knife and thermometer in hot water before each time used.
“Normal milk contains practically no organisms but the straight lactic acid bacteria. These germs produce no gas and no bad odors, but purely lactic acid and the curd formed therefrom is such as is represented in [Fig. 17].
Fig. 15.—Improvised Wisconsin Curd Test.
C, Can used to hold sample; P, pipette for measuring rennet; K, knife for breaking curd.
Fig. 16.—A, Milk; B, Broken Curd in Whey; C, Matted Curd.
Fig. 17.—Curd from a Good Milk. Large, Irregular Holes Mechanical.
Fig. 18.—Curd from a Tainted Milk. Large, Irregular Holes Mechanical; Small Pinholes Due to Gas.
Fig. 19.—Curd from Foul Milk.
“Milk contaminated by the introduction of dust, dirt, fecal matter, or kept in imperfectly cleaned cans becomes fouled with gas-producing bacteria that break down the milk sugar and so produce gases and usually undesirable odors.... Therefore milks showing the presence of gas or bad odors in any considerable degree are milks that have been more or less polluted with extraneous organisms or carelessly handled, and as a consequence such milks show a type of curd revealed in [Figs. 17], [18], and [19].
Whey.
—The residue left from milk in the process of the making of cheese is known as whey. Whey consists of that portion of milk which is not precipitated by the rennet and which separates when the casein of milk is coagulated and sets in the process of cheese making. The whey contains the principal portion of the water in milk, the most of the milk sugar therein, and small quantities of butter and soluble nitrogenous portions (albumin) and solid particles which remain suspended in the solution. It may, therefore, be properly considered as milk from which the greater part of the nitrogenous portions and fat particles has been separated. The value of whey as a food product consists chiefly in the milk sugar which it contains. It is not very largely used for human food but is valued as a food for young domesticated animals, especially pigs and poultry.
Composition of Whey.
—The whey resulting from the manufacture of cheese contains nearly all the foods of the whole milk with the exception of the casein and fat. It is composed of from 6 to 8 percent of solids consisting chiefly of milk sugar, some albumin, a little fat, and about 0.6 percent of mineral matter.
Koumiss.
—Koumiss originated in Asia Minor in the production of a fermented drink from mare’s milk, which is richer in milk sugar than the lactic secretions of most other mammals. By the fermentation of the milk sugar mare’s milk is converted into a fermented beverage containing a small percentage of alcohol. In this country koumiss is made almost exclusively from cow’s milk and by special fermentation at a low temperature. It is a beverage valued especially by convalescents and invalids and frequently is capable of nourishing the body in diseases which affect the digestive organs when other foods fail of assimilation. It is also a cooling and delicious beverage for those in health when properly prepared and stored.
Modified Koumiss or Kephir.
—Koumiss made from cow’s milk with the previous addition of milk or cane sugar to increase the alcoholic content cannot be regarded as a natural product but rather one to which the term “modified” may be applied. The greater part of koumiss made in the United States from cow’s milk is of this modified variety. Cow’s milk contains on an average about 4 or 5 percent of sugar and does not yield a fermented beverage of a sufficient alcoholic content without reducing the actual sugar content of the beverage below the point of palatability. Cane sugar is usually employed as the modifying agent. While modified koumiss cannot be regarded as of equal value with the natural article made from mare’s milk it is a palatable and wholesome beverage when produced and stored under proper conditions. The quantity of alcohol produced in any case is not very great and the change in composition which renders koumiss so easily assimilable in many cases cannot be due alone to the alcohol formed but to the fermentative changes produced by enzymic action which takes place in the other constituents of koumiss, especially casein during the process of fermentation.
Koumiss or kephir, which is the name applied to koumiss made from cow’s milk, is also prepared with the addition of honey, in the place of sugar, and small quantities of wheat flour, not exceeding 20 parts to 1500 parts of other constituents. Koumiss is sometimes artificially fortified by the addition of small quantities of alcohol, but this practice must be regarded as extremely reprehensible. The alcohol of koumiss is incident to its fermentation and should not be increased beyond the normal amount. One of the important points in the making of koumiss is the control of the temperature which, during fermentation, ought not to rise above 50 degrees in order to get the best results. Koumiss may be made in the bottle in which it is kept, in fact, it is best made so, and its fermentation then resembles that of champagne. During the process of fermentation the bottle should be shaken at least once a day in order that the part which coagulates cannot be unevenly distributed throughout the mass. The bottle should be strong enough to resist the pressure produced by the carbon dioxid which is formed and the cork should be securely tied in. As in the case of champagne it is best to place the bottle with the cork down. Before using, the bottle containing the koumiss should be well shaken in order to thoroughly mix the contents which form a creamy, foamy mass extremely palatable, highly nutritious, and valuable not only as a beverage but in many cases of disease and disordered digestion as a food. In fact the value of koumiss for medicinal purposes, that is for medicinal food, is not thoroughly appreciated by the medical profession. This may be due to the fact that the art of making koumiss is not generally known, and while the general principles upon which its manufacture is based have been set forth it requires an expert to make a palatable and useful article (“British Dairy Farming” by Jas. Long). It is worthy of suggestion now that the use of horses for draft purposes has practically been superseded by the automobile and the trolley that the production of real koumiss from mare’s milk might become a very useful field of industry in the United States. It is perfectly certain that the genuine article must possess properties which are not wholly found in the imitations of koumiss which are so common in this country. It is well understood by physicians that a natural product produced from natural material is always superior in character both as a food and medicine to the synthetic or artificial product. Whenever, therefore, a fermented beverage produced from natural sources is contaminated by artificial products the resulting compound is not so useful nor digestible. For instance, wine which is made partially from sugar and beer made partially from dextrose, although they may be healthful and wholesome beverages, are inferior in quality and character to the real product made from grape juice or barley malt.
Buttermilk.
—The residue left in the churn in the manufacture of butter is termed buttermilk. There are two distinct varieties of buttermilk, namely, that resulting from the churning of unsoured cream and that remaining from the churning of soured and ripened cream. The first kind of buttermilk does not differ in its characteristic essentials from skimmed milk and therefore is not considered here. The second class of buttermilk is far more common and is a beverage of pleasing acid taste. When made from properly ripened cream it is wholesome and delicious, especially in summer time. Its composition is that of cream subjected to enzymic action during the ripening process by which an agreeable degree of acidity is produced due to lactic acid, together with the incidental changes which take place in the composition of other parts of the liquid due to enzymic action. Buttermilk also usually contains small particles of butter itself which escape aggregation during the final process of churning. In well prepared buttermilk, however, these particles of butter are not very numerous and they add nothing to the palatability, although they do add something to the nutritive properties of the beverage. The buttermilk represents that portion of milk which is one of the chief constituents of cream as far as bulk is concerned, freed practically from its butter fat. It does not differ greatly, therefore, in its chemical properties from skimmed milk, although there is a slight difference in the relative percentages of the milk solids in cream as compared with the same constituents in whole milk. The composition of buttermilk is shown in the following table:
COMPOSITION OF BUTTERMILK.
| From Sweet Cream. | From Sour Cream. | |
|---|---|---|
| Percent. | Percent. | |
| Water, | 89.74 | 90.93 |
| Fat, | 1.21 | 0.31 |
| Milk sugar, | 4.98 | 4.58 |
| Protein, | 3.28 | 3.37 |
| Ash, | 0.79 | 0.81 |
| Acidity, | ... | 0.80 |
Bonnyclabber.
—Bonnyclabber is a term applied to milk which has become soured by lactic fermentation, producing a gelatinous coagulation of casein which is sufficiently firm at times to prevent the liquid from being poured. Clabber may be regarded as a natural cheese curd except that the fat is chiefly on top. It is a beverage or food of a very agreeable taste to most persons and is often eaten with sugar. In the summer it is often formed during hot murky weather, especially of that character which produces thunder storms. For this reason it is a common supposition that thunder or lightning sours milk. The thunder and lightning, however, have nothing to do with this process. The condition of the atmosphere which produces an environment favorable to electrical disturbances of this kind also favors in the highest degree the growth of the organisms which produce the lactic ferments. Hence thunder storms and the rapid souring of milk are frequently coincident leading to the popular impression as above mentioned. Inasmuch as the souring of milk usually takes place after the cream has risen the composition of clabber is practically that of skimmed milk modified by the lactic fermentation which has taken place.