The milk of different animals varies very much in color, taste, and nutritive value. That of the cow is a little heavier than water—its specific gravity being, on the average, about 1·030, water being 1·000. It is composed of three constituents—namely, butter, curd, and whey—each of which is also composed of a number of substances. These three constituents are of unequal weight, or specific gravity, and their separation is the chief process carried on in the dairy. The butter is the lightest and the curd is the heaviest constituent.
The following table represents the composition of the milk of different animals:—
| COMPOSITION OF THE MILK OF DIFFERENT ANIMALS. 1,000 PARTS CONTAIN— | |||||||
| Specific Gravity, or Density. | Water. | Solid Ingredients. | Cheesy Matter. | Sugar. | Butter. | Mineral Matter. | |
|---|---|---|---|---|---|---|---|
| Woman | 1032·67 | 889·08 | 110·92 | 39·30 | 43·68 | 26·66 | 1·30 |
| Cow | 1030 | 864·20 | 135·80 | 48·80 | 47·70 | 31·30 | 6·00 |
| Goat | 1033·53 | 844·90 | 155·10 | 35·14 | 36·91 | 56·87 | 6·18 |
| Ewe | 1040·98 | 832·32 | 167·68 | 69·78 | 39·43 | 51·31 | 7·16 |
| Mare | 1033·74 | 904·30 | 95·70 | 33·35 | 32·76 | 24·36 | 5·23 |
| Ass | 1034·57 | 890·12 | 109·88 | 35·65 | 50·46 | 18·53 | 5·24 |
| Bitch | 1041·62 | 772·08 | 227·92 | 116·88 | 15·29 | 87·95 | 7·80 |
Milk examined through a microscope is a colorless fluid, containing a large number of little vesicles, or bags, filled with butter—a mixture of oily and fatty matters. When the milk stands for some time, the globules, being lighter than the other constituents, ascend to the top, and, mixed with a certain proportion of milk, are removed as cream. The curd is termed in scientific parlance casein, and is in fresh milk in a state of solution—that is to say, is dissolved in milk in the same way that we dissolve sugar in water. When milk becomes sour, either naturally or by the addition of rennet, it can no longer hold casein in solution, and the curd consequently separates. Casein is the substance which forms the basis of cheese. The substance that remains after the removal of the butter and cheese is called serum, or whey, and is composed of a sweetish substance termed sugar of milk, and certain saline bodies, termed the ash, dissolved in water.
The butter and the sugar of milk are employed in the animal economy in the production of fat, and are what have been styled by physiologists heat-producers and fat-formers. The casein resembles the gluten of wheat in composition; it belongs to the class of food substances termed flesh-formers. The ash, or mineral part of the milk, is chiefly employed in forming the bones of the young animals it is destined to nourish.
The quality of milk is influenced by the quantity and quality of the food given to the animal. The milk of cows fed on distillery wash, turnip, and mangel tops, coarse herbage, and other kinds of inferior food, is always of inferior quality. Hence it is of great importance that dairy stock be kept in good old pastures in summer, and fed on Swedish turnips, mangel-wurtzel, and oil-cake during winter. It is true economy to supply dairy cows with abundance of nutritious food; and it should be constantly borne in mind that the milk from two well-fed cows will give more butter than can be obtained from the produce of three badly-fed animals.
The butter is the constituent of milk which is most affected by the nature and amount of the animal's food; and butter is precisely the article which is of the greatest importance to the Irish dairy farmer, as the quantity of cheese prepared in this country is inconsiderable. When, therefore, it is found that a cow pastured on inferior land, or badly fed in the byre, yields a large supply of milk of a high specific quantity (which, however, is rarely the case), it must not be concluded that the result is satisfactory; for if such milk be tested by the lactometer it will certainly be found wanting in butter. The average composition of English milk, according to Way, is:—
| Water | 87·02 |
| Butter | 3·23 |
| Casein | 4·48 |
| Sugar of milk | 4·67 |
| Ash | 0·60 |
| ——— | |
| 100·00 |
In several analyses of milk published by Professor Voelcker, the highest proportion of butter is stated to be 7·62. In that of cows kept on poor and over-stocked pastures less than 2 per cent. was found. I have examined in my capacity of Food Analyst to the City of Dublin several hundred samples of milk, in not one of which have I found the proportion of butter to amount to more than 5·6 per cent. In no sample did I find a higher per-centage of solid matter than 13·15, or (when pure) lower than 12·08. The quality of the food of the milch cow exercises a great influence on the quality and yield of her milk. Aliments rich in fat and sugar favor the production of butter, and augment the supply of milk. Locust-beans, malt, and molasses are good milk-producing foods; but the chief condition in the production of milk rich in butter is simply that the animals which yield it must be fed with abundance of nutritious food. Nor must it be supposed that the richness of milk is due to the smallness of the yield, for whenever the quality of the secretion is inferior, it is almost certain to be deficient in quantity. Those cows which give the richest milk, generally yield the largest quantity.
Yield of Milk.—According to Boussingault, a cow daily yields on the average 10·4 parts of milk per 1,000 parts of her weight. Morton, in his "Cyclopædia of Agriculture," p. 621, states that Mr. Young, a Scotch dairy keeper, obtained 680 gallons per cow per annum. Voelcker found that some common dairy stock gave each of them fifty-two pints of milk per diem, whilst three pedigree cows yielded respectively forty-nine pints.