Food, Weather, Other conditions being the same milk is richer in winter than in summer, in cooler than in warmer weather, and on rich dry food than on pasture, except that on first turning the cows to pasture a richer milk may be obtained for a time. Probably anything which induces a greater consumption of food has a tendency to increase at first the richness of the milk secreted. In most cases, however, such increase in richness is found to be only temporary, the more permanent in fluence of better feeding being to improve the quantity rather than the quality of milk pro duced.
Partial All of the statements regarding the composition of milk refer to the product of a complete milking. In partial or fractional milking the first portions drawn are comparatively poor, especially in fat, and the last portions or 'strippings* are much richer.
Milk Milk fat, like other fats, is a mixture of gycerides (compounds of glycerin and fatty acids). It differs from other animal fats in containing a smaller proportion of stearic acid and large proportions of the acids of lower molecular weight. A detailed study of the constitution of milk fat by Browne gave the following results: Advance oL After the third or fourth month the milk tends to increase in richness as lactation advances. A study of nearly 50 lactation periods of individual cows at the New York State Experiment Station yielded the following average results, the ob The last four acids are the 'volatile fatty acids," the large amount of which serves to dis tinguish milk fat (butter) from other fats which as a rule have only a fraction of a per cent of these acids. In the milk the fat exists in the form of minute suspended globules which vary considerably in size, but average about of an inch in diameter. A drop of aver age milk contains over 100,000,000 fat globules.
Amount of Fat in The percentage of fat in milk varies more than that of any other solid constituent or, indeed, of all the other solids combined. In the mixed milk of herds where the influence of individual cases of ad vanced lactation is minimized, we may consider the usual range to be from 3 to 6 per cent of fat and from 8.5 to 9.5 per cent of solids not fat. Hence the varying richness of cow's milk is due principally to differences in fat content, and as fat is also the constituent of greatest pecuniary value, a "rich') mil'. is essentially one
containing a high percentage of fat.
Nitrogenous Compounds or Proteins of The greater part of the nitrogen in milk (usually over three-fourths) is in the form of casein, a compound protein which contains phosphorus in organic combination and is prob ably also combined with lime salts. Casein is readily coagulated by acids or rennet and the curd thus formed encloses the greater part of the fat which the milk contained. Of the nitro gen compounds other than casein, albumin is by far the most abundant. Milk albumin is not coagulated by rennet, nor by acids at ordinary temperatures, but is coagulated by heat. In addition to casein and albumin milk contains small amounts of other protein bodies including the enzymes or ufiorganized ferments which play an important part in cheese-making and probably aid the digestion when milk is con sumed in the fresh state without previous heat ing. Among the nitrogenous compounds other than proteins which have been found in milk may be mentioned small amounts of lecithin, amino acids, gvitamine0 and traces of am monia and urea.
Amount of Proteins in Formerly, through faulty methods of separation, the re ported percentages of proteins were often very inaccurate and were usually too high. The amount of protein matter as determined by modern methods is usually between 3 and 4 per cent, being higher in those samples which are rich in fat. Average milk with 13 per cent of solid matter usually contains about 4 per cent of fat and 3 1/3 per cent of proteins. In richer milk the increased amount of solids is usually made up of about three-fourths fat and one-fourth proteins. In other words, the per centages of fat and proteins tend to rise and fall together approximately in the proportions represented by the formula — Proteins= 2.00 + 1/3 Fat. Variations in fat, especially if due to temporary causes, are not always accom panied by so much variation in proteins as the formula would indicate. On the other hand, the percentage of proteins is apt to exceed that indicated by the formula, in very advanced lactation. As a rule when the amount of solids not-f at exceeds 9 per cent, the excess is due chiefly to excess of proteins.