Page:The New International Encyclopædia 1st ed. v. 13.djvu/554

* MILK. 500 MILK. the casein, which gradually separates from the soluble conslituents, inclosing much of the fat. Casein is held in solution in milk by the presence of lime salts, and lime water causes it when curdled to separate in a much more finely di- vided condition. The albuniin of milk is not curdled by acids or rennet, but is acted upon by heat. Fibrin, similar to that of blood, globu- lin, nuclein, and several other nitrogenous bodies, have been found in milk in small quantities, but are of little importance. The sugar in milk, chemically known as lactose, is not so sweet as cam- sugar. It is in solution. The ash of milk consists of a mixture of a num- ber of salts, but is composed ])rincipally of the phosphates of lime and potash, the chlorides of potash and soda, and small amounts of phosphate of iron and magnesia. Some of the phosphate of lime appears to be associated with the casein, which also contains some sulphur. Most of the salts are in solution. Besides the constituents named, milk contains normally a coloring mat- ter, a trace of citric acid, urea, and several other bodies. The fat and the allmmin of milk are the most variable constituents, the ash and tlie sugar the least so. The casein bears a quite constant ratio to the fat, rising and falling with it. Variatio.ns IX Milk. The richness of milk is to a certain extent an individual character- istic: i.e. some individuals normally give rich milk, while others, for no apparent reason, give milk containing several per cent, more water. The richness of milk has been increa.sed by do- nu-s(icatinn, care, and breeding, and certain breeds of cows, sheep, goats, etc., have been pro- duced w'uich give a characteristically rich milk. The quality also varies with the stage of lacta- tion. The milk given early in the lactation period is usually poorer than that secreted later, and grows richer toward the close of the period until the animal 'goes dry.' Young animals usually give poorer milk and less of it than after the third or fourth parturition, and the milk from the first part of any milking is ptxircr than the last part, or "slrippings." The kind of food has little effect on the composition of milk, pro- vided it is wholesome and the amount sufficient. Food influences the proportion of the different fatty acids composing the fat. and so has an effect on the hardness and other qualities of but- ter. But the rather prevalent notion that the milk fat. for instance, can be permanently in- creased by feeding has been shown by much careful investigation to be a fallacy. Little is known of the physiological processes by ^yhich the constituents of the food are transformed into milk constituents. In some cases there ap- pears to be a direct transmissicm of the con- .slituents fnmi (he food to the milk, as is notice- able when cows eat garlic, onions, etc. Tlie ex- periments of .Jordan at the New York State Kxp<'riment Station have shown that milk fat is not derived solely from the fat of the food, for cows fed upon food from which the fats were practically completely extracted continued to se- crete milk of normal composition for long periods, and. juilging from the mainliiiance of their weight, did not draw upon their body fat to sup- jily this ingredient, fnder the conditions of the experiments, the carbohydrates seemed to be utilized to .some extent in the elaboration of milk fat. The more the process of milk secretion is imderstood the more apparent it becomes that richness and the volume of the yield are individual characters, and if cows have a ten- dency to give poor milk no amount of feeding will overcome it. The remedy lies in getting better cows. For general statements regarding the com- position of the milk of ditferent breeds of cows, see C'.TTLE. As an illustration of the variation of the milk of ordinary cows of mixed breeding, the data obtained by Van 81yke in New York from the analysis of the mixed milk of 1.5.000 cows each month from ilay to October may lie cited. The total solids ranged from 11.17 to 1.3.91 and averaged 12.07 per cent., and the fat from 3.04 to 4.01) and averaged 3.75 per cent. The content of total solids and of fat was lowest during the summer months and increased in the fall. In the analysis of over three thousand samples of milk at the Massachusetts Experiment Station, the total solids varied from 10.02 to 19. .55 and averaged 13.57 ])er cent., and the fat from 1.5 to 10.70 and averaged 4.32 per cent. The anal- ysis of eight hundred sanii)les made at the ex- periment stations in ditferent part.s of the coun- try varied in total solids from 9.3 to 19.7 per cent., averaging 12.8 per cent., and in fat from 1.7 to G.5, averaging 3.7 per cent. Milk Fermentations. Milk is subject to a great variety of fermentatiims. for it is a favor- able medium for the growth of many kinds of bacteria, yeasts, and other fungi which cause numerous changes in its constituents, T'hus. while one class of organisms curdles milk by the production of lactic acid, another class gives it an alkaline reaction, at the same time curdling it; others impart to it a deep blue, violet, yel- low, green, or red color, by the production of pigments in the milk: others give it a bitter or other unpleasant taste: another class produces alcohol from the milk svigar. and is taken advantage of in the preparation of such bever- ages as koiuniss and kepliir (((q.v.). and still others cause putrefaction. The most common and familiar change is the souring of milk, due >isu- ally to the action of lactic-acid bacteria. Under ordinary conditions normal milk nearly always undergoes some sort of lactic fermentation on standing. The production of lactic acid soon curdles the milk and obscures all other forms of fermentation, and the acid stops the growth of other bacteria so that no subsi'c|nent effects are usually seen. The popular belief that the elec- tricity in the air during thunderstorms sours milk appears to l)e unfounded, but its rapid sour- ing at such times is due to the climatic condi- tions prevailing, which hasten bacterial growth. The same difliculty in keeping milk is experienced during very hot weather. Curdling is not al- ways due to the formation of acid; milk appar- ently '.sour' may have no acid taste. In such cases the cause is die to ;ilkaline fermentation, induced by another class of organisms. The milk becomes coagulated into a soft slimy mass, which is usually bitter and has an alkaline or neutral reaction. Ordinarily this form of fermentation is not very apparent, as the organisms causing it grow slowly and the lactic-acid organisms get the start of them. The organisms which produce butyric acid in milk attack and decompose the fats, giving a rancid odor. In the ordinary han- dling of milk the latter are of little importance, but it is supposed that the,v have an important