THE CHEMISTRY OF MILK The fat found in milk globules consists of mixed glycerides; that is, glycerin in combination with various fatty acids; e.g.: cA02 .Butyric Acid • . Palm i t ic Acid Ci,,Hm302• ..Stearic Acid Such a compound may be separated from butter by simple processes. The composition of the fat molecule is more complicated, since as many as fourteen fatty acids may be obtained from milk fat, and these may possibly be represented proportionately in every molecule. The only difficulty arises from the data which go to show that the smallest milk globules yield more oleic and fewer volatile acids than do the large ones. In any ease, existing methods afford as yet an imperfect insight into the composition of milk fat, which, moreover, possesses a specific character. The question has hardly as yet been raised as to whether it is immaterial to the organism from what source it receives its supply of fat or whether only the quantity of the different fatty acids is important. In a treatise on milk nutrition this question would play an important part. From experiments in the fat-using industries, the cholesterin contained in milk fat seems to be an important factor in the quality of the emulsion. A small portion of fatty acids (about 0.06 Gm. per litre of milk) is present, not as a simple glyceride, but as lecithin, which is perhaps even combined with a proteid as lecith-albumin. This need not necessarily exist in the milk globules, but the larger part is supposed to pass into the cream as it rises. Besides this still disputed ease, we know of three proteids found in milk, the casein, laetalbumin, and lactoglobulin, to which, apparently, there should be added the lactomucin, from which a carbo hydrate is said to be split off by acids. The lactomucin appears to be present more abundantly in colostrum and in centrifugal sediment. The so-called opalisins, on the contrary, are nothing but the casein remaining in solution after precipitation.
The caseins are acid-like proteids containing phosphoric acid (nucleo-albumin or phosphoglobulin), which through rennin undergo an unexplained change of such a nature that their combinations with lime, which do not give an alkaline reaction, are precipitated by a solu tion of lime salts at body temperature (rennin coagulation). They are
distinguished —though only cow casein has been tested—from most proteids by the absence of a carbohydrate group. Recently, however, by treatment with ozone, there has been obtained from casein a reduc ing substance, forming an osazone, which may be a sort of carbohydrate.
The following table compares the proportion of the proteid-nuclei in cow casein with that of these nuclei in the globin found in the h[elno globin of horses: The caseins are insoluble in water, but soluble in bases, acids and salts. In milk they are dissolved by alkalies, especially by alkaline earths. There are, however, numerous intermediate stages between solution, which is indestructible by any mechanical means, and sus pension, in which very minute portions of undissolved casein are kept in a finely divided state by means of calcium casein. Casein, in this state, has been appropriately designated by the Germans as "Kase stoff ," though this is not intended to determine what components take part in a solution of this kind. Such casein does not coagulate upon boiling, it is carried clown by all heavy precipitates, and it cannot pass through heavy filters.
Cow casein contains 53 per cent. of carbon, 7.06 per cent.. of hydro gen, 15.65 per cent. of nitrogen, 0.76 to 0.79 per cent. of sulphur, and 0.85 per cent. of phosphorus. It is at least a tetrabasic acid, and com bines with 2.73 per cent. of sodium oxide, using phenolphthalein. It gives all the albumin reactions, even those of Molisch and Adainkiewicz, because the carbohydrate group is lacking, and only a weak reaction for sulphur. It dissolves quickly in boiling water and alkalies, and when digested by pepsin it yields albumoses. From these by further digestion a precipitate, called pseudonuclein, is deposited, which is in turn dissolved only after vigorous digestion. The pscudonucleins are not simple substances, but mixtures of albumoses and the pseudonucleic acids. These are rich in phosphorus per cent.).