CHEESE conjunction with lactic acid, or the extent of its agency in generating lactic acid, has not heen determined. Moist caseine exposed to the air soon putrefies, yielding sulphide and carbonate of ammonia, and an oily body having a disagreeable smell, together with butyric and valeric acids, the undecomposed caseine dissolving in the ammonia which is formed. Caseine plays an important part in the making of cheese, although as a constituent its quantity is often less than that of butter. An analysis by Volcker of an average sample of good milk gave: Water 87-80 Butter 8-75 Caseine 8-31 Milk sugar and extractive matter 4-86 Mineral matters (ash) 0-78 100-00 The cheese which was made from this milk had the following composition : Water 87-85 Butter 23-91 Caseine 25-00 Extractive matter, lactic acid, &c 4-91 Mineral matter containing common salt 3-33 100-00 In cheese making, the coagulation of the milk may be effected in either of two ways : by adding an acid, which is done in Holland, or by subjecting the caseine to a peculiar fermen- tation induced by the action of rennet, which is the usual mode. Rennet is usually prepared from the fourth stomach of the calf, by salting and drying. The stomach should not be washed, but turned and carefully wiped with a cloth, sprinkled with salt, and dried at a moderate temperature in the open air, stretched upon a small hoop or forked stick. It is prepared for use by steeping it either in whey or brine. Whey is preferred for the reason that it more readily assists in inducing lactic acid fermen- tion ; but it should first be freed from the al- buminous matter which it contains, by boiling and straining. The steeping occupies about a week, during which time the rennets should be squeezed and rubbed to extract the active prin- ciple. A wooden vessel should never be used, as it is almost certain to impart putrefactive properties to the rennet which are injurious to the cheese, but the steeping should be done in earthen jars. The English method is to steep the rennets in a brine strong enough to bear an egg, adding six rennets, one sliced lemon, and an ounce of saltpetre to two gallons of brine. The brine liquor is usually prepared one or two months before it is used, as it is be- lieved that this age improves its coagulating qualities. The theory of the action of rennet is not yet considered to be well established. Liebig, although admitting the necessity of the presence of a ferment to initiate by a catalytic force the fermentation of milk sugar and its conversion into lactic acid, rejects the theory of Pasteur that the process is the result of the constant development of a minute fungus which has been called micrococcus. W. Hal- lier, however, following Pasteur, has found that newly made cheese contains numerous fer- ment nuclei, which he considers as a kind of putrefactive yeast. According to his investi- gations, these nuclei are developed, as are the nuclei of beer yeast, from spores of penicillium, differing from them only on account of the dif- ference in the fluid which affords them nour- ishment. In beer the penicillium, according to Hallier, is developed into cryptococcw, while in fresh milk it is converted into micro- coccus, and when the milk becomes sour into arthrococcus. He says that cows' milk contains ready-formed micrococcus cells, but that coagu- lation does not take place till the conditions for their development are favorable. The addition of rennet, which contains cells of a similar character ready to take on an active condition, produces a development of the micrococcus cells. Volcker and others have demonstrated by numerous experiments that milk is often coagulated by the action of rennet before it be- comes sour; and although it is undoubtedly true that caseine owes its solubility to a com- bination with an alkali, it does not follow that that combination may not be broken up even while the fluid in which it is dissolved is alka- line. It cannot therefore be maintained that the coagulation of the caseine in. the ordinary use of rennet is due to the action of lactic acid. When an acid is added to milk, it does not co- agulate till there is decided acidity. It would seem therefore that the alkaline constituent of the caseine is not immediately abstracted, because of the slight acidity of the milk in which it is held in solution. It is not unlikely that the coagulation of caseine by rennet has much similarity to its coagulation by the gas- tric juice, which by a catalytic action has the power of coagulating soluble caseine, and of again dissolving it in a condition which Maihle calls albuminose, as he does the digested pro- duct of all the albuminoids, but which Leh- mann calls peptones. The juice of the gastric tubules never effects this transformation unless it contains a free acid, but it is possible that the mucous coat of the stomach when convert- ed into rennet may possess the power of coagu- lating soluble caseine, although not of digesting it. The antiseptic properties of gastric juice are probably owing to the combined action of the pepsine and lactic acid, and it is not im- probable that the development of lactic acid in cheese making arrests putrefactive fermen- tation upon the same principle. But whatever may be the result of the investigations that are in progress, it is admitted on all hands that the transformation of sugar of milk into lactic acid is hastened by the action of rennet, although coagulation of caseine may precede the lactio acid fermentation. (See FERMENTATION.) In the practice of cheese making the subject of ferments is one that requires to be constantly borne in mind if satisfactory results are to be secured. The spores of various fungi are con-
