~Digestive milk enzym theory.~ In 1897 Babcock and the writer[202] showed that milk underwent digestive changes spontaneously when bacterial activity was suspended by the addition of such anaesthetics as ether, chloroform and benzol. The chemical nature of the by-products produced by this auto-digestion of milk resembles quite closely those found in ripened cheese, except that ammonia is not produced as is the case in old cheese. The cause of the decomposition of the casein, they found to be due to the action of a milk enzym which is inherent to the milk itself. This digestive ferment may be separated from fresh milk by concentrating centrifuge slime extracts by the usual physiological reagents. This ferment, called by them _galactase_, on account of its origin in milk, is a proteolytic enzym of the tryptic type. Its activity is destroyed by strong chemicals such as formaldehyde, corrosive sublimate, also when heated to 175 deg. F. or above. When such extracts are added to boiled milk, the digestive process is started anew, and the by-products produced are very similar to those noted in a normal cheese. Jensen[203] has also shown that the addition of pancreatic extracts to cheese accelerated the formation of soluble nitrogenous products. The action of galactase in milk and cheese has been confirmed by Freudenreich[204] and Jensen,[205] as well as by American investigators, and this enzym is now generally accepted as one of the factors concerned in the decomposition of the casein. Freudenreich believes it is able to change casein into albumose and peptones, but that the lactic-acid bacteria are chiefly responsible for the further decomposition of the nitrogen to amid form. Failure before to recognize the presence of galactase in milk is attributable to the fact that all attempts to secure sterile milk had been made by heating the same, in which case galactase was necessarily destroyed. A brief exposure at 176 deg. F. is sufficient to destroy its activity, and even an exposure at lower temperatures weakens its action considerably, especially if the reaction of the medium is acid. This undoubtedly explains the contradictory results obtained in the ripening of cheese from pasteurized milk, such cheese occasionally breaking down in an abnormal manner. The results mentioned on page 172, in which cheese failed to ripen when treated with disinfectants,--experiments which were supposed at that time to be the foundation of the bacterial theory of casein digestion--are now explicable on an entirely different basis. In these cases the casein was not peptonized, because these strong disinfectants destroyed the activity of the enzyms as well as the bacteria.