GENERAL CHEMISTRY OF MILK 91 



properties, such as optimum temperature, resistance to environ- 

 mental conditions, etc. Enzyms have never been isolated in pure 

 form. Differences in properties of coagulating enzyms may, 

 therefore, be due to admixtures of other substances, and it is dif- 

 ficult to separate them. 



Rennet extract for commercial purposes is made from the 

 fourth stomach of a young calf. The calves are not fed for twelve 

 hours before slaughtering, and the stomachs are then removed, 

 washed, and cleaned. The clean stomachs are sprinkled with 

 salt, inflated, and allowed to dry in the air. After three to twelve 

 days they are extracted with 4 to 5 per cent, salt solution. It is 

 thought that the potency of the stomachs increases up to twelve 

 days after slaughtering. For scientific purposes the glandular 

 part of the washed stomach is scraped off and the scrapings 

 digested with ten to twenty times their volume of 0.2 per cent, 

 hydrochloric acid. The acid is neutralized before the extract is 

 used. 



Rennet extracts deteriorate with age and if exposed to light 

 or warmth. Therefore they should be kept in a cool, dark place. 

 Rennin is a colloid and does not diffuse through animal mem- 

 branes, but can pass through porcelain filters. 



There are two changes produced in milk by rennet extract: 

 one is coagulation of the casein, the other digestion of the coagu- 

 lum. Whether these activities are due to one or two enzyms is 

 not known. Some authors think that there is but one enzym 

 which produces both reactions, others believe that there are two 

 distinct enzyms, similar to pepsin in the stomach. The coagu- 

 lative enzym is called rennin or chymosin, the proteolytic enzym 

 is called pepsin. The latter enzym is of great importance in 

 cheese ripening. 



Rennin does not produce curds of the same character in all 

 kinds of milk. Human milk, for example, is more alkaline than 

 cow's milk, and rennin action is only possible in human milk after 

 addition of acid, since rennin does not act in alkaline fluids. 

 Mare's milk coagulates with rennet only after addition of a soluble 

 calcium salt. As a rule, rennin acts more promptly and more 

 completely on homologous milk than on heterologous milk. For 

 example, rennin prepared from calves' stomachs acts better on 

 cow's milk than on milk of any other animal. 



The coagulation of casein by rennin is a complicated physical 

 and chemical process. According to Van Slyke and Publow the 

 reaction takes place in three phases, namely: 1, the change of 

 casein into paracasein; 2, the change of insoluble calcium salts 

 into soluble ones; 3, the precipitation of uncoagulated paracasein 

 by soluble calcium salts. 



