92 MILK 



The change of casein into paracasein is wholly dependent on 

 the action of the enzym, and can be illustrated by the following 

 experiment: Two solutions of acid casein alkali are prepared. To 

 one of these solutions rennet extract is added and to the other ren- 

 net extract inactivated by boiling. After the solutions have been 

 kept at body temperature for some time they are boiled so that 

 rennin action ceases. At this stage no visible difference can be 

 detected between the solutions, either in appearance or by test- 

 ing the reaction with litmus or phenol phthalein. The solution 

 to which raw rennet has been added has lost some of its viscosity, 

 electric conductivity is increased, and the precipitation limit of 

 ammonium sulphate is lowered. If an equal amount of a soluble 

 calcium salt (a solution of calcium chlorid is usually used) is added 

 to each solution there is a rapid change. The solution with raw 

 rennet forms a curd which separates from the fluid portion, while 

 the solution with inactivated rennet turns milky. If the reaction 

 of the casein solution was neutral there is less difference in the 

 precipitates formed, and if the solution was alkaline there is no 

 difference. If a similar experiment is made with an acid solution 

 of casein in an alkaline earth instead of an alkali, the difference 

 between the two solutions is still more pronounced. 



v The experiment shows that rennin so alters the casein that it 

 coagulates in the presence of soluble calcium salts after destruction 

 of the enzym by boiling. Calcium salts do not coagulate casein 

 unless it has been acted upon by rennin. 



/During the second phase the rennin acts upon the calcium 

 salts which are* in an insoluble condition in milk and renders 

 them soluble. This phase requires more time than the first 

 one. Action can be accelerated by addition of a soluble calcium 

 salt. 



As soon as the calcium salts have become soluble the reaction 

 enters upon the third phase. The curd becomes visible and the 

 fluid thickens. This change is brought about rapidly and does 

 not depend upon rennet action, as in the first and second phases. 

 The curd from milk always encloses insoluble calcium phosphates 

 either mechanically or in actual combination with the para- 

 casein. Milk-fat and colloidal substances are also enclosed in 

 the curd. 



The chemical reactions taking place in rennet coagulation of 

 casein are not well understood, but the fact is established that 

 without soluble calcium salts no coagulation occurs. It must be 

 assumed that there is some difference in composition between 

 casein and paracasein. Paracasein has lower precipitation limits 

 than casein and is precipitated by soluble calcium salts, while 

 casein is not. If after prolonged rennin action on casein-alkali 



