GENERAL CHEMISTRY OF MILK 93 



solutions acetic acid is added, a precipitate is formed which con- 

 sists of paracasein and paracasein-B. The latter is not pre- 

 cipitated by calcium salts and has been called whey-albumin, 

 hemi-casein-albumin, and lacto-serum-protein by different au- 

 thors. It is a protein of the nature of an albumose, and may be 

 the result of partial digestion of the casein by a proteotytic 

 enzym. 



Loevenhart assumed that when casein changes to paracasein 

 the molecules form large clusters and render paracasein more 

 readily coagulable. Ultramicroscopic observation has shown that 

 larger clusters are actually formed during the process. Whether 

 this is a chemical or merely a physical phenomenon has not been 

 determined. Van Slyke and Hart agree with Loevenhart, but 

 Laqueur takes the opposite view and assumes that casein mole- 

 cules are larger than paracasein molecules. However this may be, 

 the following facts are not disputed : 



1. Casein and calcium salts are the only substances acted upon. 



2. Rennet action effects no change of reaction. 



3. For rennin coagulation, rennin and soluble calcium salts are 

 indispensable. 



4. Casein does not coagulate before it is transformed into 

 paracasein by rennin action. 



Conditions Governing Rennet Action. Rennet enzym or ren- 

 net activity is influenced by a number of factors which have 

 received considerable study. The most important of these are: 



1. The presence of calcium salts. 



2. The presence of acids or alkalies. 



3. The presence of salts and other chemical compounds. 



4. Temperature. 



5. Dilution of milk and relative quantity of rennet used. 



6. The age and quality of the milk. 



1. The Presence of Soluble Calcium Salts. The necessity of 

 their presence for rennet action has been discussed at some length 

 previously. 



2. The Presence of Acids or Alkalies. Acids and acid salts in 

 suitable dilutions favor rennet action. Up to a certain concentra- 

 tion rennet action is accelerated in proportion to increasing 

 amounts of acids or acid salts. However, there is a limit beyond 

 which the concentration cannot be carried without producing an 

 inhibitory effect. For example, milk which has soured to the 

 curdling point or buttermilk are not coagulated by rennet. A 

 slight acidity is favorable. The following table given by Van 

 Slyke and Publow demonstrates the influence of acid. The re- 

 sults were obtained by adding to 250 c.c. fresh milk at 29 C. 

 1 c.c. of a rennet solution, prepared by dissolving a commercial 



