20 THE STORY OF MILK 



There are other tests used in scientific dairying as the 

 Fermentation Test to ascertain the relative purity of 

 milk, the Casein Test, etc., but the above are those 

 mostly used besides the Bacterial Count which is men- 

 tioned under the chapter on ''Milk Supply," and the 

 Rennet Test described under "Cheese Making." 



FERMENTS 



Two classes of ferments are of importance in con- 

 nection with milk: (1) ''unorganized" or chemical fer- 

 ments, the ''enzymes," and (2) "organized" ferments 

 such as bacteria and yeast. 



Enzymes 



Rennet. — Among the unorganized ferments. Rennet 

 or Rennin is highly important on account of its power 

 of coagulating or curdling milk by precipitation of the 

 casein. Rennet is extracted from the stomach of the 

 suckling or milk-fed calf, where it serves in digesting 

 the calf's food. It is in the market in the form of a 

 liquid extract as well as a dry powder compressed into 



pink. The color will quickly disappear, however, and a few more 

 drops of the alkali are added and stirred in several times until a faint 

 but distinct pink color remains for some time. That indicates that the 

 acid in the milk has been neutralized and the amount of the soda solu- 

 tion consumed is then read off on the scale on the burette. By dividing 

 the number of c.c. of the soda solution used by two, the tenths per cent 

 of lactic acid in the milk is found. For example, if it takes 4 c.c. of the 

 soda solution to neutralize 17.6 c.c. milk, the acidity is .2%. This de- 

 pends upon the fact that 1 c. c. of a tenth normal soda neutralizes .009 

 gram of lactic acid and that therefore the per cent of acid in the milk 

 is equal to .009 multiplied by the number of c.c. of soda solution used, 

 divided by the number of c.c. of milk and multiplied by 100. 



If 50 c.c. of milk is taken instead of 17.6 the calculation is changed 

 accordingly. 



