130 DAIRY BACTERIOLOGY 



will be required for complete coagulation. The higher the tem- 

 perature, the easier the coagulation. Thus, at 18, 30, 40 and 

 100 C., 0-6, 0-5, 0-25 and 0-1 per cent, respectively of lactic acid 

 will generally be required, corresponding to 80, 72-5, 53-5 and 

 27-5 c.c. of decinormal caustic soda per 100 c.c. respectively. 

 For this reason, milk is often warmed when the casein is to be 

 precipitated. If a temperature of 70 C. is exceeded, appreciable 

 amounts of albumin are also precipitated ; larger yields of cheese 

 are therefore obtained by using milk pasteurised at high tem- 

 peratures. On heating to over 90 C., all the albumin and globulin 

 are precipitated ; the more completely this takes place the more 

 easily will the milk coagulate, for all solid particles, including fat 

 globules, serve to stiffen the coagulum. The same milk will 

 therefore coagulate at a lower acidity .when pasteurised at a high 

 temperature than when pasteurised at a low temperature. Acid 

 coagulum differs from rennet coagalum in not contracting so much, 

 so that it does not separate such large quantities of whey when 

 allowed to stand undisturbed. This affords a means of distin- 

 guishing between the two types of coagulation. ,In the case of 

 a bacterium which coagulates milk, a titration will at once decide 

 whether a quantity of acid sufficient to be entirely responsible for 

 the coagulation has been produced ; if not, then the bacterium 

 must also have produced a coagulating enzyme like rennet. The 

 coagulating power of rennet is increased by the addition of acid ; 

 the reason for this is not only that the acid promotes the action of 

 the enzyme, but also that it forms soluble calcium salts which 

 facilitate the precipitation of the paracasein. The coagulation of 

 pasteurised milk by rennet may thus be promoted by the addition 

 of a fair amount of acid, e.g., in the form of buttermilk 1 , and also 

 by the addition of calcium chloride (100 c.c. of a 40 to 50 per cent, 

 solution to 100 litres of milk). The contraction of the rennet 

 coagulum is also promoted to a certain degree by the addition of 

 acid, the whey separating most readily at a concentration of 

 about J- per cent, of lactic acid, when all the casein is converted 

 into the mono-calcium salt ; for this reason, the cheese dries 

 better if prepared from slightly acid milk than if prepared from 

 fresh milk. If the milk is so sour that there is a suspicion that 

 the resulting cheese will be too dry, all that is necessary is to 

 dilute it with water before adding the rennet ; in this way, the 

 concentration of both the free, acid and the soluble calcium salts 

 will be decreased. Conversely if a higher acidity is desired, the 



1 In the making of Cheddar cheese from pasteurised milk, Sammis and 

 Bruhn (Bureau of Anirn. Ind., 1913, Bull. 165) recommend besides the 

 addition of starter milk containing f per cent, of acid the use of 1 part of 

 normal hydrochloric acid per 100 parts of milk. 



