MILK PRODUCTS 333 



into small pieces and soaked in dilute brine at the ordinary 

 temperature. About a gallon of water and J Ib. of salt (5 per 

 cent, solution) are used to each pound of stomach. After 

 standing for four or five days, with occasional stirring, another 

 J Ib. of salt is added, and the liquid is strained off and filtered. 

 This forms the ordinary " extract of rennet." Commercial 

 preparations generally contain a certain amount of boracic 

 acid or other preservative, to make the rennet keep well ; but 

 they all interfere more or less with the action of the ferment. 



As prepared above, the extract of rennet contains about 

 10 per cent, of salt. The addition of further quantities causes 

 precipitation of the enzyme, but does not destroy its activity. 

 The precipitate may be strained off and dried in the air, and 

 preparations of this kind are called rennet powders. They 

 consist mainly of salt (about 96 per cent.), the other ingredients 

 being 2 per cent, of nitrogenous matter, and i per cent, each 

 of water and non-nitrogenous organic matter. 



The action of rennet, which has already been described 

 (p. 75), does not take place in alkaline solution. On the 

 contrary, a certain degree of acidity is necessary, or at least 

 highly favourable, hence the preliminary " ripening " of the milk 

 (P- 33 r )- The process must not, however, be carried too far, 

 otherwise the acid itself will cause coagulation of the casein ; 

 and, for the purposes of cheese-making, this is not considered 

 desirable. According to Fleischmann, the optimum tempera- 

 ture for the action of rennet is about 106 F. ; but in practical 

 cheese-making a somewhat lower temperature is generally 

 employed. The " strength " of the rennet is measured by the 

 time required to coagulate a certain quantity of milk of known 

 acidity and temperature. 



Ripening of Cheese. Rennet, prepared in the manner 

 described above, necessarily contains several, possibly many, 

 other ferments besides rennin. It probably contains pepsin, 

 or at least proteolytic ferments. When excessive quantities 

 are used, or if the action be unduly prolonged, a peptonising 

 action (p. 71) sets in, changes similar to those which take 

 place in the process of digestion occur, and in time the curd 

 would be again dissolved. 



