42 THE BOOK OF CHEESE 



cubated at 70 F. They produce a smooth solid mass 

 without a sign of gas holes and without the separation 

 of whey from the curd, and develop in milk a maximum 

 acidity of about 0.90 of one per cent when titrated as 

 lactic acid. (For titration see Chapter V.) This species 

 is usually present in small numbers in fresh milk. There 

 are many varieties or strains of the species with differing 

 rates of activity and measurable differences in acid pro- 

 duced but with approximately the same qualitative 

 characters. Most commercial starters for cheese- and 

 butter-making belong to this group of species, although 

 special mixtures with other organisms are prepared 

 for special purposes. In addition to this group, most 

 varieties of cheese contain some members of the colon- 

 aerogenes group. When the milk is in proper condition, 

 the activity of this group should be held in check by the 

 early and rapid development of acid. Free development 

 of members of this group usually shows itself in the 

 presence of gas holes in the curd. 



57. Starter. The practice of using pure cultures 

 in cheese-making has brought about the development 

 of factory methods of producing day by day cultures of 

 the organisms desired, in quantities sufficient to inoculate 

 the total quantity of milk used in manufacture. For 

 this purpose milk is mostly used and the product is known 

 as " starter." For cheese-making purposes, a starter 

 is a substance used in the manufacture of dairy products 

 having a predominance of lactic acid-forming micro- 

 organisms in an active state. There are two general 

 classes of starter: (1) Natural starter; (2) commercial 

 starter. 



58. Natural starter. Milk, or other similar substance, 

 which has become sour or in which large numbers of 



