1910.] The Use of Starters in Dairying. 



717 



should be cooled down to 50 0 Fahr. or even lower. Close 

 attention to temperature during the ripening process is 

 imperative, if a superior article is to be obtained. 



Starters. — A starter is really a culture of bacteria which 

 change the sugar of milk (lactose) into lactic acid. These 

 bacteria occur in abundance in sour milk, butter-milk, sour 

 whey, cheese, etc., and the first three of these may be used for 

 this purpose, provided they have a clean sharp flavour and 

 are not tainted in any way with other injurious bacteria. 

 Commercial starters can be obtained in the form of a liquid 

 or powder, but the liquid starter is the one chiefly used in 

 this country. 



These commercial starters are pure cultures of lactic acid 

 bacteria, and can be made in the following way : — Take three 

 test tubes containing nutrient gelatine, and maintain at a 

 temperature of ioo° Fahr. to keep the gelatine liquid. Make 

 a loop on the end of a platinum wire, sterilise it, and place 

 a loopful of sour milk, which has a clean, sharp, acid flavour, 

 in No. 1 tube and stir after adding. Transfer a loopful of 

 inoculated gelatine from No. 1 to No. 2, stir again and do 

 likewise from No. 2 to No. 3. The contents of Nos. 2 and 3 

 are poured into sterilised plates with covers on, and spread 

 evenly over the bottom. This is called "laying the plates" 

 or practically " seeding" them. Each, or at least several, of 

 the species of bacteria flourish on the nutrient gelatine, and by 

 multiplication form colonies after their own particular kind, 

 provided they are incubated at a suitable temperature (70 0 

 Fahr.). After examining these colonies under a microscope 

 to see if the desired form of bacterium is present, one of these 

 colonies is selected and cultivated further on gelatine, agar- 

 agar, and in sterile milk, to make sure that none other than 

 the lactic acid forming bacteria are present. The required 

 organism having been isolated, it is propagated from day to 

 day in a suitable medium like separated milk, which has pre- 

 viously been pasteurised, and in this way a pure culture 

 is obtained. This, so to speak, concentrated culture of the 

 lactic acid bacteria is what we get when a " starter" is bought. 

 1 It is generally found best to prepare sub-cultures from com- 

 i mercial starters to get rid of any flavour of the medium on 

 which the bacteria have been cultivated, and this is best done 



1 



