COMPOSITION AND CHARACTERISTICS 31 



ticular organism. In the bacteriological laboratory, 

 where alone absolute steriUty of utensils and medium, 

 and entire exclusion of foreign infection are possible, 

 the culture may remain pure while this inoculation and 

 propagation are repeated over and over again. But 

 when the propagation is carried on in the house or the 

 dairy, for instance in preparing starters or buttermilk. 



Streptococcus lacticus (Storch, No. 18) Bacterium lactis acidi (from Storch) 



such absolute cleanliness is impossible and in the long 

 run infections will creep in from the air or from the 

 utensils and after a while it becomes necessary to 

 start with a new '^pure culture." How often such 

 renewal must be resorted to depends largely upon the 

 surroundings and the care of the operator. Usually it 

 must be done after a week's time, although it is sur- 

 prising to find milk preparations made by the simplest 

 processes equal in purity to those prepared with the 

 assistance of bacteriological science and technique. 

 This is, for instance, the case in Bulgaria, where the 

 famous Yoghourt sour milk is prepared pure without 



