STARTERS 253 



culture of lactic acid bacteria is used. The application of pure 

 cultures of lactic acid bacteria for the ripening of cream is the 

 result of extensive research by the eminent Danish investigator, 

 Dr. Storch who demonstrated its value as early as 1890, 1 and 

 later by Dr. Weigrnann of Kiel, Germany. 



While these commercial lactic acid cultures are frequently 

 spoken of as pure cultures, they are generally not pure cultures ; 

 they consist more often of mixed cultures of several different 

 species of desirable lactic acid bacteria and particularly of 

 Streptococcus lacticus and Bacterium lactis acidi, occasionally 

 they also contain species of yeast. 



Some of the commercial lactic acid cultures are put on 

 the market in liquid form, usually in a medium of nutrient 

 bouillon (beef broth), whey bouillon, or sterilized milk. Others 

 are prepared in the form of a dry powder which consists of 

 liquid cultures to which has been added some powder, such 

 as ground milk sugar, starch, chalk, etc., in sufficient quan- 

 tities to absorb the excess moisture. 



The liquid cultures have the advantage of greater purity, 

 but must be used within a few days of their preparation. Old 

 liquid cultures have usually lost their virulence and are 

 worthless. 



The dry or powdered cultures are commercially more prac- 

 tical, inasmuch as the bacteria they contain, retain their viru- 

 lence for a reasonable period of time. They are therefore 

 adapted to transportation to distant destinations. Their keep- 

 ing quality is by no means unlimited, however, they cannot 

 be held over from one season to another, but should be used 

 within a few weeks of their preparation. They are somewhat 

 slower in regaining their activity, than the liquid cultures and 

 require several propagations before they can be depended on 

 to produce a usable starter. One of the most serious disadvan- 

 tages of the dry starter cultures lies in the fact that they are 

 seldom really pure, and quite often they are seriously con- 

 taminated with other and frequently undesirable micro-organ- 

 isms. The contamination is due, in part, to the bacterial im- 

 purities in their absorptive medium and partly to the often 



1 Eighteenth Annual Report, Danish Experiment Station, 1890. 



