RELATION OF MICROORGANISMS TO BUTTER 479 



when the acidity is 0.6 per cent and above; the volatile acids formed by 

 the members of the colon-aerogenes and coccus groups impart a sharp, 

 pungent taste. In milk of like acidity fermented by pure cultures of 

 Bact. lactis acidi, the acid is scarcely evident to the taste and there is no 

 sharpness, due to- the absence of volatile acids. This same difference 

 appears in the butter made from the two kinds of milk. 



The low flavor of the butter made from cream ripened by pure 

 cultures was one of the factors that prevented the rapid introduction 

 of the cultures in this country. The demands of the butter market 

 have changed and the mild flavored butter, which is now considered 

 to be the finest, can be made by the use of pure cultures in the fermenta- 

 tion of pure sweet cream. 



COMMERCIAL CULTURES. In this country the preparation and 

 distribution of cultures for the ripening of cream is largely in the 

 hands of commercial firms; hence, the term "commercial culture" 

 is applied to them. The different pure cultures are propagated in 

 the laboratory of the maker; they are sent out either as liquid cultures, 

 a small mass of milk or bouillon inoculated with the organism, or in 

 a dry form, the latter being prepared by mixing a culture of the organism 

 with an inert substance, such as milk sugar, milk powder, or starch, 

 and drying at a low temperature. In a liquid the organisms are 

 exposed to the effects of their own by-products, and the vitality of 

 the culture is rapidly lost. Such cultures must be used when fresh 

 in order to give good results, and they cannot be kept in stock by the 

 manufacturer or dealer. The resistance of Bact. lactis acidi to desic- 

 cation is great; it thus lends itself to the preparation of the dry cultures, 

 in which the organisms remain in a dormant condition and retain 

 their vitality for long periods. 



Most of the cultures now sold are pure, as this term is used in bac- 

 teriology, still others contain non-acid-forming organisns intentionally 

 added or introduced accidentally during the process of preparation. 

 If the lactic bacteria are present in such cultures in large numbers, 

 the impurities are usually of small practical significance. In the past 

 so-called "duplex" cultures have been sold which were supposed to 

 contain an acid-forming organism and a second organism that was 

 to enhance the flavor of the product. Such cultures are no longer sold. 



For the propagation in the creamery the contents of the container 

 purchased are added to a small mass of milk that has been heated 



