340 MICROBIOLOGY OF MILK AND MILK PRODUCTS. 



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 fermentation of pure sweet 

 cream. 



COMMERCIAL CULTURES. In this country the preparation and dis- 

 tribution 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 resist- 

 ance of Bact. lactis acidi to desiccation is great; it thus lends itself to the 

 preparation of the dry cultures, in which the organisms remain in a dor- 

 mant 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 organisms, 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 to 

 destroy all non-spore-forming bacteria and other microorganisms; the 

 milk, after being inoculated, is incubated at favorable temperatures and 

 when curdled can be used for the inoculation of a second and larger 

 quantity. The process of inoculating a quantity of milk is carried out 

 daily. It is impossible for the butter maker to propagate the culture so 



