478 MICROBIOLOGY OF MILK AND MILK PRODUCTS 



to accelerate the souring of cream antedates by many hundred years the 

 science of bacteriology. 



The next logical step in the development of the process was the use of 

 the same types of bacteria from day to day. Cultures of these were ob- 

 tained by allowing a quantity of milk to sour, and if it had the desired 

 flavor, a small amount of it was added to another quantity of milk that 

 had been heated, in order to destroy the acid-forming bacteria it con- 

 tained. By the daily preparation of some heated milk, and the inocula- 

 tion of it with the soured milk previously prepared, the butter-maker 

 could use the same types of bacteria for an indefinite time for addition 

 to the cream. 



It had been found by Hansen that, in order to control the flavor of 

 beer, pure cultures of yeasts must be used for the fermentation of the 

 wort. The success of this method in the brewing industry led to the 

 introduction of pure lactic cultures for the fermentation of cream. The 

 use of such cultures was suggested independently by Storch, a Danish 

 bacteriologist and by Weigmann, the director of the dairy experiment 

 station at Kiel in Germany, in 1890. Many cultures were isolated and 

 tested as to their effect on the flavor of butter. Those found to be 

 desirable could be maintained in the laboratory, and could be furnished 

 to butter-makers to be used and propagated in a manner similar to the 

 method employed with the impure and less constant home-made starters. 

 The pure cultures of lactic bacteria are widely used at present in the 

 butter-producing countries of the world and their use is being constantly 

 extended, as butter makers come to recognize the importance of con- 

 trolling the ripening of cream. 



It was found that the butter made from cream ripened by pure lactic 

 cultures did not possess as high a flavor as did the finest butter made 

 from naturally ripened cream. This led to the search for organisms 

 that could be used alone, or together, with the lactic bacteria, and which 

 should give the high flavor desired. Such cultures were found, but 

 their use did not prove practical, either because they did not maintain 

 their properties on continued cultivation, or because of their effect on 

 the keeping quality of the product. The difference in flavor in the case 

 of butter made from naturally ripened cream and that from cream rip- 

 ened by pure lactic cultures is undoubtedly due to the products of the 

 B. coli-aero genes group. 



The acid in spontaneously soured milk is very evident to the taste 



