202 AGRICULTURAL AND INDUSTRIAL BACTERIOLOGY 



the desirable microorganisms and their growth products 

 usually overwhelms undesirable types of bacteria and leads 

 to the development of a satisfactory product. 



Recent studies have made it evident that the changes 

 brought about in a satisfactory starter are not the results 

 simply of the growth of Streptococcus lacticus. Another 

 organism closely related to it must ordinarily grow with it. 

 Cream which has been ripened by means of a pure culture 

 of the ordinary Streptococcus lacticus does not produce but- 

 ter with as satisfactory a flavor and aroma as that produced 

 by cream which has been inoculated with a mixture of these 

 organisms. The ripening produced by a starter is, there- 

 fore, an excellent example of associative action. The true 

 Streptococcues lacticus apparently produces very little 

 change in milk or in cream other than the development of 

 the pure lactic acid from lactose. The associative organism, 

 also a coccus (either Streptococcus citrovorus or Strepto- 

 coccus paracitrovorus) can by its own growth bring about 

 very little change in milk. When grown in the presence of 

 Streptococcus lacticus, however, the activity of this organ- 

 ism is greatly stimulated and small amounts of volatile 

 acids are developed. It is to the absorption of these and 

 perhaps of other growth products that the butter owes its 

 characteristic aroma and flavor. 



EFFECTS OF PASTEURIZATION UPON THE SOURING OF MILK 



Some of the earlier opponents of pasteurized milk urged 

 that the process of pasteurization, if properly performed in 

 a manner to eliminate all disease-producing bacteria in the 

 milk, would likewise kill all of the lactic acid bacteria, and 

 that the only organisms remaining alive would be spore- 

 producing bacilli and clostridia. These would then begin 

 to grow, after the milk had cooled, and bring about putre- 

 factive changes. However, the work of Ayers and his 

 associates in the Dairy Division of the Bureau of Animal 



