RELATION OF BACTERIA TO BUTTER 97 



of the fresh butter aud the same butter some weeks later 

 should be determined. 



Exercise. Each student will examine quantitatively a sample of 

 fresh creamery butter. The portion sampled is to be placed in the 

 refrigerator and reexamined three or four weeks later. 



Starters. With increasing knowledge of bacteria and of 

 the various kinds finding favorable conditions for growth in 

 milk and cream, it became evident that the flavor of the 

 butter depended largely upon the types of bacteria present. 

 Tn order to emphasize the effect of specific forms on the flavor 

 of the product, it is necessary to insure their predominance in 

 the cream by the addition of pure cultures. The three types 

 most commonly present in cream are: (1) lactic-acid-forming 

 organisms of the desirable type (B. lactis acidi Leichmann) ; 

 (2) acid-producing organisms of less desirable types (B. coli 

 communis, B. lactis aerogenes) ; (3) organisms liquefying casein 

 (B. fluorescens liquefaciens, etc.). 



Pasteurized or perfectly sweet fresh cream should be in- 

 oculated with pure cultures of the above types of bacteria, 

 both alone and in mixtures. The growth should be allowed 

 to continue for some time and the flavor of the ripened cream 

 noted. The cream should be churned and the flavor of the 

 butter determined. 



These trials may be carried out under more practical con- 

 ditions by preparing the various starters in the laboratory, 

 and the remaining portion of the work should then be done 

 under creamery conditions. 



Commercial starters. With the determination of the im- 

 portance of the types of bacteria in influencing the flavor of 

 the butter, it became evident that selected forms should be 

 grown in masses and added to the cream to insure their 

 predominance. The introduction of the " pure culture " into 



