KKLA1ION OF BACTERIA TO BUTTER 95 



and the acidity is 0.6 per cent, the acidity of the liquid j>;irt 

 of the cream amounts to 0.85 per cent. In a 40 per cent 

 cream it will be found impossible to develop much over 0.7 

 per cent of acid. Such an increase in acid can only occur 

 when the bacteria have greatly increased in number. 



Exercise. Each student will make a quantitative and qualitative 

 examination of sweet cream and ripened cream. The sweet cream 

 should be that obtained from very fresh milk. The ripened cream 

 should have developed the acidity necessary for churning. Prepare 

 from each, by the loop method, lactose-litmus-agar plates and also 

 lactose-agar plates. Note which cream shows the greatest diversity 

 of species. 



Relation of butter flavor to bacterial development. Two 

 types of butter are made, one from sweet cream and the other 

 from cream which has been allowed to develop a greater or 

 less amount of acid, ripened cream. These types of butter 

 differ primarily in flavor. Small samples of cream may be 

 readily churned in Mason fruit jars. The jars should not be 

 filled more than half full, and may be shaken by hand or 

 in a shaking machine. As nearly as possible the process 

 should be the same as that employed on a large scale, 

 washing, salting, etc. 



The relation of flavor to bacterial products can be more 

 specifically shown by dividing a quantity of perfectly sweet 

 cream. One portion is to be clVurned without further treat- 

 ment ; to the other a large amount of starter should be added 

 (25-30 per cent), and the mixture churned. The resulting 

 butters should be examined as to flavor. 



Exercise. The samples of cream examined in the previous exer- 

 cise should be churned and the flavor of the butters noted. 



Quantitative analysis of butter. Butter is much more 

 difficult to sample than milk on account of difficulty in se- 

 curing a thoroughly even emulsion. Two methods are used. 



^^ 



OF THE ' 



UNIVERSITY 



