2/2 FERMENTATIONS IN MILK 



temperature. Those isolated from fresh milk often give 

 rise to lactic acid, feebly at first, but their power in this 

 respect increases very considerably when they are culti- 

 vated in milk for some time. The soured milk has a 

 pure, clean taste, occasionally with a fine distinct aroma, 

 the acid being chiefly right handed lactic acid with 

 little or no acetic or volatile acid by-products. Str. 

 lacticus, or forms of it, are the most useful of all lactic 

 bacteria for dairy purposes, and the majority of the 

 " starters " utilized in manufacture 

 of butter and cheese are more or 

 less pure cultures of this type. 



Streptococcus lacticus, Kruse (Fig. 

 46). The following morphological and 

 biological features are characteristic of 

 the type : 



Size and form. A streptococcus or 

 FIG. ^.streptococcus lacticus, short bacterium . s M to i u in length. 



Kr. ( X 800). . , ~ , , j 



Stains by Gram s method. 



Gelatine. The colonies are white and yellowish sometimes, 

 very minute, and grow only below the surface. 



Stab growth is slight, usually beaded or granular ; no surface 

 growth. 



Agar. Very small, little or none on surface. 



Potato. Little or no growth. 



bouillon. Very slight growth, with the faintest turbidity and 

 sediment. 



Milk. Acid without gas : curd dense, smooth, and firm, with- 

 out separation of whey. No peptonization of the casein. 



Included in Group i., and very nearly allied to Strepto- 

 coccus lacticus i are a number of other bacteria, differing 

 in some particular characters from the above description. 

 Streptococcus Kefir., Mig., from Kefir, and some others 



