CHAPTEE XLVIII. 



BACTERIA IN BUTTER AND CHEESE-MAKING. 



THE most important products prepared from milk, which is not 

 consumed as such, are butter and cheese. The former may be 

 obtained from sweet milk, sour milk, sweet cream, or sour cream. 

 Cream may be permitted to separate spontaneously from milk, but 

 this is rarely done today, when most cream is obtained by centrifuging 

 the milk in a centrifuging apparatus or cream separator. This has 

 the advantage that the separation occurs more rapidly and is more 

 complete; in fact, apparatuses have been constructed which will 

 leave in the skimmed milk only 0.06 to 0.12 per cent, butter fat and 

 will remove from the whole milk with the cream over 96 per cent, of 

 the butter fat. Butter is usually prepared from sour cream. The 

 souring of the cream may have been permitted to occur spontaneously, 

 generally within thirty to thirty-six hours, by the action of lactic acid 

 bacteria, or it may have been brought about by the addition of a 

 so-called "starter," or "Saurewecker" (German). This starter con- 

 sists of whole milk or skimmed milk which has become sour spon- 

 taneously or a culture of lactic-acid bacteria. The employment of 

 the artificially prepared starters owes its origin to the investigations of 

 Storch, Weigmann, and Conn. Storch, in Denmark, and Weigmann, 

 in Germany, have shown that the souring of the cream was due to 

 lactic-acid bacteria, and Leichmann had demonstrated that it was par- 

 ticularly due to the Streptococcus lacticus. Cultures of this organism, 

 also containing some other bacteria and yeast cells which have the 

 power to produce an agreeable aroma in the butter, have been used 

 more and more during the last twenty years in souring the cream for 

 the preparation of butter. It was found, however, that the aroma 



I microorganisms must be used very carefully, since some of them, 

 particularly under certain conditions, have a tendency to split some 

 af the butter fat, forming butyric acid and making the butter rancid, 

 [t has also become a practice frequently to pasteurize the milk or 

 cream which is subsequently soured by the starter. Aside from 

 hygienic considerations the advantage of this is that the souring is 

 produced by definite organisms and not by a variety contained 

 originally in the milk, which might impart undesirable flavors or 

 other objectionable properties to the butter. The sour cream is 

 subsequently churned in apparatuses of various types, in which it is 

 subjected to violent agitation and in consequence the globules of 

 butter fat become confluent and most of the watery part of the cream 

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