584 ANNUAL REPORT OF THE Off. Doc. 



acid bacteria. VVtieu butter is made in the old way, tlie cream is 

 allowed to stand until it ripens spontaneously, no attempt being 

 made to control the process. As the result of the fermentations 

 that occur in the ripening process, many complex changes take place 

 in cream, the details of which are not fully understood. The most 

 obvious results brought about by cream-ripening may be included 

 under three general heads; (1) The formation of lactic acid, (2) the 

 development of products that have charactertistic odors, and (3) the 

 formation of substances that yield a characteristic taste on the 

 tongue. To what extent these effects are produced by si)ecial organ- 

 isms, we have little detailed knowledge, aside from the work of lactic 

 acid bacteria in producing lactic acid. The real sources of the flavors 

 of ripened cream we do not know specifically, nor do we know what 

 these specific compounds are that give rise to the flavors. The 

 amount of lactic acid formed is used as a measure of the extent 

 or degree of cream-ripening, but other forms of fermentation are 

 known to be present at the same time, at least during the early 

 portion of the ripening process. Butter may be made from cream 

 that has not been ripened at all, that is, from sweet cream, or it 

 may be made with the help of artificial acid added to cream, but in 

 neither case do we make a product that is in flavor like butter made 

 from ripened cream. 



27. How Lactic Acid is Produced in Cream Ripening. 



In order to have the lactic acid fermentation of cream ripening, 

 lactic acid bacteria must be present in the cream and the cream 

 must be kept at a temperature favorable to their growth. We may 

 leave the cream to receive the bacteria by chance, or we may intro- 

 duce them into the cream purposely. In the old-style method of 

 butter-making, the former method is employed. We have already 

 seen (section 12, p. 567), that milk nearly always contains lactic acid 

 bacteria. During the operation of creaming, these bacteria usually 

 develop to sucli an extent that, when the cream is exposed to higher 

 temperatures, fermentation proceeds rapidly. However, under such 

 circumstances, the rate of fermentation is not uniform in different 

 lots of cream; at one time more lactic acid is formed, and at another 

 less, during a given time, because the number of bacteria will in- 

 evitably vary greath^ when their introduction is left to chance. 



The formation of lactic acid can be controlled in respect to time 

 and quantity, when we introduce the lactic acid bacteria purposely 

 in sufficient quantities. Material, containing large numbers of lactic 

 acid organisms, which is used to add to milk or cream for the purpose 

 of causing lactic acid fermentation, is known as a '^starter." There 

 are two varieties or sources of starters, (1) natural, and (2) pure 

 cultures. 



