THE RELATION OF MICROORGANISMS TO BUTTER. 341 



as to maintain the original purity, but with care in the heating of the milk 

 the sterilization of all utensils and the maintaining of proper temperatures, 

 the contamination that occurs will not injure the culture for practical 

 work. The cultures propagated under such conditions gradually deteri- 

 orate and recourse must be had sooner or later to a fresh culture. The 

 contamination that is of the greatest practical significance is undoubtedly 

 that with other acid-forming bacteria rather than with the forms that 

 remain in the milk after heating. 



Many of the cultures gradually lose their fermentative properties, 

 and do not form acid rapidly and in sufficient amounts to insure ex- 

 haustive churning and to produce the desired degree of flavor in the product. 

 Cultures frequently become slimy or ropy on propagation. This is not 

 necessarily due to contamination with specific slime-forming organisms 

 but rather to a change in the lactic organism itself. Such an abnormality 

 usually persists for only a short period and the conditions that govern its 

 appearance and disappearance are not known. It is asserted by practical 

 butter makers that the development of too high an acidity in the cultures 

 as they are propagated in the creameries permanently impairs the value 

 of the culture. 



The cultures 'are propagated in skim milk. Where this is not avail- 

 able, unsweetened, condensed milk has been employed. Efforts have 

 been made to grow the bacteria in some other kind of medium than milk, 

 but without success. The starter is said to be ripe or in the best condition 

 for use soon after curdling, or when the acidity is 0.5 to 0.7 per cent, as at 

 this time it contains the maximum number of living cells. The practical 

 man thus uses the curdling as an indication of the ripeness of the starter. 

 The curdled milk should show no free whey, and the curd should be 

 easily broken up to form a creamy mass that can be uniformly incor- 

 porated with the cream. The temperature of incubation and the amount 

 of initial inoculation determine the rapidity with which the acid fermen- 

 tation will progress, the maker seeking to regulate these so that the culture 

 shall be ripe at the desired time each day. 



USE OF PURE CULTURES IN RAW CREAM. The cream as it reaches 

 the creamery contains a greater or less number of acid-forming bacteria 

 that ultimately will cause it to ripen and the flavor of the butter will be 

 due to the by-products of the mixture of bacteria. If, through the addi- 

 tion of a pure culture, the relative number of organisms that are known 

 to be favorable is greatly increased, the flavor of the product should be 



