480 MICROBIOLOGY OF MILK AND MILK PRODUCTS 



to destroy all non-spore-forming bacteria and other microorganisms; 

 the milk, after being inoculated, is incubated at favorable tempera- 

 tures and when curdled can be used for the inoculation of the second 

 and larger quantity. The process of inoculating a quantity of milk 

 is carried out daily. It is impossible for the butter-maker to propagate 

 the culture in such a way as to maintain its original purity, but if the 

 milk is heated sufficiently, if all utensils are sterilized, and if the culture 

 is kept at a temperature that is especially favorable for the organism, 

 the contamination that may occur will not injure the culture for prac- 

 tical work. The cultures propagated under such conditions gradually 

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

 The contamination that is of the greatest 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 

 exhaustive churning and to produce the desired degree of flavor in 

 the product. Cultures frequently become slimy or ropy on propaga- 

 tion. 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 propa- 

 gated 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 or milk powder have 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 incorporated with the cream. 

 The temperature of incubation and the amount of initial inoculation 

 determine the rapidity with which the acid fermentation will progress, 



