10 BULLETIN 319, U. S. DEPARTMENT OF AGRICULTURE. 



In many creameries the cream is allowed to sour spontaneously. 

 In this case many bacteria other than the true lactic bacteria will 

 take part in the acid formation, and in addition to lactic acid the but- 

 termilk may contain in small quantities acetic, succinic, and formic 

 acids, and sometimes traces of alcohol. The lactic bacteria form 

 lactic acid, with only slight traces of other organic acids, no alcohol, 

 and no gas. In well-managed creameries the acid fermentation is as- 

 sisted and controlled to some extent by the use of a starter. This may 

 be milk allowed to sour spontaneously, or buttermilk from the previous 

 day's churning, but careful buttermakers build up starters from com- 

 mercial cultures sold in the form of powders, tablets, or fluid cultures, 

 as varieties of lactic-acid bacteria selected with special reference to 

 the production of a desirable flavor in butter. The buttermaker puts 

 this culture into about a quart of milk which has been steamed for an 

 hour or more to reduce the bacteria to the lowest possible number. 

 After standing overnight the milk will usually be curdled, but gas 

 bubbles and other evidences of contamination may be observed. A 

 small portion of this milk is transferred to another bottle of milk 

 prepared as before, and this process is continued until the acid fer- 

 mentation has become sufficiently active to eliminate the contaminat- 

 ing bacteria, and the milk curdles with a clean, acid taste and without 

 signs of gas or "wheying off." This small starter, or "mother 

 starter," is carried along indefinitely by daily transfers to freshly 

 steamed milk. If reasonable precautions are taken to prevent con- 

 tamination after a thorough heating of the milk, this culture will re- 

 main pure and vigorous for an indefinite time. 



To prepare the starter actually used in ripening the cream, a larger 

 lot of milk — 25 to 50 gallons or more, according to the amount of 

 cream — is heated for an hour or more. This is usually done in a 

 special apparatus (sold by creamery supply houses) which consists 

 of a large can inclosed on the sides and bottom by a steam jacket and 

 fitted with a belt-driven stirrer. Milk either skimmed or unskimmed 

 is heated by turning steam into the jacket; during the heating the 

 milk is stirred constantly. After the pasteurization is completed 

 cold water is run into the jacket and the milk cooled to about 

 24-27° C. (75.2-80.6° F.). A bottle of the mother starter is added 

 and the can is covered and allowed to stand overnight. This gives a 

 large and active pure culture of lactic-acid bacteria to start the acid 

 formation in the cream. Better results are obtained if the cream is 

 first pasteurized. 



When lactic-acid bacteria grow in milk the lactose is converted into 

 lactic acid with slight traces of certain other organic acids. This acid 

 breaks up the combination of calcium phosphate and casein which 

 holds the casein in solution, and the casein is precipitated as a firm, 

 jelly like mass. When this occurs in cream the fat globules are en- 



