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, 
jellylike mass. When this occurs in cream the fat globules are en- 
