8 DEPARTMENT BULLETIN 319, U. S. DEPT. OF AGRICULTURE 
milk which has been steamed for an hour or more to reduce the bac- 
teria 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 fermentation has become suffi- 
ciently active to eliminate the contaminating bacteria and the milk 
curdles with a clean, acid taste and without signs of gas or the sepa- 
ration of a watery whey. This small starter, called " mother starter," 
is carried along indefinitely by daily transfers to freshly steamed 
milk. If reasonable precautions are taken to prevent contamination 
after a thorough heating of the milk, this culture will remain 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 is cooled to about 24° 
to 27° C. (75.2° to 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 precipitates the casein as a firm, jellylike mass. When this 
occurs in cream the fat globules are entangled in the precipitated 
casein. In the process of churning the casein is broken into fine par- 
ticles and the fat globules are collected into large granules that float 
on top of the buttermilk. Buttermilk, then, is the water of the milk 
holding the sugar, acids, ash, and other soluble constituents in solu- 
tion and the finely divided particles of precipitated casein in suspen- 
sion. The amount of fat in the buttermilk depends on the complete- 
ness with which the fat is removed in the churning. Even with the 
best methods a little of the fat in the form of very small globules 
remains in the buttermilk. On standing, the suspended casein settles 
slowly to the bottom. 
The composition of an average buttermilk is about as follows (20, 
p. 119) : 
Per cent 
Fat 0.5 
Casein 2.4 
Albumin • 6 
Lactose 5. 3 
Ash . 7 
Total solids 9.5 
Chemically, buttermilk differs but little from skim milk. Only a 
slight rearrangement is necessary to bring about the physical change 
in the casein. If the milk has been pasteurized at a high tempera- 
