22 BULLETIN 661, U. S. DEPARTMENT OF AGRICULTURE. 
preventing separation. The precipitating temperature, under normal 
conditions, is largely controlled by the process itself and ordinarily 
is not high enough to cause material injury to the quality of the 
casein. 
ACIDITY OF BTTTEEMILK. 
The acidity of buttermilk at the time of precipitation is of im- 
portance only when the curd is not well washed, the effect of which 
is discussed under " Washing the curd.'' When high-acid buttermilk 
curd is well washed no appreciable difference in the strength of the 
casein can be distinguished. High-acid buttermilk is more liable 
to give trouble in handling and so should be avoided whenever it is 
possible. 
THE MANUFACTURE OF SKIM-MILK CASEIN. 
SULPHURIC-ACID METHOD. 
The usual method of making skim-milk casein is that of adding 
crude commercial sulphuric acid to the fresh skim milk after heating 
the latter to a temperature of 120° F. (48.89° C). The advantage 
of this method is that the skim milk can be disposed of quickly and 
at a time when the quality of the curd is in the best condition. 
The fresh skim milk is run into a vat, generally of wood because 
of its cheapness and the deteriorating effect of acid upon a metal vat. 
It is there heated with direct steam to a temperature of 120° F. 
(48.89° C) and never higher than 125° F. (51.67° C). Commercial 
sulphuric acid, of the strength used for making Babcock tests (1.83 
specific gravity), is then added at the rate of one pint to every 1,000 
pounds of milk, at the same time stirring the mixture continuously. 
Before using, each pint of acid should be diluted with about a gallon 
of water, always pouring the acid into the water and not vice versa. 
After the addition of the acid the skim milk is stirred gently until a 
good separation of the curd is obtained, as indicated by the clearness 
of the whey. This usually takes but a few minutes, provided suffi- 
cient acid has been used. In case the curd does not separate well, leav- 
ing a clear whey, more acid can be added to get the proper results. 
The quantity of acid required for a good, clear separation varies some- 
what, depending upon its strength and the condition of the milk. 
The proper quantity can be determined easily if the vat is marked 
off so as to show how much milk is on hand for each run. By vary- 
ing the quantity of acid used from day to day the proportion that 
gives the best results can be ascertained. The clear whey is run 
through the gate valve at once, and the curd remaining in the vat is 
rinsed with cold water to remove the excess of whey and the free 
acid remaining. When the curd in the vat has drained it is placed in 
