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 BUTTERMILK. 



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. (18.89° 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 



