TESTS USED IN CHEESE-MAKING 445 



below 65° F., it is well to use for dilution water that 

 is at a temperature of about 80° F. 



(4) Filtration of casein. — After the casein is com- 

 pletely precipitated, one adds pure, soft (preferably, 

 distilled) water to the flask until the 200 cc. mark is 

 reached. The flask is then vigorously shaken 10 or 15 

 seconds, in order to distribute the acid through the 

 mixture as uniformly as possible. The contents of the 

 flask are then poured on a clean, dry filter, and the 

 filtrate caught in a cup. The funnels, filter and cups 

 used to catch filtrate should all be dry before being 

 used. It is well, generally, to allow the filtration to 

 continue until practically all of the liquid has run into 

 the cup. 



(a) Rapidity of filtration. — The usual time of filtra- 

 tion should not exceed 3 to 5 minutes. The rapidity 

 depends upon the temperature of precipitation and the 

 completeness of the separation of casein. In general, 

 the higher the temperature of the mixture when pre- 

 cipitated with acid, the more rapid should be filtration, 

 other conditions being uniform. In case of insufficient 

 acid, the filtration is slower. 



(b) Appearance of filtrate. — The filtrate should 

 be quite clear, though this is not always a sure indica- 

 tion that the right amount of acid has been used. 

 Sometimes the filtrate may be clear when not quite 

 enough acid has been used, in which case the filtration 

 is apt to be slow. In case of milk rich in fat, a slight 

 turbidity may appear, due to fat-globules in the filtrate. 

 The filtrate should be free from all marked signs of 

 turbidity or anything like milkiness. If such a filtrate 

 appears, a new sample of milk should be taken and 

 the operation repeated from the beginning, more acid 

 being used than before. With a little experience, 



