30 MILK AND DAIRY PRODUCTS PART 



of cotton wool, it is possible to compute with fair accuracy how 

 much dirt is present. Fliegel, Bernstein, and Gerber have con- 

 structed various forms of apparatus for this purpose. 



In Bernstein's apparatus (Fig. 9) the milk passes through 

 a small cotton-wool filter, and by simple observation of the 

 filter an idea of the quantity of dirt in the milk can be obtained. 



C. CHEMICAL EXAMINATION. 

 I. DETERMINATION OF THE ACIDITY OF MILK. 



A knowledge of the degree of acidity of a sample of milk is 

 of great importance in judging of its freshness or its suitability 

 for some particular purpose, such as the making of a certain 

 sort of cheese. Further, it is important to determine the 

 acidity of sour cream in order to control the process of ripening. 



The following methods are especially worth attention when a 

 determination of the degree of acidity of milk has to be made. 



1. The Reaction of the Milk to Litmus Paper. 



Normal, fresh milk has an amphoteric reaction : that is, it 

 turns blue litmus paper feebly red, and red litmus paper blue. 

 A distinctly acid reaction to litmus, therefore, indicates a milk 

 in which the normal degree of acidity has been passed, whilst 

 a predominating alkaline reaction shows that the milk is 

 abnormal, and may either have come from diseased animals or 

 have been mixed with water. 



2. The Boiling Test. 



If the milk coagulates on being heated in a test-tube, it is an 

 indication that it is already strongly acid. Normal milk 

 coagulates on boiling when it contains about 0*26% lactic acid ; 

 this is equivalent, according to Soxhlet-Henkel (see below), to 

 12 c.c. N/4 NaOH per 100 c.c. of milk. 



3. The Alcohol Test. 



Equal quantities of milk and alcohol (60 per cent, by weight 

 or 68 per cent, by volume) are mixed in a small test-tube. 



