CHAP, xii CHEMICAL TESTS OF MILK 203 



culture of lactic acid bacilli is used in a sterilised milk is lactic 

 acid the only acid produced. 



The acidity of milk is generally recorded as so many 

 degrees of acidity. The degree of acidity of a milk sample is the 



N 

 number of c.c. of alkali required to render 100 c.c. of 



milk neutral to phenol -phthalein. The acidity of fresh milk 

 varies considerably. With over 50 samples of quite fresh 

 milk collected at the byre, the writer found the acidity to 

 vary between 11 and 23'5 degrees of acidity. 



Since lactic acid is produced by bacteria, and old stale 

 milk has often a higher acidity than fresh byre milk, the 

 determination of the acidity has been suggested as a quick and 

 easy test by which to judge if milk samples contain an exces- 

 sive number of bacteria. Thoerner, for example, has suggested 

 as a practical limit for wholesome milk an acidity equal to 



N 



1- of the volume of the milk in c.c. of alkali. This is the 



10 



same as saying 20 degrees of acidity, equal to an acidity of 

 0*18 per cent of lactic acid. As recorded above, the writer has 

 found this exceeded by perfectly fresh byre milk samples. 

 Newman 1 recommended the determination of acidity as a 

 valuable and reliable test of the staleness or otherwise of a 

 milk. He advocated a standard of not more than 24 to 25 

 degrees of total acidity. The Committee of the American 

 Association of Medical Milk Commissions dealing with Chemi- 

 cal Standards recommended that the maximum acidity of 

 milk allowed should be 0*18 per cent calculated as lactic 

 acid. 



The value of the estimation turns upon whether there is 

 any relationship between the number of bacteria and the 

 degree of acidity. The writer investigated this question care- 

 fully, 2 and found no relationship between the acidity and either 

 the number of bacteria or the number of B. coli. For example, 

 when the three highest and the three lowest acidity results 

 with fresh byre milk were taken, the results obtained were 

 as follows : 



1 Public Health, 1905, xviii. p. 157. 

 2 Report of Medical Officer, Local Government Board, 1909-10, p. 474. 



