MILK 27 



cities the legal limit is 0.2 per cent. A sour taste is not 

 present until the acidity exceeds 0.3 per cent. 



The acidity of colostrum is about three times as great 

 as that of milk. As the colostrum changes to milk, the 

 acidity gradually decreases until it reaches the point 

 normal for milk. Toward the end of lactation the acidity 

 is further decreased, and in "strippers" is very low, or 

 the reaction may even be alkaline. 



The reaction of the milk of individual cows is usually 

 below normal in acidity, or may even be alkaline, in ordi- 

 nary inflammations of the udder, tuberculosis of the 

 udder, and probably also when the udder is eliminating 

 abnormal substances, as in cowpox; but this is by no 

 means always the case. In streptococcic mastitis the 

 milk may be more acid than normal. The reaction alone 

 of the milk of individual cows cannot therefore be relied 

 upon to discover diseased conditions. It is hardly neces- 

 sary to mention that nothing can be learned on this point 

 from determining the reaction of market milk. 



High acidity in market milk is usually the result of 

 excessive fermentation of the lactose, and is an indication 

 that the milk is stale, or was produced under unclean 

 conditions, or was not properly cooled and cared for. 

 The addition of boric acid or formaldehyde also increases 

 the acidity. Salicylic acid has less effect on the reac- 

 tion because it is usually not added in very great quan- 

 tity. The acidity may be reduced by the addition of alka- 

 lies (bicarbonate of soda, chalk, potash) , but the addition 

 of these substances to milk is illegal. Addition of water 

 and heating (loss of CO 2 ) also reduce the acidity. 



(For methods of determining acidity, see pages 254 

 to 257.) 



