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:) also reduce the acidity. 
(For methods of determining acidity, see pages 254 
to 257.) 
