Acidity in Milk. 
65 
phosphates, for free carbonic acid has very slight, if any, effect 
on the colour of alizarin, and the alkali therefore produces its 
full effect directly this stage has been reached. 
To take an illustration which will show the application of the 
test in its most marked form : A sample of whey which has been 
bottled and allowed to ferment under pressure, may, when un- 
corked, and when active effervescence has ceased, show, with 
phenolphthallein as an iridicator, as much as 80 degrees of acidity. 
But, with alizarin, the acidity registered would be only from 30 to 
40 degrees ; thus demonstrating that the volatile acidity, due to 
the presence of carbonic acid, may under such conditions 
amount to nearly as much as the fixed acidity, caused by lactic 
acid and the acid phosphates. Indeed 40 degrees is the highest 
amount of acidity in whey due to fixed acid which the writer 
has ever observed ; when that amount of acidity is reached the 
further generation of lactic acid is arrested, unless the free acid 
be neutralised by an alkali or an alkaline earth, as is done in 
the process for manufacturing the acid for commercial purposes. 
The acidity of perfectly fresh milk may be as low as zero. 
Indeed in cases of local or constitutional disease the reaction of 
the milk may be distinctly alkaline. Such milk is absolutely 
unfit for cheese-making, as it rapidly undergoes the butyric fer- 
mentation, amongst the products of which are hydrogen and 
carbonic acid ; and this leads to the permeation of the cheese 
by these gases, with the results, so well known to cheese-makers, 
of " heaving," " puffing," &c. Hence the great importance to 
the cheese-maker of being able to estimate the acidity of the 
milk which he employs, for if he finds it deficient in acidity he 
has the indication to push on the development of acidity, which 
he may do in several ways. 
The amount of acidity which milk should possess for cheese- 
making varies from three to seven degrees, according to the 
time of year. In the spring, when the cows are feeding princi- 
pally on the abundant herbage of early pastures, the casein of 
the milk will be deficient in tenacity, and a higher degi'ee of 
acidity will be desirable in the milk than in the summer and 
autumn, when, from the grasses being seeding and the pasture 
comparatively dry, the tenacity of the casein will be much 
higher. Without the means of accurately measuring the acidity 
of milk in this respect, the work of the cheese-maker is mere 
blind empiricism ; but with the aid of the acidometer he has the 
whole process under complete control. 
A great deal of loose and inaccurate talk is often to be heard 
amongst so-called "practical" cheese-makers about the import- 
ance of getting the curd out of the whey before acidity is deve- 
VOL. H, T. 8.— 5 F 
