64 
Acidity in Milk. 
purple colour has been produced. The number of drops required 
for this purpose gives the number of degrees of acidity which 
the milk possesses. 
It is only possible here to add a few words in illustration of 
the information which the test as thus made is calculated to give, 
and of the conditions under which acidity is developed in milk 
and its products. 
If a sample of milk that has stood for a few hours in a warm 
place be examined by the test above described, it will be found 
to exhibit an acidity which may vary from 4 to 6 or 7 degrees. 
This acidity will be due to two distinct bodies : to lactic acid 
and acid salts (chiefly acid phosphates), and to carbonic acid. 
The former of these compounds, which may be called the fixed 
acidity, is due to the fermentation of the sugar present in the 
milk, and to its conversion into lactic acid. This acid in its 
turn acts upon the basic salts of the milk (alkaline and earthy 
phosphates), and converts them into acid salts. The carbonic 
acid, which may be called the volatile ingredient of the acidity, is 
produced by a further breaking up of the lactic acid. The two 
together make up the total acidity. 
It is this total acidity which is determined by the test which 
has been above described, and it is very questionable whether any 
advantage is to be obtained by a separate determination of its 
two component parts, inasmuch as the principal effect which the 
development of acidity has on the milk, viz. that of rendering the 
casein insoluble, is produced by carbonic acid as well as by lactic 
acid, though, as might be supposed, from the difference in the 
relative intensities of these two acids, witli diffei'ent degrees of 
energy. The relative feebleness of free carbonic acid makes it 
much more difficult to determine with precision the amount of 
it which may be present in anj^ given liquid ; but a fair approxi- 
mation to it may be made by usiug an alcoholic solutioA of 
alizarin as an indicator instead of a solution of plienolphtliallein, 
as above described. The latter of these two reagents is affected 
by free carbonic acid, as it is by all other free acids or acid salts 
of any energy ; the purple colour of an alkalised solution being 
equally discharged. But the action of alizarin is different. If 
a drop or two of this reagent be added to a solution such as 
whey, the acidity of which is due to the presence of free carbonic 
and lactic acids as well as acid salts, a bright yellow colour is 
communicated to the solution. If now the acidity is gradually 
neutralised by an alkali a point will be reached when the yellow 
tint passes through an intermediate brownish tint into purple. 
This transition indicates the stage of neutralisation of that 
portion of the acidity which is caused by the lactic acid and acid 
