There are two fundamentally different methods of expressing acidity:
(a) titratable acidity expressed as percent lactic acid, and (b)
hydrogen ion concentration or pH. The former measures the total
acidity but does not measure the strength of the acids. The pH indicates
the strength of the acid condition.
The true neutral point is at pH 7.0; pH values below 7.0 indicate
an acid reaction; pH values above 7.O indicate an alkaline reaction.
One pH unit means a tenfold difference in strength; for example,
a pH 5.5 indicates an acidity that is ten times as great as pH 6.5.
It is the pH that determines such processes as curdling of milk
the action of enzymes, the growth of bacteria, the color of indicators,
taste, etc. Today pH is easy to measure.
The customary practice of using a 9 cc. sample in titrating dairy
products involves a weight discrepancy. Since the acidity test is
used for comparative purposes, this discrepancy is not serious.
However, when we compare different products (i.e.. milk and cream,
milk and condensed milk), this discrepancy must be remembered.
When titrating dairy products to the pink color of the phenolphthalein
endpoint, we are titrating to about a pH 8.3 or 8.4, which is quite
appreciably on the alkaline side of the true neutral point. On the
other hand titrating to A pH of 7.0 will result in a different %
acidity. Most "normal" acidities reported in the early literature
were determined with the phenolphthalein indicator. Thus, the true
acidity is reported higher than it actually is.
If we add distilled water to the measured sample of milk, a lower
titratable acidity value is obtained. This is due to the fact that
there will be less precipitation of tri-calcium phosphate.
The acidity of milk from individual cows ranges from 0.10 to 0.26
%. Herd milk varies less in acidity because of commingling,, but
occasionally herds are found where the acidity of the fresh milk
is 0.18 % and as high as 0.23 %.
The acidity of fresh milk is due to phosphates, casein and whey
proteins, citrates and carbon dioxide.
The acidity of milk may change during the lactation period but no
definite trend can be stated except that (a) the acidity of colostrum
is high, and (b) the milk towards the very end of the lactation
period frequently has a lower acidity.
It has been impossible to increase the acidity of milk by feeding
silage or even by feeding such inorganic acids such as inorganic
as sulphuric acid or phosphoric acid.
Mastitis, even in mild or sub-clinical form, causes the acidity
of the milk to be lower. In rare cases mastitis causes a high acidity
in the milk.
The acids produced by bacteria growing in milk are mainly lactic
and acetic acids.
The number of bacteria must increase to several millions per mL
before there is a measurable increase in acidity.
The acidity test must be used with considerable discretion, if used
at all, for grading raw milk at the plant intake, because (a) fresh
milk varies widely in acidity, and (b) millions of bacteria are
required to produce the first rise in acidity. This bacterial growth
can only occur if the farm milk was held un- refrigerated at 55
F or more. for several hours. If this were the case the required
recording thermometer would show a refrigeration problem. This "high
acidity" milk would also have show a SPC in excess of 500,000 per
The expected acidity of cream is frequently calculated from the
acidity of the milk and the fat content of the cream. This calculation
does not hold unless the acidity of the cream is determined by measuring
g cc. of cream, adding 9 cc. of water, and then titrating as usual.
Failure to recognize this fact has led to unjustifiable neutralization
of sweet cream.
If the acidity of condensed milk products is determined for the
purpose of forming an opinion as to the raw material used, then
the sample should be measured as in the case of milk or skim milk.
Enough distilled water should be added to the measured sample to
dilute the condensed product to the original concentration of the
The acidity of ice cream mixes is higher than the acidity of milk
in the same proportion that the serum solids content of the mix
is higher than that of milk.