The pH is a measure of the hydrogen ion (H+) activity in aqueous solutions. The most exact measure of pH is obtained electrometrically by means of a pH-meter. The pH in normal fresh milk from healthy cows is 6.6 ± 0.1. Values higher than 6.7 indicate mastitis infections, whereas values below 6.5 indicate the presence of colostrum or bacterial deterioration. Milk with deviating pH should be rejected from the milk powder production, as the heat stability most probably will be inferior.

The acidity in milk is measured, for example by titration with a 0.1 n NaOH solution, and indicates the consumption of NaOH necessary to shift the pH-value from 6.6 ± 0.1 (corresponding to fresh milk) to a pH-value of 8.2 - 8.4 (phenolphthalein).

Lactic acid is an organic acid with one carboxylic acid, CH₃-CHOH-COOH, having a molecular weight of 90. One ml 0.1 n NaOH therefore corresponds to:

 90 x 0.1 / 1000 = 0.009g of lactic acid

If the titration requires e.g. 14.5 ml 0.1 n NaOH, the result is often expressed as:

14.5 x 0.009  ≈  0.13% lactic acid (see Fig. 101)

Fig. 101  Comparison of titratable acidity based on solely on alcali comsumption 

However, fresh milk contains practically no lactic acid, and the consumption of NaOH is used to change the pH-value of the following components:

Carbondioxide	equivalent to	0.01% lactic acid
Citrates	-	0.01% lactic acid
Casein	-	0.07% lactic acid
Albumin/globulin	-	0.01% lactic acid
Phosphates	-	0.03% lactic acid
Titratable acidity	equivalent to	0.13% lactic acid

The determination of "acidity" in fresh milk by means of titration is therefore more a measure of the buffer action of milk than anything else.

It is thus necessary to talk about the developed acidity, which is the result of bacterial activity producing lactic acid during milk collection, transportation, and processing.

The developed acidity will, needless to say, be more pronounced if the milk is not cooled.

In order to avoid the uncertainties about the degree of titratable acidity or developed acidity, it is necessary to use direct determination of lactic acid during the processing. This is done in order to find whether any of the installed equipment is responsible for developing acidity expressing activity of not only the bacteria alive after a pasteurization, but also previous activity of bacteria killed during the heat treatment.