Electrical Conductivity Changes in Milk Caused by Acidification: Determining Factors

¹ Istituto Sperimentale Lattiero-Caseario (ILC), Lodi, 20075 Italy

Electrical conductivity of milk is mainly due to its soluble salt fraction. Lactose does not conduct current, and fat decreases conductivity. The contribution of proteins and peptides is of minor importance.

In addition, pH decrease causes hydrogenation of monohydrogen phosphate ions to dihydrogen phosphate ions, which have lower molar conductivity. Thus, addition of lactic acid to a phosphate buffer solution (pH 6.5) decreases conductivity. However, fermentation of lactose to lactic acid sharply increases conductivity; also, acidification of milk changes equilibria of buffer systems and solubilizes casein-bound calcium and phosphorus salts. This phenomenon increases conductivity sharply, as shown by the acidification of diafiltered milk that is free from soluble salt fractions.

The relationships among such equilibria increase the electrical conductivity of milk by acidification, which is why milk is a suitable substrate for the automatic monitoring of lactic acid bacteria growth by conductimetric methods.

Key Words: milk • conductivity • acidification • buffer system

Submitted on July 27, 1993
Accepted on December 6, 1993