MONITORING THE MILK ACIDIFICATION BY THE CONDUCTOMETRIC METHOD

Rodica Caprita, A. Caprita

Abstract
Milk has conductive properties because it contains charged compounds, especially mineral salts. The determination of the electric conductivity of individual milk components allowed the identification of individual contributions to overall conductivity. The addition of NaCl to milk caused an increase in conductivity that was only 77.78% of that in aqueous solutions, due to the equilibrium between soluble and colloidal salts, and other soluble substances, which interfere with the salts and decrease its conducting power. Milk acidification and the decrease of pH are due to the lactose fermentation when lactic acid is formed. The phosphoric acid, the citric acid, and their acid salts have a buffering role in milk, so that the incipient milk acidification is at first buffered. Lactic acid added to fresh milk decreased the pH and increased the electrical conductivity of milk. The increase in electric conductivity for addition of lactic acid to raw milk is from 5.2 mS/cm to 5.7 mS/cm, and for addition of lactic acid to deionized water is from 0 mS/cm to 0.56 mS/cm. We observed that when the same quantity of lactic acid is added to milk the curve for electrical conductivity increase is parallel to that of aqueous solutions, but with lower absolute values. The lower incrementally increase in conductivity is due to the uptake of protons by the buffers present in milk. HPO₄^2- changes into H₂PO₄^- as pH decreases, which has a lower molar conductivity. The variation of the electrical conductivity incrementally with variation of lactic acid concentration makes possible the use of this biophysical parameter for monitoring milk acidification.

Key words: pH, electric conductivity, lactose, milk, lactic acid