Analysis for Citric Acid in Presence of Casein

Abstract

Factors affecting the determination of citric acid in presence of casein were studied with the pyridine-acetic anhydride method. Although the opalescence introduced by aliquots of diluted skimmilk affected results by less than 2.5%, the turbidity obtained on analysis of solutions of isolated casein could cause gross overestimation. A pH above 12.3 in the test aliquot assured a clear reaction mixture with both skimmilk and casein solutions. It is therefore recommended that test aliquots containing casein should comprise sufficient alkali to ensure a clear reaction. Casein prepared for use in the course of analytical work requires very thorough washing. Acid-precipitated casein was found to require 7 resuspensions in fresh washing solution at pH 4.5 for complete removal of occluded citrate, lactose, and Ca. Adsorption of citric acid by acid-precipitated casein was demonstrated, accounting for the lower citrate concentration in acid sera as compared to skimmilk.