Direct Determination of Citric Acid in Milk with an Improved Pyridine-Acetic Anhydride Method

Abstract

Summary The determination of citric acid with pyridine and acetic anhydride has been investigated at reaction temperatures from 17 to 60° C. The optimum proportions of pyridine, acetic anhydride, water, and acetic acid for maximum color intensity and stability are given for each temperature. The procedure has been modified to eliminate the violent nature of the reaction, even when the analysis is done at a reaction temperature of 60° C. Details of a method for the determination of 25–200 μ g. of citric acid, at a reaction temperature of 32° C., are presented. In comparison with previously published methods based on the reaction, the recommended technique results in improved sensitivity, stability, and reproducibility without requiring careful timing. The method has been successfully applied to the routine analysis of milk and milk products. Milk and serum can be analyzed directly, after suitable dilution. Corrections for the interference caused by fat in homogenized milk, and by trichloroacetic acid in T.C.A. serum, can be made easily. Results of direct analysis of milk were from 5 to 15% higher than those for the corresponding sera and are believed to represent the true values for the citric acid content of milk.