The determination of citric acid in milk and milk sera

The accuracy of methods that utilize the colour-forming reaction between pyridine, acetic anhydride and citric acid for the determination of citric acid in milk and milk sera has been examined. A method based on that of Saffran & Denstedt (1948), in which a trichloroacetic acid (TCA) filtrate is used, gave values 3–5% low for the citric acid content of both milk and milk ultrafiltrate. The method of Merier & Boulet (1958), in which aqueous samples are analysed, gave an accurate value for the citric acid content of milk. The same technique but applied to aqueous-NaOH samples (Marier & Boulet, 1960) gave values about 7% high for the citric acid content of milk and about 3% high for the citric acid content of ultrafiltrate. However, a method using the technique of Marier & Boulet and TCA filtrate gave accurate values for the citric acid content of both milk and milk ultrafiltrate.