Determination of True Protein in Milk with Formaldehyde-Trichloroacetic Acid Precipitation-Kjeldahl Nitrogen

A new Kjeldahl nitrogen method for the determination of true protein in milk which could effectively remove the interference of melamine and other non-protein nitrogen was proposed. 3 mL of formaldehyde was added into 5 mL samples, then the mixture was vortexed for 1 min and heated in 70 ℃ water bath for 10 min. 5 mL of 150 g·L-1 trichloroacetic acid solution was added to precipitate proteins. After filtration, proteins in the precipitate were measured by automatic Kjeldahl nitrogen analyzer. The data found was analyzed using paired t-test and showed. No significant difference between bovine serum albumin standard solution determined by this method and the GB method, between true protein in milk determined by this method and Coomassie brilliant blue G-250 method, between protein of interference group and control group in interference test. Values of recovery of disruptors in filtrate found was in the range of 98.4%-100%.