Dye Binding Method for Protein Content of Dairy Products

Abstract

Abstract The Udy dye binding method for protein in milk has been under collaborative study since 1965. The test is based on the formation of an insoluble complex between Acid Orange 12 (C.I. 15970) and basic amino acid residues of the protein. A known amount of dye is added to the sample, the complex is removed, and the excess dye is measured by colorimetry. The amount of protein in the original sample is calculated from the amount of dye used in complex formation. In previous reports, it was reported that the interlaboratory coefficient of variation is 1% or less, i.e., 0.03% protein in the sample. The coefficient of variation for the Kjeldahl method is 3%. Suitable analytical control is achieved through optical determination of dye concentration in the reagents and by testing known milks. Mercuric chloride is a suitable sample preservative, but potassium dichromate decolorizes the dye. It is recommended that this method be adopted as official final action for determining protein contents of fluid milk, half-and-half, nonfat dry milk, ice cream mix, chocolate drink, and buttermilk.