Improved dye method for estimating protein

Abstract

Abstract Several improvements and extensions of the Udy dye method for estimating protein content in natural products are discussed. The many outstanding physical and chemical properties of the monosulfonic azo dye, acid orange 12, are presented. This dye is used in a stablizing pH 2 buffer system to react with the protein’s basic groups that originate from the basic amino acids (BAA) histidine, arginine and lysine. A complete analysis requires less than 5 min. The ionic reaction rate is limited to the exposure rate of each binding site. Regression equations relating nitrogen content and bound dye are presented for several oilseeds, grains, legumes and animal products. Random mixtures of products, having wide dif‐ferences in dye‐binding capacity, are not amenable to this method of protein estimation. Because nonpro‐tein nitrogen is not measured, and the critically essential amino acid, lysine, is measured only when it is nutritionally available, it is suggested that the amount of dye bound by the protein’s BAA offers a better nutritional index than conventional nitrogen procedures.