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Milk Serum Proteins. I. A Quantitative Biuret Test for Milk Serum Proteins¹

Department of Dairy and Food Industry, University of Wisconsin, Madison

ABSTRACT

The biuret reaction was applied to a quantitative colorimetric test for milk serum protein solutions.

The method is simple and rapid, the solutions are free of turbidity and the response is nearly linear. Full development of the Cu-protein color requires about 30 min., after which the color is stable at least 24 hr.

Lactose and calcium phosphate were found to interfere in direct application of the test to whey. Dialysis of the whey was necessary to obtain normal values.

Concentrations of NaCl, Na²S0⁴ or ethanol as used in protein fractionation interfered with the test. Standard curves were prepared for protein solutions containing known concentrations of these reagents.

The color response of casein is different than that for milk serum protein; therefore, mixtures of these proteins cannot be analyzed by the biuret test.

¹ Published with the permission of the Director of the Wisconsin Agricultural Experiment Station