Rapid Determination of Protein in Milk by Dye Binding

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Abstract
A rapid test for protein content in fluid milk, developed by Udy, was studied collaboratively. The test is based on the spectrophotometric measurement of excess Acid Orange 12 remaining in solution after the dye-protein complex has been removed. The dye, at a concentration of 1.300 g/L, is carried in a 0.05M phosphate buffer; 40 ml dye is mixed with 2.24 ml milk. The mixture is shaken and filtered, and the supernatant is read in a previously calibrated spectrophotometer set at 480 mμ. A flow through cuvette of 0.3 mm path length is used. Collaborative results on 39 samples in 5—7 laboratories show a small but significant variation between laboratories. This variation is smaller than that obtained by the Kjeldahl method. Within-laboratory variation was less with the dye binding procedure than with the Kjeldahl procedure. Based on these results, it is recommended that the dye binding method be adopted as oflicial, first action

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