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Bromcresol Green Paper in the Manufacture of Grain-Curd Casein

Research Laboratories, Bureau of Dairy Industry, United States Department of Agriculture

ABSTRACT

In recent work on grain-curd casein, it was found that the end-point of the precipitation,—pH 4.1—may be simply, quickly, and accurately determined by the use of bromcresol green paper. The paper was prepared by dipping S. & S. 589, White Ribbon, filter paper into a 0.4 per cent solution of bromcresol green in 25 per cent ethanol, the solution being adjusted to pH 4.6, potentiometrically measured. The papers were than dried and cut into strips of the usual test paper size. In making a test, a strip of the indicator paper is wet with the whey and stuck on a window pane away from the direct rays of the sun. The color of the paper, viewed by transmitted light, should be compared at once with permanent dry paper standards viewed by reflected light. The protein error is very slight—not more than 0.1 pH unit in whey—and turbidity may be disregarded.