A method is described for the colorimetric determination of lactic acid in milk and unsweetened milk products. The protein and lactose is removed after precipitation with copper sulphate and calcium hydroxide at 45° C, the lactic acid in an aliquot of the filtrate is oxidized to acetaldehyde in sulphuric acid in the presence of copper sulphate, a purple colour is developed with p–hydroxydiphenyl, and the colour intensity is measured in a photoelectric absorptiometer or other suitable instrument.Recoveries of from 96 to 103%, averaging 100%, were obtained when 20–400 mg. lactic acid/100 ml. were added to fresh milk.The proposed method has an advantage over existing methods in the ease and speed with which large numbers of determinations can be carried out simultaneously whilst retaining the essential accuracy.
INTERFERENCE WITH THE COLORIMETRIC DETERMINATION OF LACTIC ACID (BARKER-SUMMERSON METHOD) BY NITRATE AND NITRITE IONS
