A COLORIMETRIC METHOD FOR THE ESTIMATION OF REDUCING GROUPS IN MILK POWDERS

Abstract

A method has been developed for the detection and estimation of certain reducing groups in milk powders. The method is based on the reduction of potassium ferricyanide at 70 °C. and pH 5 and colorimetric estimation of ferric ferrocyanide. Titration with 2,6-dichlorophenolindophenol or potassium iodate failed to show any difference between fresh and stale milk powders. In contrast the ferricyanide test indicates much higher values in fresh powders than do the titration methods, and it also reveals a marked increase in samples that have developed stale, musty odours and flavours. Lactalbumin and casein show reducing activity that increases on heating. Of the amino acids tested, only tryptophane gives a positive reaction. It is concluded that the reducing groups are present in the protein molecule and become accessible on denaturation.