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Ultraviolet Spectrophotometric Determination of Protein Content in Milk

Department of Agricultural Chemistry, The University of Tokyo, Japan

ABSTRACT

With the adoption of end-on type photo-multiplication system, optical densities of milk can be measured directly. Individual milks and skimmilks prepared from them were subjected to the ultraviolet spectrophotometric analysis. Two significant peaks around 230 and 280 mµ, and the readings at the maximal absorbances were compared with the protein contents determined by the standard Kjeldahl method. Optical densities of both peaks showed a close correlation with values obtained by the standard Kjeldahl method, but the coefficient of correlation was slightly higher and the standard deviation a little lower with the readings at the longer wavelength (285 mµ) than at the shorter one (243 mµ). Coefficients of correlation at the shorter and longer wavelengths of 20-fold-diluted Holstein milks were 0.9965 and 0.9976; whereas, the standard deviations from the regression formulae were 0.019 and 0.016%, respectively. The protein content of Holstein milk can be measured as precisely as with the dye-binding method, using Diacid Light Green GS. Preservatives such as mercuric chloride, formalin, or chloroform did not affect the results significantly.