An ultraviolet spectrophotometric method to determine milk protein content in alkaline medium

Abstract

An ultraviolet (UV) spectrophotometric method to estimate the total milk protein content is proposed. The method is based on the capacity of strong alkaline solutions to change the spectrum of the amino acid tyrosine to higher wavelength values in the UV region. In the range between 248 and 256 nm the absorbance is a linear function of the wavelength and the slope coefficient is directly proportional to the protein concentration. Thus, the measurements of absorbance at two wavelengths were used to estimate the protein content. For samples where the ratio of lipid to protein was ≤0.05, the following general expression can be used: [Protein] (g litre −1) = (A 248−A 256) ( A ̊ 248− A ̊ 256) where A 248 and A 256 are the absorbance values of the sample at 248 and 256 nm and A ̊ 248 and A ̊ 256 are the values for the standard protein at the same wavelengths. Due to the interference of the fat globules, when the ratio lipid/protein is ≥0.05, it is necessary to introduce a correction coefficient, as follows: [Protein]={ (A 248−A 256) ( A ̊ 248− A ̊ 256 }·{ 1 [1+a·exp(b( A 256 A 248 ))] } where a = 1.76 × 10 5 and b = 12.29