A Comparison of Two Methods for Estimating Milk Proteins by Ultraviolet Spectrophotometry

The purpose of this study was to compare two ultraviolet spectrophotometric methods for the determination of milk proteins; the difference in absorbance at 215 and 225 nm, and the absorbance at 280 nm. Seven purified milk proteins were dried to constant weight under vacuum, dissolved, and diluted to five concentration levels in 0.9% NaCl, phosphate buffer and sodium dodecyl sulfate buffer. Duplicate concentration series were measured spectrophotometrically and the results were analyzed by regression. From the absorbances for the series of seven proteins studied, the 215-225 nm measurements were from 2 to 16 times as sensitive as those at 280 nm. For both methods, precision was about 5%, and the accuracy was around 5% also. When the same protein was measured in different solvents at 215-225 nm, the absorbances among different solutions of the same protein showed a slightly significant difference. The error for estimating different proteins by using one standard at 215-225 nm was less than at 280 nm, but the error was too large to make that technique practical.