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Fluorimetry as a Method of Determining Protein Content of Milk

Dairy Products Laboratory, Eastern Utilization Research and Development Division, USDA, Washington, D. C.

ABSTRACT

A commercial spectrophotofluorimeter was used to determine the relationship between the intensity of the fluorescence of milk in the ultraviolet region and its protein content. The effect of pH, temperature, milk contaminants, and milk protein aggregation on the fluorescence was studied. By dissociating the milk micelles with a diluent containing citrate, phosphate, and urea, stable fluorescing systems were obtained for study. A double-reciprocal plot of the data obtained showed a good correlation between fluorescence intensity and protein content as determined by Kjeldahl methods. In examining the milk of 88 cows of differing breed, age, and period of lactation it was found that the Standard error of the estimate was 0.079% protein. It was concluded that the intensity of fluorescence of milk in the ultraviolet could be used to make accurate and rapid determinations of the protein content of milk, but a number of disadvantages must also be considered before recommending the technique for general use. These include the large dilution of milk samples required, the necessity for rigid temperature control, adverse effects of milk contamination, the relatively small slope of the standard curve, and the fact that commercial fluorimeters suitable for routine use are not available at present.