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Some Observations on the Hull Method for Measurement of Proteolysis in Milk¹

Department of Microbiology, Oregon State University, Corvallis

ABSTRACT

Although the widely used (2, 3) Hull method (1) for the determination of hydrolysis of milk protein is rapid and convenient, certain difficulties have been encountered in this laboratory during its application. Highly turbid, rather than clear, solutions were often obtained during the final stage of the assay. In addition, when using autoclaved milk, bacterial proteolysis could not be accurately determined. These difflculties and their corrective measures will be described.

According to the Hull procedure, a clear trichloroacetic acid filtrate is obtained from the milk sample to be assayed. The pH of the filtrate is increased by the addition of a sodium carbonate-sodium methaphosphate solution. Finally, upon the addition of Folin-Ciocalteau phenol reagent, a clear blue solution results. Using a suitable blank, the intensity of the colored solution may be measured photometrically and related to a standard curve of tyrosine equivalents.

During the present study, a white precipitate frequently formed a few minutes after the phenol reagent was added.

¹ Supported by NIH Grant EF-69.