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A Comparison of the Orange G Dye and Kjeldahl Methods for Determining Milk Protein¹

Department of Dairy Science, Ohio Agricultural Experiment Station, Wooster

ABSTRACT

The orange G dye method as developed by Fraenkel-Conrat and Cooper (2) and modified by Udy (4, 6) for estimating proteins in wheat flour and milk, was compared to the standard macro-Kjeldahl procedure, using a total of 104 samples of whole milk. The dye unbound by the protein was measured in an Evelyn photoelectric colorimeter using a special absorption cell (5). The galvanometer readings were converted to protein values by use of a graph. By increasing the time of mixing the sample and dye with a mechanical tumbler from 5 to 10 min., more uniform results on triplicate determinations were obtained.

Milk from Holstein, Jersey, and Guernsey cattle of different ages and in different stages of lactation in two Ohio Agricultural Experiment Station herds was used in this comparison. Samples were collected at three different times approximately 1 mo. apart, but the same cows were not represented each time.

¹ Journal Article 99-58, The Ohio Agricultural Experiment Station.