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Sources of Variation Affecting Relationship of Milk Protein Determinations by the Orange G Dye and Kjeldahl Methods

Department of Veterinary and Animal Sciences

D. J. Hankinson

Department of Food Science and Technology University of Massachusetts, Amherst

ABSTRACT

Research has been conducted in the last few years on methods of measuring the per cent protein in the milk of dairy cows. The various dye-binding methods are generally considered to be superior for processing large numbers of samples, due to a combination of desirable features such as speed, cost, equipment, training of technicans, and relative accuracy and repeatability. One disadvantage of the dye-binding methods is that results are usually obtained in the form of scalar readings from a spectrophotometric device and these readings are not directly convertible into protein per cents. The usual method of conversion is to analyze a series of duplicate samples by a dye-binding method and by some other method (most frequently by macro-Kjeldahl analysis for N content). The results of the two methods must then be related mathematically, to permit the rapid conversion of the spectrophotometric readings into protein per cents.

An important application of this method is to obtain an accurate estimate of the protein content of milk produced by a single cow at two consecutive milkings, i.e., the sampling procedure customarily used in the production testing programs to obtain 24-hr milk production.

¹ Present address: Dairy Cattle Research Branch, USDA, Beltsville, Maryland.