Standardization of the Harland-Ashworth Test for Whey Protein Nitrogen

A standard procedure for performing the Harland-Ashworth whey protein nitrogen determination is presented. In this method, standard curves relating turbidity to the whey protein nitrogen content are prepared by using appropriate mixtures of salt filtrates prepared from two lots of nonfat dry milk, one containing a low, and the other a high, level of undenatured whey protein. Collsborative results secured from 17 laboratories using this method on six samples of nonfat dry milk indicated sufficient precision to warrant the use of this method as a basis of specifications for nonfat dry milk.