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Whey protein nitrogen index (WPNI) is a well-known method of classifying nonfat dry milk powder (NFDM) based on its heat treatment. This classification scheme provides one criterion for the selection of NFDM for food applications. However, the effects of variation in NFDM composition on WPNI are not well documented. The objective of this study was to determine the effects of changing protein content in low heat and medium heat NFDM on WPNI value. Edible lactose powder (ELP) and permeate powder (PP) from skim milk ultrafiltration were used to downward standardize NFDM from a protein content of 35.5–30% protein on a wet basis. These standardized powders were analyzed for WPNI by American Dairy Products Institute (ADPI) method. Powders were also analyzed for protein, ash, fat, moisture and lactose by standard methods to describe the composition. Using linear regression, WPNI was found to be positively associated with protein content for both low and medium heat NFDM from several suppliers. For example, a low heat NFDM (from supplier B) with initial protein content of 34.3% (wet basis) and WPNI of 6.38 mg soluble whey protein nitrogen (classified as a low heat powder) had its WPNI reduced to less than 6.0 mg of soluble nitrogen per g of powder when standardized to a protein content of less than or equal to 31.89% (wet basis) with either ELP or PP. This would reclassify it as medium heat powder. We conclude that standardization of NFDM with lactose or permeate will change its WPNI value and may effect its heat classification. We propose a modified approach to calculate WPNI based upon soluble whey protein nitrogen per g protein. This new WPNI value (protein corrected) would be independent of powder protein content and hence be more indicative of the actual heat treatment it is intended to reflect.

Impact of protein standardization of milk powder with lactose or permeate on whey protein nitrogen index and heat classification