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

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. whey protein nitrogen index / protein standardization / edible lactose powder / permeate powder / nonfat dry milk powder 摘要 – 乳糖和超滤透过液对乳粉中乳蛋白标准化后对乳清蛋白态氮指数和热分类的影 响。乳清蛋白态氮指数是划分脱脂乳粉热处理程度的良好方法。该分类方法为脱脂乳粉 在食品工业中的应用提供了选择标准。但是脱脂乳粉成分的变化对乳清蛋白态氮指数的 影响还没有予以证实。本文旨在研究低热及中热脱脂乳粉中不同蛋白质含量对乳清蛋白 态氮指数的影响。使用食用乳糖粉或者脱脂乳的超滤透过液, 将脱脂乳粉的湿基蛋白含 量标准化到 35.5%–30.0%, 其乳清蛋白态氮指数测定采用美国乳品协会提供的测定方法, 同时按照标准方法测定其蛋白质、灰分、脂肪、水分和乳糖含量。线性回归分析结果表 明:乳清蛋白态氮指数与低热及中热脱脂乳粉 , (不同供应商提供) 的蛋白质含量均呈正相 关。例如:低热脱脂乳粉 (B供应商) 在原始蛋白含量为 34.3% (湿基) 、可溶性乳清蛋白态 氮指数为 6.38 mg 时被认为是低热乳粉, 但是当用食用乳糖粉或者超滤透过液将湿基蛋白质 含量降低至 31.89% 时, 则每克乳粉中可溶性乳清蛋白态氮的含量小于 6.0 mg, 这样可能就被 归类为中热脱脂乳粉。这就表明使用乳糖或者超滤透过液进行脱脂乳的标准化可能改变其 乳清蛋白态氮指数, 并影响其热分类结果。因此, 并影响其热分类结果。因此, 建议采用每克 蛋白中可溶性乳清蛋白氮来改进乳清蛋白态氮指数的计算方法。该种方法中乳清蛋白态氮 指数 (蛋白质进行了校正) 与脱脂乳粉中的蛋白质含量无关, 因此更能反应出热处理的真实