Natural Variation of Milk Serum Proteins as a Limitation of Their Use in Evaluating the Heat Treatment of Milk

Eighty-one samples of fresh bulked milk, collected in ten regions of the United States in winter, spring, and fall seasons, were analyzed for nitrogen distribution, sulfhydryl content, and susceptibility of the serum proteins to heat denaturation. The variability in content of serum proteins and in their heat denaturability imposes serious limitations on the use of serum protein analyses for assessing dry milks of unknown history for specific uses. The sulfhydryl content not only exhibits a natural variability but is further affected by aging of the milk and contamination with copper.