Effect of Heat upon Size and Composition of Proteins Sedimented from Normal and Concentrated Skimmilk

Abstract

Unheated and heated skimmilk and concentrated skimmilk were subjected to ultracentrifugation at four different forces ranging from 11,700×g to 144,000×g and the resulting supernatant and sediment fractions were examined to determine protein particle size distribution and chemical composition. An ultrafiltrate-dilution technique was used to determine sedimentation of the protein aggregates from concentrated skimmilk.Heating skimmilk at 88C for 10min had no significant effect upon the protein particle size distribution or chemical composition of the protein sediment fractions.Concentrated skimmilk protein aggregates were larger-sized and exhibited higher Ca/N and P/N ratios than those from either unheated or heated skimmilk. Heating concentrated skimmilk at 88C for periods up to 10min resulted in additional increases in both Ca/N and P/N ratios in the sediment fractions when nondiluted concentrate was employed. However, the sediment fractions obtained from heated concentrated skimmilk (diluted with their respective ultrafiltrates prior to centrifugation) had smaller Ca/N and P/N ratios than those from the unheated concentrate.Concentration of skimmilk and subsequent heating of the concentrate causes aggregation of protein particles, with incorporation of greater amounts of calcium phosphate into these aggregates.