Protein Aggregation in Conventional and Ultra High-Temparature Heated Skimmilk

Ultracentrifugate supernatant fractions from raw and heated skimmilk were examined by Sephadex G-100 gel-filtration and zonal electrophoresis to investigate the protein aggregation produced by heating skimmilk at 90 C for 10 or 30 rain, and at temperatures, to 149 C for as long as 16 sec. Heating at 90 C caused preferential formation of intermediate size />75 to 100 S 4425 to 500 S protein particles; whereas, the ultra high-temperature heat treatments favored formation of larger size ~ 425 to 500 S particles. Ultra high-temperature heat treatments produced only slight alteration of Sephadex gel-filtration patterns for the smallest size 475 to 100 S protein components; whereas, all of the 475 to 100 S protein components in 90 C heated skimmilk were sufficiently aggregated as to be totally excluded from Sephadex G-100. Polyacrylamide gel electrophoresis data indicated that the rate and extent of heatinduced protein aggregation to particle sizes in excess of about 75 to 100 S closely parallel those for whey protein denaturation. The findings support the concept that ultra high-temperature heat treatments of 146 to 150 C may produce small amounts of nonsedimenting casein components by disaggregation of casein micelles.