Large Protein Particle Changes in Ultra High-Temperature Sterilized Concentrated Skimmilk

Abstract

Sterilized concentrated skimmilk was diluted to approximately 14% total solids and fractionated in a Model SS3 Servall centrifuge equipped with a Szent-Gyorgyi and Blum continuous flow system. The purpose was to determine changes in the large protein particles formed during heat sterilization after storage of the concentrated skimmilk at 4C for 12, 82, 159, and 194 days. After 12 days 35.6% of the total protein sedimented at 300×g and an additional 17.3% sedimented when the supernatant was recentrifuged at 1,000×g. After 194 days 27.1% protein sedimented at 300×g and an additional 18.9% at 10,000×g. Total protein sedimented at 194 days was 46% compared with 52.9% at 12 days, although the gravitational force on the supernatant at 194 days was 10 times that at 12 days. Photomicrographs revealed shape and size of large particles by anoptral phase contrast. Densitometric measurements of polyacrylamidegel electrophoretic patterns showed that some whey proteins had moved with β-casein after heating but particles 0.75μ diameter or larger which sedimented at 1,000×g were mostly or possibly all casein. After centrifugation at 1,000×g, 47 to 54% of the total protein remained in the supernatant, including casein micelles less than 0.75μ diameter and whey proteins. Added oxalate rapidly dissociated sedimented protein particles.