Thermal stability of skim milk/whey protein solution blends

Abstract

Dried protein fractions consisting predominantly of α-lactalbumin, β-lactoglobulin or a whey protein concentrate (WPC) were dissolved in skim milk ultrafiltration permeate at 3.44% (w/w) protein. Each solution, denoted as α-FS, β-FS or WPCS, was blended with skim milk at ratios of 100/0–60/40 (g skim milk g −1 whey protein solution), to increase the ratio of whey to casein protein without changing the concentration of total protein. The blends were adjusted to pH values in the range 6.6–7.1 and heat stability determined by measuring the heat coagulation time at 140°C. At initial pH 6.6–6.7, blends of skim milk and α-FS were slightly more heat stable than normal skim milk; gel electrophoresis indicated that thermal association between α-la and casein may have augmented heat stability. From pH 6.8–7.1, the blends were considerably less heat stable than normal skim milk, possibly due to the formation of soluble complexes between α-la and κ-casein and accompanying heat-sensitive, κ-casein-depleted casein micelles. In most cases, enrichment of skim milk with β-lg, by blending with β-FS or WPCS, caused a drastic reduction in heat stability; this was attributed to the low heat stability of β-lg. An exception to this trend was the high heat stability of a 70/30 blend of skim milk and β-FS or skim milk and WPCS; electrophoresis indicated that the heat stable entity was a complex between β-lg and casein.