Effect of individual whey proteins on the rheological properties of acid gels prepared from heated skim milk

The effect of fortification of whey-protein-depleted (WPD) milk with different levels of beta-lactoglobulin (beta-LG), alpha-lactalbumin (alpha-lac) or mixtures of beta-LG and alpha-lac on the rheological properties of acid skim milk gels was investigated. Milk samples were heated at 80degreesC for 30 min before acidification with glucono-delta-lactone. Acid gels prepared from heated WPD milk had lower G values, longer gelation times and lower gelation pHs than those prepared from heated skim milk. The G values of acid gels from the WPD milk were not as low as those observed for unheated milk, which may be attributed to the residual whey proteins in the WPD milk. Addition of increasing levels of alpha-lac to the WPD milk prior to heat treatment caused only small increases in the G values of the acid gels, and had a small effect on the gelation time and gelation pH. Similarly, the addition of alpha-lac to WPD milk with added beta-LG caused small increases in the C value of the acid gels when compared with that of the WPD/beta-LG milk sample regardless of the genetic variant of beta-LG. The addition of increasing levels of beta-LG to the WPD milk prior to heat treatment and acidification caused marked increases in the C value and the gelation pH and a reduction in the gelation time. This effect was observed whether the beta-LG was added by itself or in combination with alpha-lac. There were some differences in behaviour between the different beta-LG variants. The addition of increasing levels of beta-LG B or beta-LG C to the WPD milk prior to heating and acidification caused an almost linear increase in the G value. In contrast, the addition of beta-LG A caused a progressive increase in the G value with added protein levels up to about 0.9% (w/w) but little further change at higher addition levels. (C) 2003 Elsevier Science Ltd. All rights reserved.