A rheological study of acid-set "simulated yogurt milk" gels prepared from heat- or pressure-treated milk proteins

Abstract

The application of high pressure as an alternative to heat treatment in the acid-set gelling of proteins was studied using a simulated yogurt milk (SYM) system, containing phosphocasein and whey protein isolate (WPI) in a ratio of 4:1. Gels were made by acidification of SYM with glucono-δ-lactone (GDL) at 40 o C to pH 4.6 and their properties measured by dynamic rheology using a Bohlin CVO rheometer. Gelation was studied in heat - (90 o C × 10 min) or pres- sure - (700 MPa × 20 min) treated SYM or SYM containing heat - (78 o C × 30 min) or pressure - (0-700 MPa × 20 min) treated WPI. For a constant time (20 min) and temperature (25 o C), the extent of whey protein denaturation was dependent on the applied pressure. Although pressures of ≤ 400 MPa caused as much as 57% denaturation, they did not support acid-set gelation when pres- sure-treated WPI was incorporated into SYM. Pressurisation of WPI at 600 and 700 MPa, which re- sulted in 86.5 and 91.4% denaturation, respectively, resulted in the formation of cohesive gels when SYM was acidified with GDL. The acid-induced gelation profiles of SYM pressurised at 700 MPa × 20 min and SYM containing WPI pressurised under the same conditions were different, suggesting that the kinetics of aggregation were different, presumably due to the disruption of casein micelles in the SYM system during the pressurisation step. Gels prepared from SYM containing pressure-treated WPI were weaker, i.e., they had lower values for G' throughout acidification, than those prepared from SYM containing heat-treated WPI. The gelation properties of heated SYM containing native or pressurised WPI were similar, indicating that the combination of pressurisation of WPI followed by heating SYM does not have an additive effect in relation to acid-induced gelation. Heating was more efficient at producing casein/whey protein interaction products that were suitable for the formation of gels on acidification. high pressure / heat treatment / whey protein isolate / phosphocasein / acid gel