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

The application of high pressure as an alternative to heat treatment in the acid-set gelling of milk proteins was studied using a simulated yogurt mil k (SYM) system, containing phosphocasein and whey protein isolate (WPI) in a ratio of 4:1. Gels were made by acidification of SYM with glucono-delta - lactone (GDL) at 40 degreesC to pH 4.6 and their properties measured by dyn amic rheology using a Bohlin CVO rheometer. Gelation was studied in heat - (90 degreesC x 10 min) or pressure - (700 MPa x 20 min) treated SYM or SYM containing heat - (78 degreesC x 30 min) or pressure - (0-700 MPa x 20 min) treated WPI. For a constant time (20 min) and temperature (25 degreesC), t he extent of whey protein denaturation was dependent on the applied pressur e. Although pressures of less than or equal to 400 MPa caused as much as 57 % denaturation, they did not support acid-set gelation when pressure-treate d WPI was incorporated into SYM. Pressurisation of WPI at 600 and 700 MPa, which resulted in 86.5 and 91.4% denaturation, respectively, resulted in th e formation of cohesive gels when SYM was acidified with GDL. The acid-indu ced gelation profiles of SYM pressurised at 700 MPa x 20 min and SYM contai ning WPI pressurised under the same conditions were different, suggesting t hat the kinetics of aggregation were different, presumably due to the disru ption of casein micelles in the SYM system during the pressurisation step. Gels prepared from SYM containing pressure-treated WPI were weaker, i.e., t hey had lower values for G' throughout acidification, than those prepared f rom SYM containing heat-treated WPI. The gelation properties of heated SYM containing native or pressurised WPI were similar, indicating that the comb ination of pressurisation of WPI followed by heating SYM does not have an a dditive effect in relation to acid-induced gelation. Heating was more effic ient at producing casein/whey protein interaction products that were suitab le for the formation of gels on acidification.