Rheological properties of acid casein gels made by acidification with glucono-δ-lactone

The effects of casein to whey protein ratio (wt. ratio 2.1:0.9, 2.4:0.6 or 2.7:0.3 casein:whey protein), incubation temp. (25, 35 or 45°C) and heating temp. (70, 80 or 90°C) on the rheological properties of milk protein solutions (TS concn. of 3% w/w) acidified with glucono-δ-Iactone (GDL) were studied using response surface methodology (RSM). Gelation was monitored using a time sweep test. Strain and frequency sweep tests were performed to characterize the final gel (200 min after starting acidification). The experimental data on the frequency sweep tests were correlated according to the following power-law: G′ = aωb. The (a) value is a measure of structural strength and the (b) value, which indicates the frequency dependency of elastic modulus (G′), shows the type of structure involved. Strain sweep tests were carried out to measure linear viscoelastic range, flow point (τf) and yield stress (τy) of the final casein gel. A high incubation temp. resulted in large decreases in (a), τf and τy values. Increasing the heating temp. also increased (a), τf and τy values. Finally, correlations between rheological parameters from time, strain and frequency sweep tests were determined. Significant (P <0.05) positive correlations were observed among G′200 (G′ of final gel), tg (the time of gelation onset after the introduction of GDL into the liquid sample), τf, τy, G′ at τf and (a) values. Significant negative correlations were found between (b) and G′200, τf, τy, G′ at τf or (a).