Heat-induced gelation of whey protein isolates (WPI): effect of NaCl and protein concentration

Abstract

Samples of a commercial whey protein isolate (WPI) have been dialysed to achieve constant ionic strength backgrounds of 60 and 100 mM NaCl in a buffer of 10 mM phosphate at pH 6.7. Gelation was induced by application of a temperature gradient between 60 and 90°C, rising at 1°C/min. Gel properties (gelation temperature, final gel elasticity ( G′ f)) were measured at a single frequency of 0.1 Hz over the whey protein concentration range 2.5–10% w/v and compared with those of non-dialysed dispersions of the WPI dissolved in 60 mM NaCl, 10 mM phosphate buffer (pH 6.7). The rheological measurements showed that for each whey protein concentration increasing ionic strength resulted in lower gelling temperatures and higher gel modulus values. For the dialysed samples, G′ f showed a power-law dependence on protein concentration, which was more pronounced in gels containing 60 mM NaCl than those with the higher salt concentration. Exponent values were ∼7.1 for the 60 mM systems and 4.2 for the 100 mM background. The non-dialysed dispersions showed curvature in the double log plots of G′ f versus whey protein concentrations with data parallel to the 60 mM line at low protein levels but a slope less than that of the 100 mM line at the highest protein concentration. The gelation temperature followed a similar trend, moving from close to the 60 mM line at low protein levels and crossing the 100 mM line at intermediate concentrations. A multiple linear regression model with ionic strength and logarithm of the protein concentration as variables accounts for 96% of the variance in our measurements of gelation temperatures in both dialysed and non-dialysed samples. The results rationalise the influence of background salt contamination in commercial whey protein preparations on their gelation properties.