Viscoelastic behaviour of heat-treated whey protein concentrate suspensions

Abstract

The viscoelastic behaviour of heat-treated whey protein concentrate (WPC) suspensions was studied. Suspensions with total protein (TP) content of 5% and 9% w/v prepared from a commercial WPC with 38% w/w protein were treated at two temperatures (72.5 and 77.5 °C) during selected times to obtain 60% of denatured protein content. Unheated WPC suspensions were used as control. Frequency sweeps were performed in the range of 0.01–10 Hz at 20 °C. Mechanical spectra of WPC suspensions were similar to viscoelastic fluids. However, unheated WPC suspensions showed some mechanical characteristics of colloidal crystals, like little dependency of elastic ( G′) and viscous ( G″) moduli with TP content, solid-like mechanical spectra, and difficulty to flow at low content of TP. At a selected frequency of 1 Hz, viscous modulus was more frequency ( ω) dependant on experimental conditions (TP and temperature) than elastic modulus. Mechanical spectra were modelled using power law equations ( G′ = aω x , G″ = bω y ), but only parameters of heat-treated WPC suspensions containing 9% of TP showed temperature dependency. Characteristic relaxation times (τ c) were calculated as the inverse of the crossover frequency, where G″ = G′. Heat treatments produced a decrease in the extent of the elastic behaviour because heat-treated WPC suspensions presented higher phase angle and smaller τ c values than unheated WPC suspensions for the same TP content. The τ c decreased when treatment temperature increased but only in heat-treated WPC suspensions with 5% of TP.