Rheological behaviour of dairy products as affected by soluble whey protein isolate

Rheological characteristics of aqueous solutions of whey protein isolate (WPI) and several dairy systems with added WPI were studied. The WPI solutions behaved as Newtonian fluids at concentration ranges <10% WPI. Higher concentrations up to 50% WPI demonstrated a non-Newtonian behaviour. Addition of WPI up to 10% decreased the apparent viscosity of a commercial yoghurt drink. The original viscosity was restored at addition of 15% WPI. In buttermilk, minimum viscosity was observed after addition of 6% and original value was restored at 12%. The viscosity lowering effect of WPI was also observed with commercial and laboratory prepared stirred yoghurts. Addition of the WPI was investigated either before fermentation (BF) or after fermentation (AF) of the heated unfortified yoghurt milk. Addition of WPI to commercial stirred yoghurt decreased G′G′, while G″G″ was affected only marginally. BF yoghurts behaved similarly to the commercial yoghurt, while AF yoghurts exhibited complete breakdown of the system.