RHEOLOGY OF WHEY-PROTEIN CONCENTRATE SOLUTIONS AS A FUNCTION OF CONCENTRATION, TEMPERATURE, PH AND SALT CONCENTRATION

Rheological properties of whey protein concentrate (WPC) solutions wer e studied in steady shear, using a Bohlin VOR Rheometer, as a function of concentration, temperature, shear rate, shearing time, pH, salt ty pe, salt concentration and solution age. At 22-degrees-C and pH 7, the WPC solutions exhibited Newtonian behaviour up to a concentration of 10% total solids, pseudoplastic behaviour between 10 and 30% and time- dependent shear thinning at 3.5% and above. The apparent viscosity of solutions at 22-degrees-C and pH 7 was linearly related to concentrati on up to 8%. The effect of temperature on apparent viscosity in the ra nge 5-60-degrees-C was closely described by the Arrhenius equation. Th e viscosities of WPC solutions were independent of solution age in the pH range 4-8 at all concentrations up to and including 20%, the preci se pH range narrowing as concentration increased. At pH values above o r below this range apparent viscosity became dependent on both pH and solution age, the age effect becoming more marked at higher WPC concen trations. Apparent viscosity at pH 7 increased markedly with both CaCl 2 concentration and solution age at concentrations above 0.6 m-CaCl2, the age effect in this case increasing with CaCl2 concentration. In co ntrast, NaCl concentrations of up to 0.8 m-NaCl had little effect on a pparent viscosity. The rheological behaviour of WPC solutions changed from time-independent to time-dependent shear thinning at high concent ration, at extreme pH values, at high CaCl2 concentration (after agein g) and on heating to above approximately 60-degrees-C. This change is considered to be caused by the formation of structure in solutions; a 40 % solution (at 22-degrees-C and pH 6.75) exhibited classic thixotro pic behaviour in a step-shear rate experiment.