Steady shear and oscillatory rheological measurements on solutions of commercially available whey concentrates : a thesis presented in partial fulfilment of the requirements for the degree of Master of Technology in Food Technology at Massey University
Three commercially available whey protein concentrates (WPC), Alacen 312, Alacen 392, and Alacen 475 were studied by steady shear and oscillatory rheological methods using a Bohlin rheometer at concentrations of 5-40% and temperatures of 5-9o0 c. The WPC solutions showed Newtonian behaviour up to a concentration of 15%, were slightly shear thinning at
20 and 25%, and exhibited time dependent or thixotropic behaviour at concentrations of 30% and above.
The apparent viscosity of Alacen 475 solutions of concentration less than 10 percent by weight could be calculated by ns = w(1+28C) where ns, nw and Care the
viscosity of the solution, the viscosity of water and the fractional weight concentration. For Alacen 475 solutions of 40% concentration the structure broken down by shearing at a
high shear rate of 734 s-1 recovered slowly when the shear
rate was suddenly dropped to 147 s-1 or zero.
The apparent viscosity of WPC solutions was temperature dependent. It decreased at first as temperature increased until a minimum viscosity was attained and then increased rapidly with further increase in temperature. Temperature also had a marked effect on the time dependency of 20% and 30% WPC solutions - causing time dependent shear thinning at
40 and 50°c and time dependent thickening at 60 and 70°c.
The continuous changes in structure of WPC solutions during heat-induced gelation were followed using oscillatory rheological measurements. The effects of temperature, concentration and heating time on the formation and dynamic rheological properties of WPC gels were determined and are discussed in terms of current theories on the rheology of protein solutions and gels. A gelling model for the gelation of globular protein solutions was proposed to interpret the
development in protein gel structure during the gelling process as reflected by the continuous changes in dynamic rheological properties.