Effects of fluid shear and temperature on whey protein gels, pure or mixed with xanthan

The effects of shear on whey protein isolate (WPI) gels, pure or mixed with xanthan, have been investigated at pH 5.4 by dynamic oscillatory measurements and light microscopy (LM). The shear was performed on the suspensions under a constant stress of between 0.04 and 2.1 Pa. Various temperature conditions were chosen in order to describe the effects of shear at the different states of aggregation of the WPI. Shear-sensitive aggregation phenomena were already found around 40 degrees C for the pure WPI samples. Continuous shearing during heating from 20 to 40 degrees C, prior to heat treatment at 90 degrees C, resulted in a gel with a storage modulus (G') half that of the unsheared gel, independent of the shear stress. Continuous shearing during heating from 20 to 76 degrees C resulted in a further decrease in G'. Inhomogeneities arose in networks formed from continuously sheared suspensions during heating from 20 to 50 degrees C and above. Depending on the shear stress and on the heating range of the shear, the networks showed areas of varied compactness and different classes of pores, ranging from 10 to 200 mu m. A higher G', compared to that for the unsheared gel, was found for gels subjected to shear for short periods in the vicinity of the gel point. The presence of xanthan inhibited the aggregation and demixing of the WPI, described as a sterical phenomenon. Under static conditions, the presence of xanthan resulted in a more homogeneous WPI network. Exposing the mixed suspensions to shear generally increased the inhomogeneity of the network structure. Short periods of shearing in the vicinity of the gel point affected the kinetics of the gel formation and resulted in gels with higher G' values than the unsheared gel. Continuous shearing under stresses below 0.09 Pa, during heating from 20 to 60 degrees C and above, also resulted in gels with an increased G'. Continuous shear under stresses above 0.9 Pa resulted in gels with a decreased G'. (C) 1998 Elsevier Science Ltd. All rights reserved.