Effects of shear on pure and mixed gels of gelatin and particulate whey protein

Microstructural and rheological effects of shear on pure whey protein (WPC) gels and on mixed gels of gelatin and WPC have been investigated at pH 5.4. The shear was performed just before the gel formation of the WPC, using shear rates of up to 300/s for times of up to 600 s. The microstructure was investigated by light microscopy (LM) and quantified by image analysis. The behaviour of the storage modulus (G') upon shear was analysed according to fully factorial experimental designs, where the shear rate and time were used as design variables. Pure WPC suspensions, sheared at similar to 2/s for similar to 20 s, formed gels which showed an extremum in G'. In the vicinity of the extremum, the G' showed a value twice that for a gel formed from an unsheared suspension. Image analysis on LM micrographs at different magnifications revealed that an inhomogeneous WPC network was formed from the suspensions sheared at similar to 2/s for similar to 20 s. Heavily sheared WPC suspensions (92/s for 240 s) formed gels which showed a weaker G' than the gels formed from unsheared suspensions. The behaviour of G' of mixed gels upon shear was similar to that of the pure WPC gels. The G' for the mixed gels proved to be less sensitive to variations in the shear conditions than the pure WPC gels. During cooling after the gel point of both pure and mixed gels, the loss modulus (G') showed a pronounced peak for samples sheared in the vicinity of the extremum. Mixed suspensions sheared in the vicinity of the extremum formed inhomogeneous WPC networks with large domains of gelatin. The mean pore size of the WPC network, estimated by image analysis, increased from 40 000 mu m(3), for the unsheared mixed sample, to 120 000 mu m(3) for the sheared mixed sample. Results from image analysis at different magnifications further confirmed that suspensions sheared in the vicinity of the extremum formed an inhomogeneous WPC network. (C) 1998 Elsevier Science Ltd. All rights reserved.