The effect of temperature/shear history on the thermal gelation of whey protein concentrates

Abstract

In many food processes proteins are heated and sheared prior to gelation. Experiments were conducted in which protein concentrate solutions at pH 7.0 and 5.2 were sheared tor different times prior to monitoring subsequent quiescent gelation using a Rheometrics dynamic spectrometer. A systematic method for the analysis of particle sizes in a protein gel has been developed. At pH 5.2, where denaturation controls gelation, shear during the initial stages of heating increased the gel strength, probably due to shear preventing rapid aggregation into weak networks, but long durations of shear decreased the gel strength. In contrast, the gelation of pH 7.0 protein solutions is aggregation limited. Aggregation is enhanced by shear; this is shown both by the formation of protein aggregates visible microscopically and an increase in the initial gel strength after shear. Breakage of the protein network as a result of shear has also been noted. The two processes act together; for pH 7.0 solutions the final gel strength is a more complex function of the time of shear before gelation than for pH 5.2. It is suggested that the rheology of a sheared gel might be described in terms of effective rather than actual protein concentration, i.e. that shear induces aggregation that reduces the amount of protein available to form a gel network. This method was successfully applied to gelation at pH 5.2, but was not as successful at pH 7.0.