Thermal aggregation of whey proteins under shear stress

Processed food protein such as whey protein, used as hydrocolloid, is a possibility to impart specific structural and physical properties of food. The specific protein properties can be achieved by controlled denaturation or rather aggregation. The resulting aggregate structure and thereby the functional properties at the macroscopic scale depend strongly on the process conditions. Heating conditions, shear stress and material composition, determine the reaction kinetics as well as the resulting particle size distribution and the structure of the aggregates. These conditions often cannot be investigated separately in industrial processes. Therefore, the impact of shear rate, heating time and protein concentration on the particle characteristics was investigated by a rotational rheometer at 80 °C. By increasing the protein concentration (from 5 to 30% w/w), smaller more compact and stable aggregates were produced. This is due to the higher viscosity and the higher shear stress. The aggregates appearance changes from long and crystalline like structure to a spherical shape. The influence of shear rate is dependent on protein concentration. In suspension with 5% protein the aggregate size initially increases with increasing shear rates because of the predominant effect of increasing number of collisions; and decreases subsequently due to limitation of the particle growth. At high concentrations the size of the aggregates decreases with the shear rate, because of increasing shear stress. The size of whey protein aggregates can thus be regulated by the applied shear rate during processing.