The effect of temperature and shear rate upon the aggregation of whey protein and its implications for milk fouling

In milk fouling, the rate of production and composition of the fouled layer is controlled by reactions in the bulk fluid which lead to the creation of aggregates which are deposited on heated surfaces. The influences of temperature (70 °C–90 °C) and shear rate (111 s−1–625 s−1) upon the rate of growth and size of aggregates in whey protein concentrate solutions (WPC35) have been investigated using a Couette apparatus with and without the addition of mineral Calcium and Phosphorous. At temperatures below 75 °C, the aggregates formed are small and weakly bonded, whilst aggregates formed at higher temperatures (and higher shear rates) were denser and more rigid. This was attributed to weak van der Waals bonding between the particles at low temperatures and the formation of stronger covalent disulphide bonds at higher temperatures. Growth of the aggregates is due to both protein denaturation and aggregation; temperature step-change experiments have verified that denaturation is a strong function of temperature whilst aggregation is a function of the applied shear field and the strength of the particles. Step-changes in shear rate have shown that both denaturation and particle growth rate are enhanced by increasing shear rate due to an increase in the number of particle collisions, yet the final particle size showed a complex behaviour with the increase in shear rate. The addition of minerals to the WPC solution resulted in the formation of much smaller aggregates and increased deposition onto the surface of the Couette apparatus. This was attributed to interactions between Ca and β-lactoglobulin both in the bulk and at the heated surface.