Whey protein concentrates with different lactose contents were heat-and shear-treated in a scraped surface heat exchanger at various temperatures. The properties of the resulting protein aggregates are closely correlated with the denaturation kinetics of ␤-lactoglobulin and the different mechanisms -unfolding and aggregation -which determine the overall reaction rate. At temperatures below 85°C, unfolding is slowed down especially if there is a high content of lactose. A loose, porous aggregate structure is formed and the particle size and the serum binding capacity increase. The smallest aggregates are produced when heating takes place between 85 and 95°C. In the temperature range above 100°C aggregation is the rate-limiting step and the aggregate structure is very dense and compact. The particle size increases and is no longer dependent on the concentration of lactose.

The effects of pH‐value and a reduction in calcium content on the kinetics of whey protein denaturation and the aggregation behaviour, under shear in a scraped surface heat exchanger, were examined. The denaturation rate of β‐lactoglobulin at 80 °C is considerably retarded as the pH‐value decreases from pH 6.7 to 4.5. Aggregates which are produced under shear between pH 4 and 5.5 reveal a small particle size (<5 μm) irrespective of the lactose content and the heating temperature. This is attributed to the low reactivity of the thiol groups and the small net charge of the proteins in this pH‐range. At a reduced calcium concentration the heat‐ and shear‐treatment resulted in a gritty structure with large rubber‐like particles. These are not to be taken as primary whey protein aggregates but as fragments of a fine‐stranded gel.

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Skimmed milk was fractionated via a cascade system: Graphic plotting of microfiltration (MF); ultrafiltration (UF); nanofiltration (NF); and reverse osmosis (RO). The buffering curves of each fraction were studied over the pH range 4-7. Depending on their composition, the individual permeate streams showed different buffering capacity values and pH ranges where the buffering occurred. The concentration of active buffering substances in the permeates decreased (in mmol/L) from~26.6 (MF) tõ 17.4 (UF) to 1.39 (NF) to 0.07 (RO). Contributions to the total buffering capacity for MF permeate, which represents the serum phase of milk, were~37% from whey proteins and~63% from milk salts (especially citrates, phosphates and carbonates) including lactose and water.