Effect of milk solids concentration on whey protein denaturation, particle size changes and solubilization of casein in high-pressure-treated skim milk

Skim milk (10–20% total solids, TS) was high-pressure-treated at 200–600 MPa for 0–60 min at 20 °C. At 200 MPa, and to a lesser extent at 300 MPa, the denaturation of β-lactoglobulin was reduced on increasing the %TS of the milk. However, at higher pressures, the level of denaturation of β-lactoglobulin was essentially the same at all milk concentrations. Denaturation of α-lactalbumin occurred at pressures ⩾500 MPa and was independent of milk concentration. For the 10% TS milk, the particle size decreased by ∼45% on increasing the pressure from 200 to 400 MPa, with little further change at up to 600 MPa. As the milk solids concentration increased, the effect of pressure on casein micelle size was less pronounced. An unusual behaviour was observed on treatment at 300 MPa, where the particle size distribution broadened; this effect was more pronounced at higher milk solids concentrations, and the size distribution changed from monomodal to bimodal. Much casein was non-sedimentable after pressure treatment; however, the concentration of milk solids only affected levels of non-sedimentable casein at 200 MPa, and less so at 300 MPa. Most of the whey protein remained in the milk serum even after treatment at the highest pressures and the longest holding times.