Particle size changes and casein solubilisation in high-pressure-treated skim milk

Reconstituted skim milk samples were pressure treated at 100–600 MPa from 0 to 60 min at 10–40 °C. The changes in casein micelle size, the size distribution and the level of casein dissociated from the casein micelles were monitored after pressure treatment. Small decreases in size were observed at 100 MPa, with slightly greater effects at higher temperatures or longer pressure treatments. At pressure ≥400 MPa, the casein micelles disintegrated; the effect was more rapid at higher temperatures although the final size was similar in all samples regardless of the pressure or temperature. At 200 MPa and 10 °C, the casein micelle size decreased slightly on heating, whereas, at higher temperatures, the size increased as a result of aggregation. Whey proteins were not involved in the aggregation reaction. At 300 MPa, there was an intermediate behaviour, with micelle aggregation and disintegration observed. Aggregation was observed at higher temperatures and shorter pressurisation times, whereas disintegration was observed in all samples at lower temperatures or at the longer pressurisation times at the higher temperatures. Considerable casein was rendered non-sedimentable on treatment at pressures above 100 MPa, even in the samples that had aggregated. There appeared to be a relationship between the size reduction and the level of non-sedimentable casein, although the aggregated samples deviated from this relationship.