ULTRAFILTRATION WITH A MICROFILTRATION MEMBRANE OF ACID SKIMMED AND FAT-ENRICHED MILK COAGULA - HYDRODYNAMIC, MICROSCOPIC AND RHEOLOGICAL APPROACHES

The effect of acidification method (microbiological with or without re nneting, HCI addition) on mass transfer, fouling structure and the rhe ology of the retentate was studied in the ultrafiltration of skim milk coagula using a mineral microfiltration membrane. The increase in fou ling with time appeared to determine permeate flow rates, which were h igher in biological coagula, and the protein retention rates which wer e higher in chemical coagula. Fouling was investigated using scanning electron microscopy. The rheological study showed that at the same tot al solids, biological coagula were more viscous than chemical coagula. The initial coagula (total solids 97 g/kg) all displayed pseudoplasti c behaviour at low shear velocities and Newtonian behaviour at high ve locities. Ultrafiltration of fat-enriched milk coagulum to a dry weigh t corresponding to a soft cheese (total solids 334 g/kg; fat in total solids 60 %) gave satisfactory permeate flow rates and protein retenti on rates. Performance was related to the composition of the product, t he hydrodynamic parameters used and the resulting fouling. The rheolog ical study showed that the initial coagulum behaved as a pseudoplastic body at low shear rate and for higher velocities as a Newtonian liqui d. The concentrated retenate behaved as an ideal viscoplastic body (Bi ngham body).