RENNET COAGULATION OF HEAT-TREATED RETENTATE FROM CROSS-FLOW MICROFILTRATION OF SKIM MILK

The aim of this study was to investigate rennet coagulation of heat-tr eated microfiltration (MF) retentates of skim milk. A ceramic membrane module from SCT was used in a pilot plant for crossflow microfiltrati on. The membrane had a nominal pore size of 0.1 mu m. The protein rete ntion was 93% and for whey proteins about 10%. The rennet coagulation time of retentates was investigated using dynamic rheology and compare d with visually observed rennet coagulation times. When monitoring the rennet coagulation with the rheometer, large differences in the value s of the elastic modulus were observed for retentates of different con centrations, which could not be seen when determining the rennet coagu lation time visually. Samples of retentates were heat treated with the same F value at two temperatures (100 degrees C and 121 degrees C) co rresponding to a reduction in the number of anaerobic spores of Clostr idium butyricum of 10(6). The values of the elastic modulus of heat-tr eated retentates show that the treatment at the higher temperature (sh orter time) is less detrimental to renneting ability, but even skim mi lk heat treated at 100 degrees C for 30 min will coagulate at a suffic iently high casein concentration. It should thus be possible to signif icantly reduce the number of anaerobic spores in cheese milk without s acrificing the properties of coagulation by the judicial use of time, temperature and microfiltration pre-concentration.