Microbial Retention and Membrane Fouling during Low Temperature Microfiltration of Skim Milk Using Ceramic Membranes
Julia de A. Fritsch, Cosmin Beliciu, and Carmen I. Moraru. Food Science, Cornell University, Stocking Hall, Ithaca, NY 14853
The non-thermal removal of microorganisms (both vegetative cells and spores) from milk using membrane separation technology has the potential to significantly improve the safety, quality and shelf life of milk and dairy products. CMF has been used in the last years by the Dairy Industry as a means for microbial removal, this operation being integrated in the commercial processing of milk, and performed at sub-pasteurization temperatures due to economical considerations. Yet, maximum benefits could be achieved if microorganisms were removed from milk in the early stages of its collection and processing, preferably in the raw milk stage. Technical challenges arise from the fact that due to regulatory provisions such a process must occur at temperatures <7°C, which is quite inefficient from a yield point of view. Efforts have been made to develop a cold CMF process that removes microorganisms from raw skim milk and yields economically attractive permeate fluxes. The objective of this work was to understand the formation of the fouling layer during the cold CMF of raw skim milk and to develop solutions to minimize fouling and increase the process yield. An experimental setup consisting of: a ceramic MF membrane of 1.4um pore size and Tami design, a tubular heat exchanger, and a centrifugal pump was used to CMF raw skim milk. While increased milk viscosity at the low process temperature is believed to be partially responsible for the much lower permeate fluxes obtained in this process as compared to the commercial process performed at 50-55°C, membrane fouling was identified to be the main limiting factor for the cold CMF of milk. Scanning Electron Microscopy (SEM) was used to analyze fouled and clean membranes, and a gel like structure was observed on the fouled surface, significantly altering the membrane pore size distribution. Differences in the characteristics of the fouling layers were observed as a function of the radial position of the flow channels inside the ceramic membrane. A ticker fouling layer was present towards the outer region of the membrane cross section, corresponding to the lower velocity region of the parabolic flow profile, which was considered visual proof of the effect of velocity on fouling. The particle size distribution of the components in the fouling layer was analyzed using a Brookhaven 90Plus Nanoparticle Size Analyzer. A trimodal distribution with pronounced peaks in the 100 – 500nm and 500 – 1100nm ranges was observed, indicating that casein micelles and potentially casein micelle aggregates are probably responsible for the severe membrane fouling. Microorganisms did not seem to play a significant role in fouling, as their presence in the fouling layer was not observed. It was thus concluded that membrane fouling is accentuated by a slow, laminar flow across the membrane and high transmembrane pressure conditions. High cross-flow velocities (v), which lead to turbulence and destabilization of the fouling layer, and low transmembrane pressures (DPtm) were conducive of high permeate fluxes. An average of about 52 L/(m2h) microfiltered skim milk was obtained after 45min at v = 7m/s and DPtm = 10psi and a temperature of 5°C, as compared to only 17.5 L/(m2h) after 45min of microfiltering skim milk at v = 7m/s and DPtm = 19psi. These conditions resulted in near complete removal of the milk microflora (>5 log reduction), and a composition of the microfiltered milk very close to the composition of the raw skim milk. Further, novel methods for the destabilization of the fouling layer, such as low pressure pulsatile gas surging, have been tested and showed promise as additional means to increase the economical attractiveness of the cold CMF process.