LOW CROSS-FLOW VELOCITY MICROFILTRATION OF SKIM MILK FOR REMOVAL OF BACTERIAL-SPORES

The use of microfiltration membranes in the dairy industry to remove b acterial spores has been applied for some time using the so-called ''b actocatch'' process. However, these microfiltration units have high en ergy demands since high linear velocities are required during operatio n, to avoid problems related to fouling and concentration polarization . In this work, optimization of a backflushing technique combined with reverse asymmetric membranes ('backshock' technique) was used to avoi d the fouling and concentration polarization problems, allowing the us e of low linear velocities and resulting in low-energy costs. In the n ovel 'backshock' technique, the permeate is pressurized during a very short time interval (less than 1 s) and with a frequency around 0.2-1 s(-1). The benefit of using a reverse asymmetric membrane is related t o the formation of a very open fouling layer just inside the porous su pport layer and the build-up of a concentration profile of the protein s inside the porous structures. The microfiltration of skim milk using ceramic and polymeric membranes was studied with different membrane s tructures. The backshock technique, combined with reverse asymmetric m embranes of pore size of 0.87 mu m, allows the effects of concentratio n polarization and fouling to be controlled, achieving very high (500 L h(-1)m(-2)) and stable fluxes with 100% casein transmission and a hi gh retention of spores (reduction by a factor 10(4)-10(5)) even at low linear velocities (0.5(-1) ms(-1)). (C) 1998 Elsevier Science Ltd. Al l rights reserved.