Limiting flux in skimmed milk ultrafiltration: impact of electrostatic repulsion due to casein micelles

The variations of the limiting flux during skimmed milk ultrafiltration were studied under various physicochemical environments (pH, ionic strength, chemical nature of added salts). The evolutions were explained taking into account the physicochemical characteristics (size distribution, zeta potential) of the casein micelles, that is the main components of the fouling layer formed at the membrane surface. It was shown than in the pH range from 5.6 to 6.7, the limiting flux was linearly related to the zeta potential of the casein micelles regardless of the way used to obtain the zeta potential variation (addition of HCl, CaCl2 or NaH2PO4): the higher the zeta potential of casein micelles (neutral pH), the greater the limiting flux. Under these operating conditions, the variations of hydrophobic interactions were negligible. The variation of the permeability of the deposit layer, composed of retained casein micelles, was therefore mainly governed by electrostatic interactions due to the casein micelles.