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 that 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.

Fig. 7. Limiting flux vs. pH obtained by HCl addition (: low heat milk, —: UHT milk; •: low heat milk + HCl). Fig. 8. Limiting flux vs. zeta potential (UHT: UHT milk, LH: low heat milk). Fig. 8 shows that the lower the casein micelles charge (!7 mV), the lower the limiting flux. As electrostatic interactions involving casein micelles decreased with their charge, one can conclude that the stronger the electrostatic interactions, the higher the limiting flux. When adding 75 mmol L!1 NaH2PO4 to skimmed milk, the zeta potential remained unchanged and the limiting flux was close to that of unmodified skimmed milk. Moreover, the decrease of the limiting flux of CaCl2 modified skimmed milk compared with natural skimmed milk (pH = 6.7 and !20 mV) was mainly due to the variation of the zeta potential. Fig. 8 sums up the relationship between the limiting flux and the zeta potential, whatever the The following general conclusions can be drawn: 1. Fouling layer permeability was governed by casein micelles. Soluble proteins only played a minor role. By adding various concentrations of NaCl in skimmed milk, it was evidenced that permeate fluxes did not depend on ionic strength in the range 100–800 mmol L!1, meaning that electrostatic interactions involving soluble proteins only play a minor role on the fouling layer stability. 2. Intensity modulations of electrostatic interactions were the main explanation of limiting flux variations compared to intensity modulations of hydrophobic interactions that were not detectable. This work evidenced the major role of electrostatic interactions involving casein micelles on permeate flux: the lower the zeta potential of casein micelles, the lower the limiting flux.