Impact of zeta potential and size of caseins as precursors of fouling deposit on limiting and critical fluxes in spiral ultrafiltration of modified skim milks

Abstract

Caseins, the main proteins of milk, are known to be responsible of flux limitations in tangential filtration of skim milk. These proteins are well known to be versatile with the pH. Their initial form, corresponding to the so-called “native casein micelles” is due to a complex aggregation of monomers through calcium phosphate bridges that can be disrupted by small pH variations. This paper deals with the determination of both critical and limiting fluxes during ultrafiltration of natural and modified skim milks. pH ranging from 3.7 to 11.5 allowed to consider skim milk ultrafiltration (UF) for production purpose (pH 6.6–6.7) as well as acidic and alkaline concentrated effluents. The physico-chemical characteristics of caseins, as diameter and zeta potential, were measured according to the pH variations obtained by the addition of HCl and NaOH. Then, both limiting and critical fluxes variations were correlated to these characteristics. The critical and limiting fluxes varied in a quite complicated way with the pH, leading to four different regions. At acidic pH, zeta potential was null and fluxes were controlled by large particles of caseins that precipitated in the solution. Around neutral pH, the diameter of casein micelles remained constant whereas the zeta potential varied strongly. In this region the fluxes increased linearly with the zeta potential of caseins. Then at alkaline pH, the zeta potential of caseins remained constant whereas the diameter varied showing successively partial destructuration and partial “restructuration” of new types of micelles, both different from native casein micelles. In this region the fluxes varied with the rate of large micelles. Finally, retention of lactose (main neutral solute of milk) and whole proteins, mainly responsible of COD, were measured and discussed in relationship with above-mentioned parameters.