Separation of casein hydrolysates using polysulfone ultrafiltration membranes with pH and EDTA treatments applied

Abstract

The preparation of peptide fractions from casein hydrolysates can be achieved by the use of ultrafiltration for the removal of the enzyme from the reaction mixture and/or to obtain specific fractions of peptides. However, the ultrafiltration of casein hydrolysates is subjected to severe fouling phenomena which affect both the flux decline and the rejection properties of the membrane. Polysulfone membrane material used in previous work showed specific rejection properties towards charged or hydroxylated amino acids. Polysulfone and polyethersulfone flat sheet membranes were selected for their distinctive surface properties. The ultrafiltration of a tryptic hydrolysate from sodium caseinate was studied and physico-chemical variables were introduced, namely, pH of the hydrolysate (pH 6, 8, 10) and addition of a calcium sequestrant (EDTA 20 mmol l-1). Flux decline measurements, total nitrogen rejection, molecular mass distribution profile, and amino acid composition of the permeates were determined. Although the polyethersulfone membranes showed greater permeability to water (+ 15%) at 25°C than polysulfone, no distinctive effect of membrane material was observed in the permeation flux (l h-1 m-2) during ultrafiltration of the hydrolysates. The rejection coefficients were similar for both materials. It was observed that the pH and the excess of EDTA had a much stronger effect than the material on the rejection properties (nitrogen, peptides, amino acids) and the flux decline. The two materials seemed to exhibit different surface reactivity towards charged molecules. © 1995 Elsevier/INRA.