EFFECT OF HIGH-METHOXY PECTIN ON PROPERTIES OF CASEIN-STABILIZED EMULSIONS

We report on the effect of high-methoxy pectin on the stability and rh eological properties of fine sunflower oil-in-water emulsions prepared with alpha(s1)-casein, beta-casein or sodium caseinate. The aqueous p hase was buffered at pH 7.0 or 5.5 and the ionic strength was adjusted with sodium chloride in the range 0.01-0.2 M. Average emulsion drople t sizes were found to be slightly larger at the lower pH and/or with p ectin present during emulsification. Analysis of the serum phase after centrifugation indicated that some pectin becomes incorporated into t he interfacial layer at pH 5.5 but not at pH 7.0. This was also suppor ted by electrophoretic mobility measurements on protein-coated emulsio n droplets and surface shear viscometry of adsorbed layers at the plan ar oil-water interface. A low pectin concentration (similar to 0.1 wt% ) was found to give rapid serum separation of moderately dilute emulsi ons (11 vol% oil, 0.6 wt% protein) and highly pseudoplastic rheologica l behaviour of concentrated emulsions (40 vol% oil, 2 wt% protein). We attribute this to reversible depletion flocculation of protein-coated droplets by non-adsorbed pectin. At ionic strength below 0.1 M, the i nitial average droplet sizes, the creaming behaviour, and the rheology were found to be similar for emulsions made with either of the indivi dual caseins (alpha(s1) and beta) or with sodium caseinate. At higher ionic strength, however, whereas emulsions containing beta-casein or s odium caseinate were stable, the corresponding alpha(s1)-casein emulsi ons exhibited irreversible salt-induced flocculation which was not inh ibited by the presence of the pectin. (C) 1998 Elsevier Science Ltd. A ll rights reserved.