Influence of high-pressure treatment on β-lactoglobulin-pectin associations in emulsions and gels

We report on changes induced by high-pressure treatment (HPT) in emulsification and gelation properties of beta-lactoglobulin (beta-L) in the presence of a low-methoxy pectin. Oil-in-water emulsions (20 vol% soybean oil) prepared with pressure-treated protein + polysaccharide solution mixtures (0.5 wt% beta-L + 0.02-0.5 wt% pectin) exhibit droplet flocculation which is sensitive to the applied pressure, pectin content and pH. Flocculated droplet networks cause modifications to emulsion rheology, which are most evident at low pH, particularly near the protein pI. Changes in creaming behaviour, interpreted as a diminution in the extent of depletion flocculation, are also indicative of pressure-induced P-L-pectin complexation. An electrostatic interaction is consistent with a bridging flocculation mechanism at pH < pI, whilst maximum emulsion flocculation is exhibited at the pI. Significant levels of flocculation at neutral pH also indicate a degree of complexation under conditions where both macromolecules carry net negative charges. This observation is supported further by gelation studies where, under neutral conditions, the presence of pectin during HPT greatly enhances the strength of the resulting protein gels (4-8 wt% beta-L + 0.1-0.5 wt% pectin). Photon correlation spectroscopy of the mixed biopolymer solution (pH 7.0) provides further evidence of beta-L-pectin complexation following HPT, which is otherwise nearly insignificant in untreated samples. Explanations for an attractive interaction at neutral pH are offered. (C) 2000 Elsevier Science Ltd. All rights reserved.