Emulsifying behaviour of 11S globulin Vicia faba in mixtures with sulphated polysaccharides: comparison of thermal and high-pressure treatments

The influence of heat (up to 80 degrees C for 2 min) and high-pressure (up to 250 MPa for 20 min) on the emulsifying properties of 11S globulin Vicia faba at pH 8.0 has been investigated for systems containing the sulphated p olysaccharides L-carrageenan (iota-CAR) and kappa-carrageenan (kappa-CAR). The emulsions (0.5 wt% 11S, 20 vol% n-tetradecane) made with heated or high -pressure treated 11S were found to give substantially larger droplets than those made with the native protein. Visual creaming behaviour has been mon itored as a function of storage time. There was a consistent trend of decre asing emulsifying efficiency and emulsion stability with increase in treatm ent temperature or pressure. Addition of iota-CAR or kappa-CAR (3:3-7:1 by weight) to the native protein at low ionic strength led to smaller droplets whose size decreased with increase in polysaccharide concentration and ext ent of high-pressure treatment (up to 200 MPa). Thermally treated biopolyme r mixtures gave emulsions with droplets that did not significantly change w ith increase in temperature. In all cases, the presence of iota-CAR led to a significant improvement in creaming stability. However, the presence of k appa-CAR in untreated and thermally treated (<75 degrees C) mixtures gave r apid serum separation probably due to depletion flocculation. Of the two po lysaccharides studied, iota-CAR gave the smallest droplets in fresh emulsio ns and the best stability with respect to visual creaming behaviour. The ob servations for 11S alone can be interpreted in terms of pressure or thermal ly induced unfolding of the protein, which results in a decrease in emulsio n efficiency due to dissociation of subunits or protein aggregation. It app ears that the strength of interaction of 11S with iota- or kappa-CAR is dep endent on the charge density on the polysaccharide. The presence of interac ting polysaccharide in the heated and high-pressure processed samples seems to inhibit the formation of aggregates. High-pressure treatment of the mix ed biopolymer solutions in the presence of sodium chloride (>0.01 M) destab ilises the emulsion, and so the protective effect of polysaccharide is lost . (C) 1999 Elsevier Science Ltd. All rights reserved.