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 polysaccharides 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 monitored as a function of storage time. There was a consistent trend of decreasing emulsifying efficiency and emulsion stability with increase in treatment 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 extent of high-pressure treatment (up to 200 MPa). Thermally treated biopolymer mixtures gave emulsions with droplets that did not significantly change with increase in temperature. In all cases, the presence of iota-CAR led to a significant improvement in creaming stability. However, the presence of kappa-CAR in untreated and thermally treated (<75 degrees C) mixtures gave rapid serum separation probably due to depletion flocculation. Of the two polysaccharides studied, iota-CAR gave the smallest droplets in fresh emulsions and the best stability with respect to visual creaming behaviour. The observations for 11S alone can be interpreted in terms of pressure or thermally induced unfolding of the protein, which results in a decrease in emulsion efficiency due to dissociation of subunits or protein aggregation. It appears that the strength of interaction of 11S with iota- or kappa-CAR is dependent on the charge density on the polysaccharide. The presence of interacting polysaccharide in the heated and high-pressure processed samples seems to inhibit the formation of aggregates. High-pressure treatment of the mixed biopolymer solutions in the presence of sodium chloride (>0.01 M) destabilises the emulsion, and so the protective effect of polysaccharide is lost. (C) 1999 Elsevier Science Ltd. All rights reserved.