Foam stabilization by protein-polysaccharide complexes

Foaming properties of covalent complexes of the nonionic polysaccharide dextran with three proteins (bovine serum albumin (BSA), lysozyme, beta-casein) have been investigated by monitoring small pressure changes above collapsing foams in a closed chamber. Assessment of foamability has been based on (i) the total foam volume produced and (ii) the pressure change associated with total foam collapse. Foam stability has been assessed in terms of(i) the time for loss of half the foam volume and (ii) the first-order rate constant associated with the time-dependent pressure change. The differential pressure monitoring technique has been found to be a more sensitive method of measuring foamability than the simple foam volume determination. A good correlation has been found between foam stability results based on the pressure monitoring technique and those based simply on the rate of foam volume collapse. Protein-polysaccharide hybrids were prepared by extended dry heating of mixtures of protein and polysaccharide at 60 degrees C under conditions of controlled relative humidity (40% or 79%). Foams were produced from dilute aqueous solutions of protein or protein-polysaccharide complex at pH 7 by sparging with nitrogen gas at constant temperature (15-30 degrees C). It has been found that complexation of lysozyme with dextran leads a very substantial enhancement in foaming properties, whereas complexation with BSA leads to only a small improvement, and complexation with beta-casein has a negative effect. The effects of complexation were enhanced with increasing molecular weight of the polysaccharide. In accordance with earlier work on emulsifying properties of similar hybrids, it appears that there is an optimum dry-heat treatment time for the enhancement of protein functionality by covalent complexation.