The influence of galactomannans with different amount of galactose side chains on the gelation of soy proteins at neutral pH

The influence of diverse galactomannans, differing mainly on the degree of branching (amount of galactose side chains along the main mannan backbone), on the heat-induced gelation of soybean proteins at pH 7, was investigated using dynamic oscillatory rheological measurements at low strain amplitude and microstructural analysis by confocal laser scanning microscopy. Rheological tests were performed during gel formation, induced by either isothermal heating or by heating/cooling at a constant rate. Two different protein concentrations were analysed, one in the vicinity of the critical gel conditions and the other corresponding to a well developed gel, whereas the galactomannan concentration ranged from 0 to 0.5%. The presence of the galactomannan promoted the gelation to occur for protein concentration below the critical gelation of soybean proteins alone, decreased the gelling temperature and had a positive effect on the gel strength of the heat-induced gels. These effects were more pronounced as the degree of branching decreases. The consequence of demixing and phase separation was dependent on biopolymer concentration and galactomannan branching, resulting in an array of microstructures, spanning emulsion-like, bicontinuous and aggregated morphologies. Structure development within the galactomannan-rich phases, dependent on the branching degree and on the capability of the galactomannan for self-association, may have played a role in the phase separation and viscoelasticity of the final gels. It was demonstrated that by using soybean proteins and galactomannan mixtures at above phase separation concentrations and controlling the polymer concentration and the length of the unsubstituted polysaccharide backbone, tailor-made viscoelasticities and microstructures can be obtained with useful applications in food formulation.