Pressure- and heat-induced gelation of mixed β-lactoglobulin/polysaccharide solutions: scanning electron microscopy of gels

Abstract

This study concerns the microstructure of pressure-induced gels (P-gels, 450 MPa, 25°C, 30 min) of mixed β-lactoglobulin/anionic polysaccharide solutions (12% w/w protein, pH 7.0, plus 0.1–1% w/w sodium alginate, low methoxy pectin or high methoxy pectin). Mixed biopolymer solutions were compatible at ambient temperature and atmospheric pressure in the concentration range studied. Scanning electron microscopy of mixed P-gels from 12% protein-0.1% polysaccharide (PS) solutions revealed microalveoles with a honeycomb structure, suggesting that pressure promoted the formation of water in water emulsion with two distinct phases of different protein and PS concentrations. Increasing PS concentration from 0.1 to 1% (w/w) progressively converted the micro-alveolated structure of P-gels into a coarser porous structure with large pores and “pillars”, and protein microparticles located in the pores. The trend to phase separation and protein microparticulation increased with PS concentration and varied in the following decreasing order: ALG≫LMP≫HMP. Phase separation phenomena and the resulting P-gel microstructure probably depend on the net negative charges carried by the PS and on the imparted solution viscosity. Heating the mixed protein–polysaccharide solutions at 87°C for 40 min also induced phase separation, porous structures and protein microparticulation. The results suggest that phase separation in the presence of an anionic polysaccharide is triggered by β-lactoglobulin unfolding and/or aggregation.