Microstructure and stability of skim milk acid gels containing an anionic bacterial exopolysaccharide and commercial polysaccharides

The structure and stability of acid milk gels containing an anionic exopolysaccharide (EPS) produced by Lactobacillus rhamnosus RW-9595M in combination with different commercial polysaccharides were studied. Gel internal structure was visualised using confocal scanning laser microscopy; protein, EPS and polysaccharides were labelled using different fluorophores to resolve their specific location. EPS at 0.01% resulted in lower syneresis (2.5%) than control (3.7%); micrographs showed EPS co-localised with protein and reinforcing network connectivity. However, 0.05% EPS increased syneresis to over 17%, although no significant changes in gel strength were detected. Pectin or agar–agar alone at 0.05% resulted in very low syneresis (<1.8%). Both polysaccharides were found to form a tenuous secondary yet more ramified network, effectively reducing pore size. Mixing pectin or agar–agar with EPS at 0.05% resulted in a loss of their functionality. Anionic polysaccharides with stiff molecular structures, such as xanthan or κ-carrageenan, induced extensive protein self-aggregation and syneresis.