Tailoring the morphology and rheology of phase-separated biopolymer gels using microbial cells as structure modifiers

Four nonviable, edible single-celled microorganisms (Lactobacillus delbrueckii subsp. bulgaricus, Saccharomyces cerevisiae, spirulina and chlorella) were used as micron-sized particles to control the microstructure and rheology of phase-separated gelatin (6 wt %) and maltodextrin (6 wt %) gels. Mixed biopolymer solutions containing 2 wt % cells were quenched from 90 °C to a holding temperature (Th) of 40 °C for 2 different holding times (th) of 30 s or 10 min. Samples were then quenched to 25 °C for rheological measurements and microstructural analysis. Presence of cells altered the phase separation kinetics and resulting microstructure. Notably, L. bulgaricus and S. cerevisiae yielded a bijel-type bicontinuous structure. Addition of cells generally limited the loss in gel elastic modulus (G′) normally associated with thermally-induced protein-polysaccharide phase separation. Strain sweep measurements suggested that phase-separated gels became stiffer in the presence of the cells. The increase in the rate of phase separation typically seen with addition of NaCl was greatly curtailed by the presence of the microorganisms, but only if they were negatively-charged (yeast and algae). Addition of the positively-charged bacteria intensified the adverse effect of NaCl on G′. These results demonstrated that nonviable, edible single-celled microorganisms may be used as a novel form of natural structure modifiers for applications ranging from gel fillers to Pickering stabilization of water-in-water emulsions. Finally, these results present the first instance of an edible bijel-like structure.