Characterization of a bioflocculant produced by lactic acid bacterium Leuconostoc mesenteroides NBRC 100496

Extracellular polymers produced by microorganisms are utilized as bioflocculants. In this study, bioflocculant-producing lactic acid bacteria were screened from seven strains based on the flocculation activity of their cultures. The Leuconostoc mesenteroides NBRC 100496 culture exhibited strong flocculation activity, with a 79.5% flocculation rate against kaolin clay. The optimal concentration of sucrose as a carbon source for the cultivation was investigated by measuring the flocculation activity of the L. mesenteroides culture. The addition of 30% sucrose was appropriate for kaolin flocculation. Kaolin exhibited a negative zeta potential, and the excessive presence of trivalent cations (Al³⁺ and Fe³⁺) caused charge inversion on the kaolin surface. The L. mesenteroides culture flocculated kaolin even under conditions where Al³⁺ and Fe³⁺ were present in excess. The glucan involved in flocculation was purified by isopropanol precipitation and alkaline treatment. The purified glucan lost its flocculation activity after dextranase treatment. Dynamic light scattering analysis revealed that the average particle size of the purified glucan was 361 nm, which was larger than that of commercially available dextrans (average molecular weight, 400,000-500,000). These results suggest that the glucan involved in flocculation exhibits α-1,6-glycosidic linkages, the particle size of the glucan is extremely large, and flocculation is caused by physical adsorption.