Culture optimization for the production and characterisation of exopolysaccharide of Leuconostoc lactis Miki-A3 isolated from traditional fermented rice beverages

Abstract

Highly viscous exopolysaccharides (EPS) derived from lactic acid bacteria are useful materials for improving the characteristics of food products and are thought to function as soluble dietary fibre. However, their productivity, structure, antioxidant activity, and other functional properties may vary among species and strains. In this study, to clarify the optimal culture conditions for producing highly viscous EPS by Leuconostoc lactis Miki-A3 isolated from rice fermented beverages, the viscosity of the culture medium was measured under various culture conditions. The EPS structures were examined through constituent sugar analysis, attenuated total reflection-Fourier transform infrared spectroscopy, ¹H- and ¹³C nuclear magnetic resonance spectroscopy, and gel filtration chromatography. Furthermore, the superoxide anion (O₂⁻) radical scavenging and Fe-reducing capacities of EPS were measured to determine their antioxidant properties. The optimal culture conditions for EPS production in Miki-A3 were 10 % (w/v) sucrose adjusted to an initial pH of 7.5, with incubation at 23 °C, which is lower than the optimal temperature for growth (30–35 °C), for 24 h. Crude EPS showed a high molecular weight (3800 kDa) and consisted exclusively of α(1→6) glucan. The produced EPS was recovered through precipitation in approximately 60 % (v/v) ethanol, which removed small-molecule contamination. High-molecular-weight EPS showed a strong O₂⁻ radical scavenging capacity. These results may be useful for the large-scale production of Miki-A3-derived EPS and studies of its potential applications. The expression of temperature-sensitive genes and consideration of fermentation materials require further analysis.