BackgroundLevan is a well-known homopolymer of fructose composed predominantly of beta-(2, 6) fructofuranosyl linkages in the backbone with occasional beta-(2, 1) linkages in the branch chains with varied applications. However, high production cost due to low yield of microbial levan has become a bottleneck for its practical applications. Furthermore, factors affecting the molecular mass of the synthesized levan by Leuconostoc spp. during prolonged cultivation is not fully elucidated.MethodsThe cultivation condition for Leuconostoc citreum BD1707 to synthesize levan was optimized by single-factor experiments and subsequently with response surface methodology (RSM). The average molecular weight (Mw) of levan synthesized by the strain L.citreum BD1707 under the optimized cultivation conditions was monitored by high-performance size exclusion chromatography (HPSEC). Finally, the enzyme with levan-degrading activity was determined by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE).ResultsThe levan yield of BD1707 reached 34.86g/L with a corresponding productivity of 7.47g/L/d under the optimal cultivation conditions deduced by RSM, i.e., cultivation at 26 degrees C and 200rpm for 112h in tomato juice supplemented with 172g/L sucrose with an initial pH value of 6.12. The Mw of levan reached a peak value of 2.320x10(7)Da at 6h of cultivation under the optimized cultivation conditions and then gradually decreased to 8.809x10(6)Da after 120h of cultivation.ConclusionThe levan yield of the strain L.citreum BD1707 could be sufficiently enhanced via cultivation condition optimization. The decrease in molecular mass of the synthesized levan was attributed predominantly to the hydrolytic activity of levansucrase secreted by L.citreum BD1707 during cultivation, with an estimated Mw of 130 KD by SDS-PAGE, while the effect of acid hydrolysis could be nearly neglected.
