Abstract
BackgroundLevan is a well-known homopolymer of fructose composed predominantly of β-(2, 6) fructofuranosyl linkages in the backbone with occasional β-(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 34.86 g/L with a corresponding productivity of 7.47 g/L/d under the optimal cultivation conditions deduced by RSM, i.e., cultivation at 26 °C and 200 rpm for 112 h in tomato juice supplemented with 172 g/L sucrose with an initial pH value of 6.12. The Mw of levan reached a peak value of 2.320 × 107 Da at 6 h of cultivation under the optimized cultivation conditions and then gradually decreased to 8.809 × 106 Da after 120 h 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.
Highlights
Levan is a well-known homopolymer of fructose composed predominantly of β-(2, 6) fructofuranosyl linkages in the backbone with occasional β-(2, 1) linkages in the branch chains with varied applications
Temperature, initial pH and sucrose concentration of Tomato juicesucrose medium (TJSM) on Levan production Figure 1a illustrated the effect of cultivation temperatures from 15 °C to 35 °C on levan production by the strain L.citreum BD1707
High levels of sucrose are beneficial for levan synthesis in TJSM by the strain L.citreum BD1707
Summary
Levan is a well-known homopolymer of fructose composed predominantly of β-(2, 6) fructofuranosyl linkages in the backbone with occasional β-(2, 1) linkages in the branch chains with varied applications. High production cost due to low yield of microbial levan has become a bottleneck for its practical applications. Levan is a well-known homopolymer of fructose composed predominantly of β-(2, 6) fructofuranosyl linkages in the backbone with occasional β-(2, 1) linkages in the branch chains and is widespread in both of plants and microorganisms. Due to its well water solubility, high molecular mass and low viscosity, microbial levan can be used as emulsifing, stabilizing, thickening and encapsulating agent in the food industry [3] and as blood plasma volume extender [4], antiobesity agent or hypocholesterolemic agent, etc., in the pharmaceutical industry [5]. Owing to its moisturizing properties, low cell cytotoxicity, promoting mammalian cell proliferation and anti-inflammatory effect, levan can be utilized in cosmetic products [6]
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