Production of dextransucrase, dextran and fructose from sucrose using Leuconostoc mesenteroides NRRL B512(f)

Abstract

The production of dextransucrase, dextran and fructose by sucrose fermentation using Leuconostoc mesenteroides NRRL-B512(F) was studied in batch operation in a bioreactor with total working volume of 1.5 dm³. The effect of temperature (20 to 40ºC), pH (5.5 and 6.7) and sucrose concentration (10 to 120 g/l) on process performance was studied. The optimum conditions for dextran and fructose production were T=35ºC and pH=5.5. Cell growth is not inhibited by high sucrose concentrations; however, for sucrose concentration higher than 40 g/dm³ separation of products from cells is difficult. Biomass (X), enzyme (E), dextran (D), fructose (F) and sucrose (S) rate equations were considered in order to derive a simple fermentation kinetic model from batch experimental data. The logistic equation provided a reasonable description for cell concentration,X. The Luedeking and Piret equation was used to describe the enzyme production rate, by considering only the growth associated term. The concentrations of products (dextran and fructose) were reasonably described by a first order kinetic law with respect to both substrate and enzyme concentrations; the substrate, S was consumed for cell growth and for dextran and fructose production. Model parameters µ_m and X_o were calculated from cell growth as a function of time. The yield Y_E/X were calculated from X_max and E_max and Y_X/S was estimated from X_max and the sucrose consumed by the bacteria. The remaining parameter k' was obtained by fitting the experimental data of substrate, dextran and fructose concentrations versus time.