Production Of Oligodextran By Using An Enzymatic Membrane Reactor

With molecular weight(Mw)in range of 5.0-8.0 kDa,oligodextrans can be used as commercial precursors of iron-dextran for anemia treatment.Traditionally,fermentation is carried out to produce large dextran molecules which will be further hydrolyzed by acid.Several ethanol precipitations are coupled for final purification of oligodextran products.But the present process results in uneven Mw product and also safety hazards.Moreover,over consumption of ethanol and wastewater discharge would bring severe burden on environment.A novel enzymatic membrane reactor(EMR)prototype has been established in this work,conducting enzyme hydrolysis in place of acid hydrolysis to produce oligodextran,where membrane separation was coupled to in-situ remove the target products.The EMR can be manipulated to control the Mw distribution of the target products and improve their uniformity.Results showed that the permeate flux and oligodextran transmission were greatly affected by the membrane materials.A polyethersulfone(PES)membrane with a molecular weight cut-off(MWCO)of 20 kDa was found to be optimal for permeating the target oligodextran as it successfully controlled the oligodextran Mw within the desired range with a narrow distribution and high permeate flux.After an integrated hydrolysis and filtration for 120 min,a high transmembrane pressure(3 bar)and a low stirring rate(160 rpm)promoted yields over 50%.Efficient removal of target products by high permeate flux is benefit for promoting the product quality by preventing over enzyme hydrolysis,while too high permeate flux may increase the rejection of target products,thus resulting in a low yield.Moreover,the resulting concentration polarization(CP)on membrane surface should be reduced to prevent large oligodextran molecule accumulation,which might diffuse through the membrane leading to a wide Mw distribution of the products.High shear rate on the membrane could mitigate CP and improve the Mw distribution,and dextranase kept stable in the range of agitation speed tested.It was found that both dextran and dextranase caused membrane irreversible fouling during the enzyme hydrolysis process,and the latter favored the enzyme immobilization and contributed to membrane pore size narrowing.By simple pre-filtration of enzyme solution,part of enzyme could be immobilized on the membrane and the impurity proteins passed through the membrane,leading to an increase in specific activity in EMR.In this novel EMR with both immobilized and free enzymes,oligodextran products with higher uniformity were obtained.It was concluded that an EMR was established by selecting a suitable ultrafiltration membrane,and its filtration behavior could be manipulated to control the oligodextran Mw and distribution,producing target oligodextran products with high quality.