When processing at low water content, i.e. at high consistency, continuous mixing requires a high amount of energy, which may not be feasible in industrial processes. In the study of Santala and her co-workers it was shown that an extruder-aided process enabled efficient xylanase action on wheat bran at low water content without the requirement for continuous mixing. The method may be applicable to other biomass sources as well.
The aim of the work was to test the hypothesis that at low water content the action of hydrolytic enzymes on biomass is enhanced when the raw material is in the form of a continuous mass instead of powder/granular form. Effects of two pre-mixing methods, blade-mixing and extrusion, on xylanase action were studied during stationary incubation (i.e. no stirring) of wheat bran of different particle sizes, also in comparison with incubation at high water content with continuous stirring.
The use of an extruder enhanced arabinoxylan (AX) solubilisation at low water content (<54%), as compared to blade-mixing. AX solubilisation was highest in the high water stirring treatment, but based on molecular weight, xylanase action on solubilised AX was similar as in the extrusion-aided process. Pre-mixing by extrusion enabled efficient enzyme action at low water content without the requirement for continuous mixing, probably due to the enhanced diffusion by the formation of a continuous mass in the extruder. It was also shown that decreasing bran particle size improved AX solubilisation in rather similar manner in all the processes studied.
Compared to traditional stirred-tank reactors, extruders and screw reactors present a cost competitive alternative reactor type for enzymatic modification of biomaterials, especially when targeting dry end products, since the extruders can operate at higher solids content, thus reducing the need for addition and removal of large amounts of water. The extrusion-aided enzymatic modification method used in this study on wheat bran may be applicable to other biomass sources as well.
Santala O., Nordlund E., Poutanen K., Use of an extruder for pre-mixing enhances xylanase action on wheat bran at low water content (2013). Bioresource Technology, 149, 191–199.
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