Impact of Water Content on Enzymatic Modification of Wheat Bran.

摘要

Enzymatic treatments of plant-based materials are generally conducted in excess water because reduction of water content usually decreases enzymatic conversion.Processing at high solids content would offer economical advantages, but in the area of enzymatic bioprocessing of plant materials for food applications, the role of water content has seldom been studied. Wheat bran is one of the most important by-products of the cereal processing industry and comprises the outer tissues of grain. Bran is a good source of dietary fibre (DF), protein and phytochemicals,but its use in food applications is limited because unprocessed bran is usually detrimental to product quality. The present work aimed to examine and develop techniques to utilize hydrolytic enzymes, especially xylanase, at reduced water content in order to increase the technological functionality of wheat bran in food applications.The impact of water content on the action of xylanase was studied by treating wheat bran with a commercial xylanase enzyme preparation at water contents ranging from 20 to 92% using different processing methods including both continuous mixing and short pre-mixing combined with stationary incubation. The actionof xylanase was measured by the solubilisation of bran arabinoxylan (AX), the main DF polysaccharide of wheat. The minimum required water content for the action of xylanase on wheat bran during continuous mixing was between 20 and30%, corresponding to a water activity (aw) of 0.83–0.89. Xylanase action was significantly enhanced at a water content of 40% (aw 0.93), at which the granular material was transformed to a continuous paste. AX was solubilised at a similar level at 40 and 90% water contents when continuous mixing was used, but at water contents of 50–80% AX solubilisation was lower. Furthermore, it was shown that the use of an extruder for pre-mixing and forming a bran-water mixture increased the action of xylanase during stationary incubation at a water content of 54%, as compared to pre-mixing with a blade-mixer. The results indicated that the formation of a continuous paste is important for efficient enzyme action at low water content, and that it is possible to increase the enzyme action by changing the granular structure of the material to a continuous paste using an extruder, without increasing the water content. The extruder-aided pre-mixing process enable defficient xylanase action at low water content without the requirement for ontinuous mixing.