Impact of water content on enzymatic modification of wheat bran

Outi Santala

    Research output: ThesisDoctoral ThesisCollection of Articles

    Abstract

    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 action of 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 and 30%, 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 enabled efficient xylanase action at low water content without the requirement for continuous mixing. Neither water content nor processing method affected the apparent average molecular weight (MW) of water extractable AX (WEAX) precipitated with 65% EtOH at water contents above 40%. When bran was treated with continuous mixing, the A/X ratio of the bran water extract decreased similarly at both water contents of 40% and 90%, suggesting that AX was solubilised from the same bran tissues regardless of the processing conditions studied. The bran treated at a water content of 40% was characterized by higher solubilisation of DF polysaccharides, smaller average particle size, lower water holding capacity and more changes in bran proteins than the treatment at a water content of 90%. The more intensive changes in the properties of bran treated at low water content were related to the compact consistency and thus higher impacts of shear exerted on the bran-water mixture. Reduction of particle size, either prior to the treatment by grinding or during the treatment by intensive mixing and shear, was shown to enhance AX solubilisation and xylanase action, presumably due to improved substrate availability as a result of increased surface area. Small particle size also favoured the transformation of the bran-water mixture from granular mass to a continuous paste, which also enhanced enzyme action. The technological functionality of modified bran was demonstrated in endosperm-flour based expanded extrudates supplemented with 20% of bran. Bran was treated with commercial xylanase and cellulase enzymes at a water content of 48% using the extrusion-aided low-water process, followed by oven or freeze drying. The modified bran ingredients increased the crispiness and reduced the hardness and bulk density of the bran-enriched expanded extrudates. The improvements in extrudate properties were attributed to the increased WEAX content and decreased water holding capacity of the modified brans. The results of the work showed that enzymatic solubilisation of bran AX and improved technological functionality of bran can be achieved by enzymatic modification at a water content of 40–50%, which is well below the point of absence of free bulk water (70–80%). The consistency of the reaction mixture, mixing method and bran particle size were found to be important factors affecting the intensity of the modification process at low water content. The results can be utilized for improving the technological functionality of bran in food applications and for developing new processes for the enzymatic modification of plant raw materials at reduced water content.
    Translated title of the contributionVesipitoisuuden vaikutus vehnäleseen entsymaattisessa muokkauksessa
    Original languageEnglish
    QualificationDoctor's degree
    Awarding Institution
    • Aalto University
    Supervisors/Advisors
    • Ojamo, Heikki, Supervising Professor
    • Poutanen, Kaisa, Thesis Advisor, External person
    • Nordlund, Emilia, Thesis Advisor
    Publisher
    Print ISBNs978-951-38-8149-8
    Electronic ISBNs978-951-38-8266-2
    Publication statusPublished - 2014
    MoE publication typeG5 Doctoral dissertation (article)

    Keywords

    • wheat bran
    • arabinoxylan
    • xylanase
    • enzymatic modification
    • solubilisation
    • water content
    • high solids hydrolysis
    • bran particle size
    • high torque mixing
    • stationary incubation
    • extrusion
    • mechanical properties
    • extrudate structure

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