Generation of xylooligosaccharides from microwave irradiated agroresidues using recombinant thermo-alkali-stable endoxylanase of the polyextremophilic bacterium Bacillus halodurans expressed in Pichia pastoris

• Overexpression of thermo-alkali-stable xylanase in Pichia pastoris.
• Improved thermo-stability of the recombinant xylanase.
• Microwave irradiation of alkali soaked agroresidues is a suitable pretreatment.
• Ultrafiltration with 10 and 1 kDa membranes fractionate XOS (DP 2–5) without loss.

The recombinant Pichia pastoris harboring the endoxylanase gene (TSEV1xyl) of Bacillus halodurans TSEV1 yielded a high titer of extracellular xylanase (502 ± 23 U ml−1) on induction with methanol. The purified recombinant xylanase (TSEV1xyl) displayed optimal activity at 80 °C and pH 9.0. The glycosylated recombinant xylanase exhibited higher thermostability (T1/2 of 45 min at 80 °C) than the native enzyme (T1/2 of 35 min at 80 °C). The agroresidues subjected to pretreatment (soaking in alkali followed by microwave irradiation) liberated xylooligosaccharides (XOS) upon hydrolysis with the recombinant xylanase. The removal of unhydrolyzed agroresidues, xylanase and xylose from the hydrolysate by two-step ultrafiltration led to the purification of XOS as confirmed by TLC as well as HPLC analysis.