Sequential Alkaline and Ultrasound Pretreatments of Oat Hulls Improve Xylanase Production by Aureobasidium pullulans in Submerged Cultivation

Abstract

Xylanases are hydrolase enzymes mainly produced by microorganisms and applied in several industrial segments. In this paper, the effects of different pretreatments of oat hulls were evaluated on the production of xylanase by Aureobasidium pullulans CCT 1261 under submerged cultivation. Oat hulls were subjected to several pretreatments: alkaline (0.5, 1, 2, 3, and 4% of NaOH, w/v), alkaline hydrogen peroxide (7.5% of H2O2, v/v), ultrasound-assisted alkaline (1% of NaOH, w/v, 90 W, 20 kHz), and sequential alkaline and ultrasound procedures. Both untreated and pretreated oat hulls were used as substrates for xylanase production by the strain CCT 1261. Changes caused by the pretreatments were observed through the characterization of oat hulls (i.e. lignocellulosic composition and scanning electron microscopy) and then related to the enzymatic production. Sequential alkaline and ultrasound pretreatments on oat hulls promoted the highest xylanase production (30.2 U/mL) and productivity (0.42 U/mL.h) in comparison to the other strategies evaluated. The sequential pretreatment of oat hulls also increased xylanase production more than 11-fold when compared to the untreated substrate. The effectiveness of the sequential procedure, and consequent increase in xylanase production, were attributed to the physical modification of the biomass surface by the ultrasound waves, as well as the lignin removal and hemicellulose maintenance by the alkaline pretreatment with 1% NaOH. Sequential pretreatment of oat hulls allowed the production of microbial xylanases in a more sustainable way using an agricultural by-product as substrate.