Enhancement of xylanase productivity using industrial by-products under solid suspended fermentation in a stirred tank bioreactor, with a dissolved oxygen constant control strategy

Abstract

Solid suspended fermentation of Bacillus subtilis D3d using different agro-industrial residues was performed in this work for the co-production of xylanase and lichenase. Based on the results, brewer's spent grain (BSG) and corn steep liquor (CSL) combination were selected for further studies. The central composite design with the aim of maximum enzyme production at minimum fermentation time indicated 2.5% BSG and 0.5% CSL at 42 h as the best conditions for highest productivity in the shake flask. The results revealed that CSL at a high level can lead to a reduction in enzyme productivity. In a 2 L stirred tank bioreactor, the productivity of xylanase reached a peak of 10 U mL⁻¹ h⁻¹ using a dissolved oxygen (DO)-constant control strategy at 50% air saturation. Lichenase production increased 1.7 times, but the productivity of this enzyme was not affected by DO levels. The physiological state of solid suspended fermentation was investigated by carbon dioxide evolution rate (CER) and oxygen transfer rate (OTR) through off gas analyzer data.