Brewer’s spent grain biotransformation to produce lignocellulolytic enzymes and polyhydroxyalkanoates in a two-stage valorization scheme

Abstract

Lignocellulolytic enzymes from low-cost sources are gaining attention as a tool to reduce production costs. Such enzymes can be obtained sustainably by diverse fungal strains via solid-state fermentation (SSF) of lignocellulosic-derived residues as substrates. Besides, these enzymes allow hydrolyzing the same residue, releasing fermentable sugars that can be transformed into value-added products. This study shows a two-stage valorization approach for the lignocellulosic leftover brewer’s spent grain (BSG): first, by producing lignocellulolytic enzymes through the SSF of BSG using three fungal strains and, second, by using the self-produced enzymes to hydrolyze the same BSG and obtaining sugar-rich hydrolysates that serve as an alternative carbon source for polyhydroxyalkanoates (PHA) production. From the evaluated set, Aspergillus niger and Thermoascus aurantiacus produced the highest xylanase activities compared with Trichoderma reesei (268 ± 24, 241 ± 10, and 150 ± 24 U per gram of dry BSG, respectively). Also, A. niger extracts resulted in the most effective for releasing sugars from BSG, obtaining up to 0.56 g per gram of dry BSG after 24 h without any pretreatment needed. Thus, the sugar-rich hydrolysate obtained with A. niger was used as a source for producing PHA by using two bacterial strains, namely, Burkholderia cepacia and Cupriavidus necator. Maximum PHA yield was achieved by using C. necator after 48 h with 9.0 ± 0.44 mg PHA·g−1 dry BSG. These results show the significant potential of BSG as raw material for obtaining value-added bioproducts and the importance of multiple valorization schemes to improve the feasibility of similar residue-based systems.