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.
Highlights
Commercial lignocellulolytic enzymes are mainly produced through submerged fermentation (SmF) systems
As it can be seen, this substrate contains a significant amount of hemicellulose and cellulose, making this leftover a potential source of sugars after a hydrolysis process
These values are in accordance with those previously found by other authors such as del Río et al [44] but slightly different compared with Mussatto et al [43] and Paz et al [23] wherein they found higher lignin contents for brewer’s spent grain (BSG)
Summary
Commercial lignocellulolytic enzymes are mainly produced through submerged fermentation (SmF) systems Such enzymatic solutions are obtained starting from high-purity substrates implying a significant increase in the total processing costs [1]. Hydrolysis allows transforming cellulose and hemicellulose into fermentable sugars that can be used to produce value-added products [3, 5] This process can be carried out by using chemical hydrolysis or other physicochemical treatments, the current trend is to use lignocellulolytic enzymes such as cellulases and xylanases [7]. Differently than their chemical counterparts, enzymatic processes tend to work under mild conditions making them more attractive from diverse points of view [6, 8]
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