Bioconversion of Glycyrrhiza glabra residue to ethanol by sodium carbonate pretreatment and separate hydrolysis and fermentation using Mucor hiemalis

Abstract

The non-valuable non-edible Glycyrrhiza glabra residue (GGR) was efficiently subjected to inexpensive sodium carbonate pretreatment for enhanced enzymatic hydrolysis and ethanol production. The process included sodium carbonate pretreatment at different concentrations (0.5 and 1 M), temperatures (60, 100 and 120 °C), and retention times (3, 5 and 8 h), followed by separate hydrolysis and fermentation using a rarely-reported fungus Mucor hiemalis. The GGR pretreated with 0.5 M sodium carbonate solution at 120 °C for 3 h exhibited the maximum digestibility yield of 94.6% which was 6.2-fold higher than that observed for the untreated GGR. Consequently, fermentation of GGR hydrolyzates by M. hiemalis led to a maximum ethanol production yield of over 94%. Composition analysis of the biomass indicated that the process could substantially remove the majority of lignin part but left the cellulose relatively intact. Moreover, in-depth analyses of the materials by rapid semi-quantitative methods (e.g., Simons’ stain) as well as FTIR, XRD, and FE-SEM investigations revealed that sodium carbonate pretreatment could not only modify biomass composition, but also expanded cellulosic surface area, increased surface hydrophilicity, enlarged the pores, and improved morphological quality of GGR; thereby, rendered it more accessible to the cellulase. The implication of the study may potentially provide the first insight into commercial utilization of GGR for large-scale bioethanol production in a biorefinery framework. © 2020 Elsevier B.V.