Enhanced ethanol production from Glycyrrhiza glabra residue by fungus Mucor hiemalis

Abstract

Glycyrrhiza glabra residue (GGR) contains 30.5% cellulose and 23.0% hemicellulose and can be considered as a promising low-cost and non-edible feedstock for production of ethanol. For the first time, GGR was subjected to inexpensive sodium hydroxide (SH) pretreatment to enhance fermentable sugars production through enzymatic hydrolysis and subsequent utilization by fungus Mucor hiemalis. The pretreatment was carried out at 5% (w/v) solid loading and different temperatures (0, 28 and 110 °C) with 2, 4, 6, 8 and 10% (w/v) SH solutions. When applying no pretreatment, the glucose and ethanol production yields through separated hydrolysis and fermentation of GGR were only 13.9% and 14.3%, respectively. However, a major improvement was achieved after alkali pretreatment of GGR and the maximum hydrolysis yield of 93.7% was observed when the substrate was pretreated with 4% SH solution at 28 °C for 24 h. Consequently, fermentation of the SH pretreated materials by M. hiemalis led to a maximum 5.9-fold increase in ethanol production yield (∼217 kg per ton of GGR), which was slightly higher than the ethanol yield through the yeast Saccharomyces cerevisiae (∼207 kg per ton of GGR). Semi-quantitative analyses of the substrate after pretreatment indicated that crystallinity reduction and expanded surface area were the main reasons for the observed improvements. In brief, the results revealed that SH pretreatment (4% w/v) at room temperature is an effective strategy to valorize GGR to ethanol through hydrolysis and fermentation by fungus M. hiemalis. © 2017 Elsevier B.V.