Advanced Biofuels and Biorefinery Platforms A Novel Pretreatment Process Using Oxalic Acid on Waste Mushroom Medium for Production Fermentable Sugar and Ethanol

Cauliflower mushroom (Sparassis crispa) is well known as a brown-rot fungi selectively degrades heartwood of wood, and that has potential anticancer and immune enhancing activity; thus its production is increasing in Korea. The mushroom is cultivated artificially using plastic pots filled with medium material, such as lignocellulosic biomass and some additives. However, most of the medium remaining after mushroom cultivation has not been utilized, except for compost. Therefore, the utilization of this waste mushroom medium could be proposed as a valuable resource, such as bioethanol and other biochemical products. In this study, we used temperature gradient for oxalic acid pretreatment to produce high fermentable sugar. We then assessed the utility of the hydrolysates for ethanol production. Solid biomass obtained after oxalic acid pretreatment was evaluated for its properties as solid fuel, in terms of biorefinery. The hydrolysates of waste mushroom medium contained high glucose and low concentrations of inhibitors. The glucose concentration in the hydrolysate particularly increased when temperature gradient was used during pretreatment, compared with that of isocratic condition. The highest increase rate of glucose was 63.16% when pretreatment was performed at 140°C for 25 min with 0.032 oxalic acid (g/g), and increased temperature to 170°C. At the same time, ethanol production from the hydrolysate was 15.72 g/l after 48h, which corresponding to an ethanol volumetric productivity of 0.33 g/l/h. Most of the lignin and some of the cellulose remained in the pretreated biomass. The calorific value in the pretreated biomass increased compared to the raw material, due to higher contents of lignin in the pretreated biomass. This study shows that waste mushroom medium has enough potential as a material for developing biorefinery. The waste mushroom medium easily converted to sugars by oxalic acid pretreatment at mild condition. In particular, the temperature gradient was a more efficient method than isocratic condition during pretreatment for production of fermentable sugar. At the same time, the hydrolysate produced high ethanol by P. stipitis without any treatment to remove inhibitors before fermentation. Additionally, pretreated waste mushroom medium could provide a suitable condition for a solid fuel, since it has been efficiently pretreated with high lignin and low ash content.