Fungal pretreatment of hardwood for cellulosic ethanol production: Formation of by-products and the potential effects on downstream bioconversion processes

Shiitake mushroom cultivation could effectively reduce the recalcitrance of hardwood, facilitating cellulosic ethanol production. However, the shiitake pretreated spent mushroom substrates (SMS) have exhibited significant variations in enzymatic saccharification and fermentation efficiencies in previous studies. Parallel with shiitake cultivation, significant increases in extractives mass were observed in the SMS. This highlights the need to examine the formation of pretreatment by-products and their potential impact on downstream bioconversion processes. In this study, shiitake cultivation resulted in 42.6–47.6 % degradation of lignocellulosic components, with continuous generation of various small molecules. Shiitake demonstrated extensive and selective utilization of these degradation products during fructification. Consequently, non-utilized molecules, including (L)-dehydroascorbic acid, triglochinic acid, and 5-hydroxy-2-methylchromone, accumulated in SMS extractives as pretreatment by-products. The SMS cellulose showed 58.8 % digestibility upon enzymatic saccharification. The resulting hydrolysate was fermented to ethanol by Saccharomyces cerevisiae rendering 83.4 % of the theoretical yield. Although water extraction effectively minimized the by-products accumulation in SMS and hydrolysate, no improvement of enzymatic saccharification and fermentation efficiencies were observed. In contrast to thermochemical pretreatment methods, the by-products derived from shiitake pretreatment had limited inhibitory effects on downstream bioconversion processes. This study provides valuable indications for further optimization of shiitake pretreatment towards industrial implementation. • Shiitake cultivation is a prominent strategy for hardwood pretreatment. • Shiitake selectively utilized lignocellulose degradation products for fructification. • Non-utilized small molecules accumulated in extractives as pretreatment by-products. • The characteristics of shiitake pretreatment by-products were investigated. • By-products had limited inhibitory effects on SMS saccharification and fermentation.