Mushroom spent straw: a potential substrate for an ethanol-based biorefinery

Abstract

Rice straw (RS) is an important lignocellulosic biomass with nearly 800 million dry tons produced annually worldwide. RS has immense potential as a lignocellulosic feedstock for making renewable fuels and chemicals in a biorefinery. However, because of its natural recalcitrance, RS needs thermochemical treatment prior to further biological processing. Ammonia fiber expansion (AFEX) is a leading biomass pretreatment process utilizing concentrated/liquefied ammonia to pretreat lignocellulosic biomass at moderate temperatures (70–140°C). Previous research has shown improved cellulose and hemicellulose conversions upon AFEX treatment of RS at 2:1 ammonia to biomass (w/w) loading, 40% moisture (dwb) and 90°C. However, there is still scope for further improvement. Fungal pretreatment of lignocellulosics is an important biological pretreatment method that has not received much attention in the past. A few reasons for ignoring fungal-based pretreatments are substantial loss in cellulose and hemicellulose content and longer pretreatment times that reduce overall productivity. However, the sugar loss can be minimized through use of white-rot fungi (e.g. Pleutorus ostreatus) over a much shorter duration of pretreatment time. It was found that mushroom spent RS prior to AFEX allowed reduction in thermochemical treatment severity, while resulting in 15% higher glucan conversions than RS pretreated with AFEX alone. In this work, we report the effect of fungal conditioning of RS followed by AFEX pretreatment and enzymatic hydrolysis. The recovery of other byproducts from the fungal conditioning process such as fungal enzymes and mushrooms are also discussed.