Anaerobic Fermentation of Woody Biomass Pretreated with Supercritical Ammonia

The degradability of ground hardwood by thermophilic anaerobic bacteria (Clostridium thermocellum with or without Thermoanaerobacter strain B6A) was greatly enhanced by pretreatment of the substrate with supercritical ammonia. Relative to C. thermocellum monocultures, cocultures of C. thermocellum and Thermoanaerobacter strain B6A degraded 1.5-fold more pretreated soft maple but produced 2- to 5-fold more fermentation endproducts because Thermoanaerobacter sp. removed reducing sugars produced by C. thermocellum during the fermentation. Dry weight losses were not totally accounted for in end products, due to formation of partially degraded material (<0.4 μm diameter wood particles) during the fermentation. One pretreated hardwood, Southern red oak, was fermented poorly because it released soluble inhibitors at the 60°C incubation temperature. Considerable (6- to 11-fold) increases in substrate degradability were also noted for supercritical ammonia-pretreated wood materials fermented in an in vitro rumen digestibility assay. Degradation of pretreated softwoods by either thermophilic or mesophilic fermentation was not measurable under the conditions tested.