pH-profiling in SPORL for high titer and yield ethanol production from undetoxified whole slurry of Douglas Fir forest residue

Reducing sugar degradation to fermentation inhibitors during pretreatment of lignocellulosic biomass is critically important to high titer biofuel production to reduce production cost. Often times, this is achieved by using low severity pretreatment at the expense of enzymatic saccharification and sugar yield. Here, we incorporate low temperature pretreatment strategy with extended reaction time (based on kinetic calculations) to maintain pretreatment severity to achieve high enzymatic saccharification of Douglas-fir forest residue, a cost effective and but very recalcitrance feedstock. Furthermore, we proposed a novel but simple approach – “pH-profiling” in SPORL pretreatment simply by delaying acid application to further substantially reduce sugar degradation. “pH-profiling” reduced furan production by approximately 70% in using SPORL pretreating Douglas-fir forest residue at 165°C for 75 min comparing with the control run at the same temperature and time while without sacrificing enzymatic saccharification of the resultant substrate. “pH-profiling” also reduced carbohydrate degradation. As a result, “pH-Profiling” substantially facilitated high solids fermentation at 21 wt% of the un-detoxified pretreated whole slurry.  A terminal ethanol titer of 48.9 ± 1.4 g/L and yield of 297 ± 9 L/tonne were achieved, comparing with 38.6 ± 7.5 in titer and 215 ± 42 L/tonnne in yield from a control run without pH profiling. Based on the amount of furan formation, the estimated potential solids loading over 35% can be implemented for efficient fermentation without detoxification.