Evaluation of pretreatment options for enzymatic hydrolysis and fermentable sugar production from sweet sorghum

Abstract

Sweet sorghum (Sorghum Bicolor L.) is a promising crop for the production of biofuels due to its high carbohydrate yields, low water requirements, and tolerance to imperfect soil conditions. Numerous options are available for the pretreatment of lignocellulosic biomass. Several pretreatment processes were investigated in order to maximize the overall carbohydrate recovery from sorghum, including mechanical pulping, extrusion, and alkali soaking. In the first part of this study, whole sweet sorghum was pretreated with Thermomechanical pulping (TMP) and Refiner Mechanical Pulping (RMP), which are common technologies in the pulp and paper industry. Enzymatic hydrolysis of TMP pretreated sorghum at 6% (w/v) solid loading resulted in the highest glucose and xylose yields of 60.8 and 12.1% respectively, when cellulase and xylanase loadings of 1.0 mL/g glucan and xylan were used. Using the same enzyme loading, enzymatic hydrolysis of RMP pretreated sorghum resulted in glucose and xylose yields of 41.3 and 10% respectively. In the second part of the study, sweet sorghum bagasse was pretreated using a twin-screw extruder at 80 and 110�C with and without the presence of 1.4 M NaOH. Among all the extrusion treatments that were tested, sorghum bagasse pretreated at 110°C in the presence of alkali resulted in the highest glucose and xylose yields of 62.7 and 46% respectively. Glucose and xylose yields were mostly unaltered with the change in extrusion temperature in the absence of alkali. However, the presence of an alkali improved the glucose and xylose yields significantly. In the treatments with alkali soaking alone, enzymatic hydrolysis of sorghum bagasse that was soaked in alkali at 40% (w/v) solid loading using 2.8, 3.5 and 4.2 M NaOH at several different conditions resulted in glucose yields ranging from 88 to 99%. Glucose yields were high with treatments containing low alkali concentration coupled with high soaking time. Xylose yields up to 78% were obtained with sorghum bagasse that was soaked in 3.5 M NaOH at 90°C for 1 h as well as those that were soaked at room temperature. Furthermore, the alkali soaking was conducted at solid state, with 40% solids, and no washing steps required.