| Summary: | As part of ongoing research to evaluate the bioconversion of softwood feedstocks to ethanol, the feasibility of using Douglas-fir residues was investigated. Whitewood feedstocks were pretreated using SC₂-catalyzed steam explosion under three pretreatment severity conditions (low, medium and high) with a goal of demonstrating efficient fermentation of the hemicellulose-rich water-soluble (WS) fraction. The chosen severity had a pronounced effect on the recovery of the hemicellulose sugars from the feedstock, as well as the proportion of monomeric sugars. Low-severity pretreatment resulted in the greatest recovery of hemicellulose sugars, with yields decreasing significantly for an increase in severity. Reduced-severity pretreatments also resulted in the recovery of fewer sugar decomposition products in the WS fraction, which translated into improved fermentation using an SSL-adapted strain of Saccharomyces cerevisiae. In contrast, the WS fraction obtained under high-severity conditions could not be fermented. Douglas-fir feedstocks containing bark (10, 20, 30 and 100% w/w) were pretreated under medium-severity to evaluate bark's impact on bioconversion. Bark had only a minor impact on the yield of hemicellulose sugars, with no negative effect on the monomeric sugar recovery. However, bark caused a significant decrease in the soluble sugar concentration. Process-derived fermentation inhibitors (e.g., furfural, HMF) also decreased, while naturally occurring inhibitors (e.g., lipophilic compounds) increased with greater bark loading. Despite this increase, bark had no detrimental impact on the rate of fermentation, and all hydrolysates were fermentable to high ethanol yield. Efforts to increase the low sugar concentration in the WS fractions were evaluated, in an attempt to increase the ethanol concentration recovered following fermentation. Increasing the sugar concentration in the WS fraction by physical means resulted in decreased rates of fermentation and reduced ethanol yields, even at low concentration factors (2- to 3-fold). As an alternative strategy, the sugar concentration in the WS fraction was augmented with carbohydrate derived from the water-insoluble cellulose component. Enzymatic hydrolysis of the cellulose component directly in the WS fraction proved unsatisfactory, due to inhibition by both carbohydrate and non-carbohydrate components. However, supplementation (1:1) of the WS fraction with the cellulose hydrolysate obtained separately in buffer provided improved sugar concentration, and significantly faster fermentation due to the effective dilution of inhibitors in the WS fraction. Using this approach, the initial hexose sugar concentration in the whitewood WS fraction was increased by 56%, and a final ethanol concentration of 23.4 g L⁻¹ was obtained.
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