Two-stage dilute acid-catalyzed hydrolytic conversion of softwood sawdust into sugars fermentable by ethanologenic microorganisms

Softwood hemlock residues generated in a sawmill were tested as a feasible agricultural feedstock for the production of high-value bio-based products using the concept of two-stage dilute acid hydrolysis. To convert the carbohydrate contained in the hemlock sawdust, two-stage dilute-acid catalyzed hydrolysis was conducted. In the first stage, sawdust was hydrolyzed at 190°C for 150 s with 1.1wt% sulfuric acid and 46.0 wt% total solids. As a result, 69.3% mannose, 74.2% galactose, 67.7% xylose, and 17.4% glucose of their theoretical maximum yields were achieved, which reveals that hemicellulose was the major target in the first-stage hydrolysis. After recovering soluble sugars by washing from the first-stage hydrolysate with water, the washed first-stage hydrolysate solids were hydrolyzed again in the second stage at more severe conditions, such as 2.6 or 3.5 wt% H₂SO₄, 205 or 210°C, and 80-135 s. After the two stages, the highest overall yields from 100 g of dry hemlock sawdust were a total of 39.3 g of fermentable sugars (36.0 and 3.3 g of hexose and pentose sugars, respectively). These results indicate that hemlock sawdust could be used to produce fermentable sugars which can be converted into other value-added bio-based products.