A Technical Review of Developments in Microbiological Utilization of Wood Sugars

The cellulosic portion of wood residue hydrolyzate and spent sulfite pulp liquor can be converted into ethyl alcohol, butanol, lactic acid, 2,3 butanediol, glycerol, arbitol, erythritol, and yeast by fermentation. Hemicellulose and lignin are by-products, and cannot presently be converted into useful products. Several pulp mills are currently producing ethyl alcohol from spent sulfite liquor by fermentation. Free sulfur dioxide and sulfurous acid inhibit fermentation so they must be removed from the liquor before it is utilized. Yields are approximately 20 gallons of 95 percent alcohol per ton of pulp. The fermentation of wood residue hydrolyzates to ethyl alcohol is in the experimental stage. Low concentration of sugars and toxic compounds in the hydrolyzate are the major problems. For butanol fermentation, it is necessary to pretreat the hydrolyzate to remove toxic compounds like furfural. Yields of neutral solvents, butanol, ethyl alcohol, and acetone or isopropyl alcohol are in the range of 30 percent to 35 percent of the fermentable sugars. The neutral solvents are obtained in approximately the following proportions: 60 percent of butanol, 10 percent ethyl alcohol, and 30 percent of acetone or isopropyl alcohol. Spent sulfite liquor can also be used as a substrate after it has been pretreated with lime. Lactic acid can be produced from the total wood hydrolyzate. With sugar concentrations of 7 to 10 percent, 82 to 88 percent of the sugars are utilized. The product is approximately 95 percent lactic acid and 5 percent acetic acid. Lactic acid can be produced from spent sulfite pulp liquor after it has been pretreated. The yield was 300 pounds of lactic acid and 80 pounds of acetic acid per ton of pulp. 2,3 butanediol has been produced from both hardwood and softwood hydrolyzates after pretreatment to remove inhibiting compounds. Yields were about 35 percent based on the sugar utilized. Glycerol has been produced from wood hydrolyzate by fermentation using sodium bisulfite as a steering agent. Because the recovery of glycerol is difficult, hydrolyzate with high sugar content is needed. Yields of glycerol amount to 28 to 30 percent; ethyl alcohol, 20 to 22 percent; and acetaldehyde, 12 to 14 percent based on total sugar fed to the system. Bench scale studies were made of the production of arabitol and erythritol from a number of yeasts. A yeast was found which would produce only arabitol and another erythritol with only traces of arabitol. Most of the cultures screened produced arabitol with varying quantities of glycerol. Yeast is currently being commercially produced by fermentation of wood hydrolyzates and spent sulfite pulp liquors. The dry yeast contains about 50 percent protein and 3 to 7 percent fat, and is produced in yields of 45 to 50 percent of the sugar consumed.

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