Production of ethanol from wood hydrolyzate by yeasts

A total of 43 Forest Products Laboratory (FPL) strains of Pichia stipitis and Candida shehatae were tested for their ability to ferment a 1:1 mixture of glucose and xylose to ethanol prior to fermentation of partially deacidified wood hydrolyzates. The starting sugar composition, pH, and concentrations of inhibitors such as acetic acid, furfural, and hydroxy methyl furfural varied from one batch to another. The delay observed in growth and fermentation depended on the amounts of inhibitors present and on the capacity of the strain to resist them. The ethanol production rates and yields obtained with C. shehatae were higher than those with P. stipitis. C. shehatae strain FPL-Y-049 produced up to 34 g/l ethanol from batch VII of wood hydrolyzates. All the glucose, mannose, galactose and xylose in the wood hydrolyzate were consumed during fermentation. Only arabinose was unused. Addition of 10 mg/l zinc to acid hydrolyzate did not affect peak ethanol production, but it did increase rates of sugar utilization and ethanol production. The fermentation of hydrolyzate with recycled cells of C. shehatae Y-049 reduced the fermentation lag and increased the final ethanol concentration. The ethanol production rate was optimum in the pH range of 5.5–6.0, and an ethanol yield of 0.41–0.46 g/g was obtained in these fermentations. Because of heterogeneity between wood hydrolyzate batches, ethanol production was found to be influenced by hydrolyzate composition, pH, acetate concentration, amount of cells, and recycling of cells.