Abstract
Glucose/xylose mixtures (90g/L total sugar) were evaluated for their effect on ethanol fermentation by a recombinant flocculent Saccharomyces cerevisiae, MA-R4. Glucose was utilized faster than xylose at any ratio of glucose/xylose, although MA-R4 can simultaneously co-ferment both sugars. A high percentage of glucose can increase cell biomass production and therefore increase the rate of glucose utilization (1.224g glucose/g biomass/h maximum) and ethanol formation (0.493g ethanol/g biomass/h maximum). However, the best ratio of glucose/xylose for the highest xylose consumption rate (0.209g xylose/g biomass/h) was 2:3. Ethanol concentration and yield increased and by-product (xylitol, glycerol, and acetic acid) concentration decreased as the proportion of glucose increased. The maximum ethanol concentration was 41.6 and 21.9g/L after 72h of fermentation with 90g/L glucose and 90g/L xylose, respectively, while the ethanol yield was 0.454 and 0.335g/g in 90g/L glucose and 90g/L xylose media, respectively. High ethanol yield when a high percentage of glucose is available is likely due to decreased production of by-products, such as glycerol and acetic acid. These results suggest that ethanol selectivity is increased when a higher proportion of glucose is available and reduced when a higher proportion of xylose is available.
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