Sunday, August 1, 2010
Pacific Concourse (Hyatt Regency San Francisco)
Numerous environmental and social benefits could result from the replacement of petroleum-based transport fuels with bio-ethanol converted from cellulosic materials. For cellulose conversion to ethanol, chemical or enzymatic conversion of the substrate to fermentable sugars is followed by fermentation by a microbe such as yeast. Recently, a new approach known as consolidated bioprocessing (CBP), which combines enzyme production, cellulose saccharification and fermentation into a single process, has been proposed that would greatly enhance the cost-effectiveness of bioethanol. However, one of the major drawbacks in CBP is the optimum temperature for saccharification and fermentation. Most cellulolytic enzymes have an optimum temperature around 50°C while most fermenting microbes have an optimum temperature ranging between 28 and 37°C. Accordingly, high-temperature fermentation is in high demand, and thermotolerant strains have been screened for the ability to ferment ethanol. In the present study, a thermotolerant yeast, Kluyveromyces marxianus, which has high growth and fermentation at high temperature, was used as a producer of ethanol from cellulose. The strain was genetically engineered to display Trichoderma reesei endoglucanase and Aspergillus aculeatus β-glucosidase on the cell surface, which successfully converts a cellulosic β-glucan to ethanol directly at 48°C with a yield of 4.24 g/l from 10 g/l within 12 h. The yield (in grams of ethanol produced per gram of β-glucan consumed) was 0.47 g/g, which corresponds to 92.2% of the theoretical yield. This study would support the development of CBP for bioethanol production.