Enzymatic Pretreatment of Sugar Beet Pulp for Ethanol Production
Much work has been done in the past two decades on the conversion of cellulosic biomass to ethanol. Interest in this field has increased tremendously in recent years because of concerns over oil prices, energy security, and the environmental impacts of fossil fuel use. Ethanol production from agricultural residues including sugar beet pulp can reduce the economic, political, and environmental impacts of petroleum use while strengthening rural economies. The majority of cellulosic ethanol studies have focused on the use of feedstocks such as corn stover, wheat straw, switchgrass, or hybrid poplar wood chips. In comparison with sugar beets, these feedstocks may have greater potential for ethanol production on a national scale but beet pulp has potential economic advantages that have yet to be fully explored. Because of the unique composition of beet pulp, it does not require the expensive thermochemical pretreatment common for other feedstocks. In addition, the resource is available at sugar processing plants and requires no additional harvest, storage, or transportation. These benefits may make beet pulp ethanol production both feasible and important on a regional basis.
Sugarbeet pulp hydrolyzes to a complex mixture of sugars including glucose, arabinose, and galacturonic acid. We are using a recombinant bacterium, E. coli KO11, and a conventional yeast, Saccharomyces cerevisiae, to ferment this mixture of sugars to maximize ethanol yields and titers. Pulp is hydrolyzed with conventional pectinase and cellulase enzymes and fermented sequentially with the two organisms or in parallel after partial hydrolysis. Maximizing solids loading rates will be a key factor in maximizing ethanol concentrations.
Major Results and Conclusions
We have been able to effectively convert all sugars present in beet pulp hydrolyzate to ethanol. Maximum sugar yields (g sugar/g pulp) occur with solid loading rates of 10% while sugar concentrations (g/L) continue to increase with loading rates as high as 16%. We are using rates of 12% which have give a slightly lower yield but significantly higher sugar, and therefore ethanol, concentrations.