Submitted to: Biotechnology Letters
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 20, 1996
Publication Date: N/A
Interpretive Summary: Agricultural biomass such as plant stalks, grasses, wood chips, and paper refuse, represents an abundant renewable resource that can be used for the production of liquid fuels such as ethanol. The first requirement before these products can be utilized is that they be broken down into simple sugars that can be subsequently fermented by microorganisms. This "break-down" or hydrolysis step is typically performed with costly enzymes produced by molds called filamentous fungi. The fermentation step can then be performed with Saccharomyces cerevisiae or baker's yeast, which is most often used for current alcohol production. However, this organism is unable to utilize many of the predominant sugars found in agriculture material. Certain strains of filamentous fungi have the unique ability to not only hydrolyze agricultural biomass, but can also ferment most of the sugars produced. The goal of this study was to screen numerous filamentous sfungi to find those that are efficient at producing ethanol and are considered safe for large-scale use. Additionally, we focused on fungi that can be further improved through genetic engineering techniques. Several fungi were identified that converted simple sugars to ethanol and were also able to convert complex materials such as oat fiber, corn germ pressings and corn fiber directly to ethanol.
Technical Abstract: We previously demonstrated that it was possible to improve ethanol fermentation in the fungus Aspergillus nidulans by heterologous expression of the Zymomonas mobilis pyruvate decarboxylase gene. While A. nidulans was a useful tool for this study, final ethanol concentrations were low and mycotoxin production limited its potential use for large-scale fermentations. The Aspergilli and Rhizopus, particularly those used in food or industrial applications, are desirable alternatives because of their safety and efficient utilization of diverse substrates. We have screened 19 Aspergilli and 10 Rhizopus strains for their ability to ferment simple sugars (glucose, xylose, and arabinose) as well as complex substrates (cellulose, oat-spelt xylan, corn cob, and corn germ pressing). One strain of A. oryzae was identified that converted 50 g/l glucose to ethanol with almost 100% theoretical yield. This strain also converted 50 g/l xylose to ethanol with a 22% theoretical yield. Five of the Rhizopus strains produced more than 23 g/l ethanol from 50 g/l glucose in only three days. Several of these strains also produced ethanol from cellobiose with approximately 80% theoretical yield. R. oryzae NRRL 1501 completely utilized 100 g/l glucose in less than three days, achieving more than 30 g/l ethanol. It was determined that R. oryzae NRRL 1501 and NRRL 3133 had an optimal fermentation temperature of 33 deg C and fermented corn fiber to ethanol in three days with approximately 40% theoretical yield.