Articulo

APPLIED AND ENVIRONMENTAL MICROBIOLOGY, Nov. 1986, p. 1055-1059

Vol. 52, No. 5

0099-2240/86/111055-05$02.00/0 Copyright C 1986, American Society for Microbiology

Direct Fermentation of Potato Starch to Ethanol by Cocultures of Aspergillus niger and Saccharomyces cerevisiaet
MOHAMED M. ABOUZIED AND C. ADINARAYANA REDDY* Department of Microbiology and Public Health, Michigan StateUniversity, East Lansing, Michigan 48824-1101
Received 21 March 1986/Accepted 1 August 1986

Direct fermentation of unhydrolyzed potato starch to ethanol by monocultures of an amylolytic fungus, Aspergilus niger, and cocultures of A. niger and Saccharomyces cerevisiae was investigated. Amylolytic activity, rate and amount of starch utilization, and ethanol yields increased several-fold in cocultureversus the monoculture due to the synergistic metabolic interactions between the species. Optimal ethanol yields were obtained in the pH range 5 to 6 and amylolytic activity was obtained in the pH range 5 to 8. Ethanol yields were maximal when fermentations were conducted anaerobically. Increasing S. cerevisiae inoculum in the coculture from 4 to 12% gave a dramatic increase in the rate of ethanolproduction, and ethanol yields of >96% of the theoretical maximum were obtained within 2 days of fermentation. These results indicate that simultaneous fermentation of starch to ethanol can be conducted efficiently by using cocultures of the amylolytic fungus A. niger and a nonamylolytic sugar fermenter, S. cerevisiae.

Large volumes of starchy feedstock such as corn represent an importantbiomass resource for fuel alcohol production, because the chemical composition and high density of starch, compared to other forms of biomass, facilitates prolonged storage and decreased transportation and pretreatment costs. Large quantities of starch-rich agroindustrial residues such as potato-processing wastes represent another important resource which could be fermented to yield ethanol. Forexample, an estimated 3.6 x 109 kg of wet potato-processing wastes is generated annually in the United States (3) which is potentially convertible by fermentation into about 126 x 109 gallons (476.9 x 109 liters) of ethanol. The production of industrial and fuel ethanol from starchy biomass commonly involves a three-step process (7): (i) liquefaction of starch by an endoamylase such as a-amylase; (ii)enzymatic saccharification of the low-molecular-weight liquefaction products (dextrins) to produce glucose; and (iii) fermentation of glucose to ethanol. Commercial amylases (frequently those produced by Aspergillus species) are used for liquefaction and saccharification of starch and represent a significant expense in the production of fuel alcohol from starchy materials. Initial studies aimed atthe elimination of the enzymatic liquefaction and saccharification step by using symbiotic coculture of amylolytic and sugar-fermenting organisms have been promising. For example, in the "Symba" process for single-cell protein production from potato-processing wastes, Jarl (4) and Skogman (11) eliminated the enzymatic liquefaction and saccharification step by using a coculture ofEndomycopsisfibuligera (an amylolytic yeast) and Candida utilis (a nonamylolytic sugar utilizer). Laluce and Mattoon (7) suggested the use of Saccharomyces diastaticus for direct conversion of manioc starch to ethanol; however, prior treatment of the starch with a-amylase was required for obtaining efficient fermentation of starch. The purpose of this study was to develop and evaluate a simultaneous
t Joumalarticle no. 11944 from the Michigan Agricultural Experiment Station.
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single-step system for the enhanced fermentation of potato starch to ethanol by using symbiotic cocultures of Aspergillus species, which hydrolyze starch to glucose, and S. cerevisiae, which is nonamylolytic but efficiently ferments glucose to ethanol.
MATERIALS AND METHODS Organisms. Amylase-producing fungi, A. niger...