Ethanol production by mixed-cultures ofPaenibacillus sp. andZymomonas mobilis using the raw starchy material from sweet potato

The conversion of raw starchy materials from sweet potato into ethanol in a mixed-culture of an amylolytic bacterium, Paenibacillus sp. and different strains of Zymomonas mobilis were studied. Raw starchy material from sweet potato was hydrolyzed by Paenibacillus sp. to glucose, which is directly used by Z. mobilis for ethanol production. A mixed-culture of Z. mobilis ATCC 29191 and Paenibacillus sp. 9 yielded the highest ethanol concentration (6.89 g/l) in cultures of small volumes. The effects of medium pH (the best pH value were between 5.0 and 6.0) and the supplementation of the medium with glucose on ethanol production were also studied. In a large-scale fermentation, the final concentration of ethanol was 6.60 g/l, which corresponds to 23.24% of the theoretical yield of the ethanol from 50.0 g/l of the starch from raw sweet potato after a 120 h fermentation period.