Performance of Rhizopus, Rhizomucor, and Mucor in ethanol production from glucose, xylose, and wood hydrolyzates

In searching for ethanol producing microorganisms also capable of fermenting pentoses, nine zygomycetes strains including three strains of Rhizopus oryzae, Mucor corticolous, M. hiemalis, M. indicus, Rhizomucor pusillus, R. miehei, and zygomycete IT were examined. Each strain was cultivated on glucose, xylose or dilute-acid hydrolyzate (DAH) as carbon sources, and the production of ethanol, lactic acid, glycerol, xylitol, and succinic acid were investigated. Great similarities but also conspicuous differences were seen between the species, to some extent linked to the genera. All strains were capable of growing on glucose or xylose as single carbon source. With the exception of the two Rhizomucor strains, all produced ethanol. All the strains produced glycerol as by-product, while Rhizopus and Rhizomucor but not Mucor produced lactic acid in significant amounts. All Mucor and Rhizopus strains and one strain of Rhizomucor produced xylitol in the xylose medium, but no xylitol was detected after growth on DAH. All Mucor and two R. oryzae strains were capable of growing on DAH. Two Mucor species, M. hiemalis and M. indicus showed greater ethanol production than the other strains. The ethanol yields by M. hiemalis on glucose, xylose, and DAH were 0.39, 0.18, and 0.44 g/g, respectively, whereas the corresponding results for M. indicus were 0.39, 0.22, and 0.44 g/g. The strains also rapidly consumed hydroxymethyl furfural present in DAH.