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Abstract
The cultivation conditions for Rhizopus oryzae grown in synthetic medium and paper
pulp spent sulfite liquor (SSL) were investigated to achieve high biomass and ethanol
yields using shake flasks and bioreactors. The fungus assimilated the hexoses glucose,
mannose and galactose, and the pentoses xylose and arabinose as well as acetic acid
which are present in SSL. The assimilation of hexoses was faster than pentoses during
cultivation in a synthetic medium. However, all sugars were assimilated concomitantly
during growth in SSL supplemented with ammonium, magnesium, calcium, phosphate, sulfate
and trace amounts of some other metal ions (SSL-S). The medium composition had an
important influence on biomass yield. The highest biomass yields, viz. 0.18 and 0.43
g biomass/g sugar were obtained, when the cells were cultivated in shake flasks with
a synthetic medium containing glucose as carbon and energy source and SSL-S, respectively.
The corresponding yields in a bioreactor with more efficient aeration were 0.22 and
0.55 g/g. In addition to the biomass, ethanol, lactic acid, and glycerol were important
extracellular metabolites of the cultivation with maximum yields of 0.37, 0.30 and
0.09 g/g, respectively. When the source of sugars in the medium was exhausted, the
fungus consumed the metabolites produced, such that the liquid medium was depleted
of potential oxidizable nutrients. In general, there was a direct competition between
lactic acid and ethanol among the metabolites. Poor medium compositions and cultivation
conditions resulted in higher yields of lactic acid, whereas the ethanol and biomass
yields were higher in rich media. SSL-S supported good growth of mycelium and a high
ethanol yield.