In a continuous-flow bioreactor study,
immobilized S. cerevisiae cells were found as the best alcohol producer
from 1% to 5% glucose than immobilized Z. mobilis cells [18].
However, there are reports also to demonstrate that Z. mobilis is a
better ethanol producer than S. cerevisiae because of its higher sugar
to ethanol conversion rate [19]. Raman and Pothiraj [20] reported
that ethanol production from cassava (Manihat esculenta
Crantz) at pH 6.0 and 36 h (residence time) of fermentation was
the highest in the Z. mobilis (9.26 g l1) mediated fermentation
(5% more) than the ethanol production rate of S. cerevisiae
(8.73 g l1). In yet another study, co-immobilized cells of S. cerevisiae
and Z. mobilis produced a high ethanol concentration compared
to immobilized cells of S. diastaticus during batch
fermentation of liquefied cassava starch [21]. The co-immobilized
cells produced 46.7 g l1 of ethanol from 150 g l1 liquefied cassava
starch, while immobilized cells of yeast, S. diastaticus produced
only 37.5 g l1 ethanol. It seems from the literature that
there is no consistency in report which organism, S. cerevisiae or
Z. mobilis, is superior in ethanol production from sugary/starchy
biomass. According to Karsch et al. [10], for industrial fermentation,
Z. mobilis seems to be inferior to S. cerevisiae, because the
advantage of the higher ethanol and the lower biomass production
of the bacterium is disadvantaged by the decrease in pH from 5.5
to 3.8 (as in the present study) during yeast fermentation thus
greatly impeding the occurrence of contaminants and making sterilization
of the medium not necessary; whereas, in case of Z. mobilis,
there might be a chance of contamination. Besides, during
ethanol fermentation, Z. mobilis encounters various environmental
stresses which adversely affect the ability of cells to perform efficiently
and consistently in converting sugars to ethanol. The major
stresses experienced by this microorganism are ethanol toxicity
and heat, which accumulate during metabolism of cell