When liquid hydrolysate before membrane separation was fed
to the recombinant S. cerevisiae, glucose and xylose were co-fermented,
as described previously. The final
ethanol concentration after 1 day of fermentation was
2.75 ± 0.56 g L–1 when the starting concentrations of glucose and
xylose were 5.65 ± 0.13 and 2.71 ± 0.12 g L–1, respectively
(Fig. 2a). The apparent ethanol yield, representing the ratio of the
amount of ethanol produced to the amount of glucose plus xylose
consumed, was 0.34. Small amounts of glycerol and xylitol were
produced (0.32 ± 0.28 and 0.06 ± 0.08 g L–1, respectively). Similarly,
recombinant S. cerevisiae co-fermented glucose and xylose in
membrane-concentrated liquid hydrolysates. The final ethanol
concentration after 1 day of fermentation was increased to 5.34–
6.45 g L–1 when the starting concentrations of glucose and xylose
were 9.42–11.00 and 4.70–5.65 g L–1, respectively (Fig. 2b, c, and
d). The apparent ethanol yields were essentially unchanged
compared with that for the liquid hydrolysate before membrane
separation: 0.41, 0.38, and 0.43 using NTR-729HF, NTR-7250 and
ESNA3 concentrated liquid hydrolysates, respectively. These values
are similar to the previous report using rice straw liquid hydrolysates
and recombinant S. cerevisiae (0.36–0.49) (Matsuda et al.,
2011). The production of glycerol and xylitol was low
(0.63 ± 1.20 and 0.11 ± 0.14 g L–1, respectively). These results show
that ethanol fermentation of concentrated liquid hydrolysates of
dilute acid-pretreated rice straw using recombinant S. cerevisiae
can be conducted efficiently.