(0.6 ± 0.1) mm3 min1, when the ethanol concentration reached
80 g L1, which was significantly lower than that of
(5.1 ± 0.1) mm3 min1 in R1, where the ethanol concentration was
approximately 60 g L1 (Fig. 7). Since there was no increasing trend
in lactic acid concentration or acetic acid concentration (Fig. 6C),
the decrease in ethanol concentration and yeast cell viability
should not be caused by bacterial contamination. Therefore, it can
be concluded that the yeast cells suffered from considerable stress
at the high ethanol concentration, resulting in a decrease of cell
growth and activity, thereby causing a decrease in ethanol concentration [6].
In order to keep the ethanol concentration in R2 at approximately 80 g L1, a third reactor was used to produce fresh yeast
cells, and the fresh yeast cells were continuously fed into R2 to
replenish viable cells and enhance the activity. In our previous
study, it had been proven that the sugar concentration between
80 g L1 and 100 g L1 resulted in the highest cells activity at 30 C
[23]. In this study, sugar concentration of 90 g L1 was fed into R3,
and the cultivation was carried out at 30 C. Ethanol concentration
in R3 was (37.1 ± 2.2) g L1 (Fig. 8A). The yeast number and cell
activity in R3 was (9.8 ± 2.8) 108 cm3 and (9.2 ± 1.0) mm3 min1,
respectively (Figs. 8B and 7). In the continuous ethanol fermentation at 30 C, the decreasing trend in ethanol concentration and
yeast cell viability disappeared (Fig. 8A and B) by feeding the fresh
yeast cells from R3 into R2. The yeast cell viability stabilized at 60%
to 70 %, and the number of viable yeast was (1.7 ± 0.4) 109 cm3
(Fig. 8B). Moreover, the cell activity was (3.8 ± 1.0) mm3 min1,
which was markedly higher than the activity without a fresh feed
(Fig. 7). The ethanol concentration in R2 was (80.1 ± 1.8) g L1
(Fig. 8A), corresponding to ethanol yield of 82.5% and productivity
of 8.8 g L1 h1. The concentration of VFA in each reactor was
relatively constant and at low level, indicating that no bacterial
contamination occurred (Fig. 8C).
Subsequently, the effect of temperature was investigated in the
continuous ethanol fermentation of thick juice. The temperature of
R3 was controlled at 30 C, while the temperatures of R1 and R2
were increased from 30 C to 35 C. As shown in Table 2, the difference of ethanol concentration in R1 was not significant when the
temperature increased from 30 C to 35 C (P ¼ 0.057). The ethanol
concentration in R2 at 33 C was not significantly different from
that at 30 C (P ¼ 0.414), while it significantly decreased to
75.2 g L1 at 35 C (P ¼ 0.016). Thus, the temperature in R2 should
be controlled at 33 C.
The continuous ethanol fermentation of thick juice can be performed year-round, due to the excellent storability [24]. Although
raw juice is not suitable for year-round ethanol fermentation due to
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