The repeated-batch fermentation which consisted of 16 cycles was
examined using the juice of FS902 in cycles 1–10, that of FS501 in
cycles 11–13, and that of KCS105 in cycles 14–16 as a carbon source,
respectively (Fig. 2). The pre-culture using the glycerol stock solution
of strains NBRC 0216 or Hitachi was carried out according to the
procedure mentioned above. For cycle 1, the pre-culture was transferred
into 10 ml of the juice of FS902 in 15 ml-plastic tubes with 0.1%
(v/v)-inoculation, and cultivated at 30°C for 3 days. In cycle 2 and
the later cycles, the cells in the tube were collected by centrifugation
at 3500×g for 5 min at room temperature, and the fermentation
broth was removed. One of the three kinds of fresh juice (10 ml) was
added into the tube containing the cells and incubated at 30°C for
1 day each. The procedure was repeated up to cycle 16. The tubes used
in this experiment were not sterilized and the manipulation was
performed under ambient atmosphere except for the pre-culture.
Consequently, the strain Hitachi produced 5.59 to 6.13% (v/v) ethanol,
and from 10.1% (w/v) sugar in FS902 during the first 10 cycles,
showing 83.3–91.4% conversion of the theoretical value. The strain
NBRC 0216 also fermented ethanol well with 78.7–89.2% conversion,
although a lower conversion of 61.4% was observed in cycle 2. The
juice of FS501 used in cycles 11–13 and that of KCS105 used in cycles
14–16 were also fermentable materials in the repeated-batch
fermentation with conversion of over 85%, although neither strains
could grew well in the juices. Generally, ethanol yields differ with
the sweet sorghum strain, growing conditions, and juice batches, as
well (17). In this study, however, good yields were obtained with
all the sweet sorghum extracts after yeast was adapted to the ethanol
producing conditions. By applying the technique reported here to
ethanol production using yeast, the range of sorghum strains which
could be used as a bioresource could be broadened.