Abstract
 This research involved a study on ethanol production from sweet sorghum grains and is a
part of the study on total use of sweet sorghum plant for fuel ethanol production. In this study, sweet sorghum
grains were treated by enzymes to produce the syrup which composed of glucose and some glucose
polymers. The ethanol production was observed later.
Two-step enzymatic treatment of sorghum grains used alpha-amylase in liquefaction and
amyloglucosidase in saccharification. The results from test-tube study showed that using 0.25 mm of
powdered grains, pH 5.0, 2.5 % alpha-amylase at 83.0 °C for 4 h were suitable conditions for liquefaction.
Conditions for saccharification were pH 4.6 and 60.4 °C with the use of 0.1 % amyloglucosidase and
incubated using minimum time of 4 h. Reducing sugar concentration obtained when using these hydrolysis
condition was 80-85 g/l when hydrolysis was carried out in 50-litre reactor with 30-litre working volume.
The syrup from the grains as substrate was tested for ethanol production in flask by using two
ethanol-producer strains, Saccharomyces cerevisiae TISTR 5339 and Zymomonas mobilis TISTR 548. The
results showed that they produced higher ethanol concentration in static condition than in shaking condition.
There was not significant increased in ethanol concentration when the inoculum from 5 to 10 %(v/v). The
results demonstrated that the grain hydrolysate (syrup) alone could be used as substrate for producing
ethanol with no added supplement. However, the syrup with the supplement of yeast extract improved to the
ethanol production from 36-43 g/l to 42-45 g/l and shorten the cultivation time by 12 h when compared to
using the syrup with no supplement


When the fermentation using the syrup with the supplement of 10 g/l yeast extract as the medium
were carried out in the bioreactor using 5 % (v/v) inoculum, both strains produced ethanol at similar
concentration of 38 g/l within 21 h for S. cerevisiae and 15 h for Z. mobilis.
The next study investigated the use of dried spent brewer’s yeast cells as the substitute for yeast
extract for ethanol production. Preliminary results indicated that using 3 g/l yeast extract as the supplement
was enough to contribute to the growth and ethanol production of both S. cerevisiae and Z. mobilis. Higher
yeast extract supplement did not help improving both ethanol concentration and productivity. The spent
brewer’s yeast cells could be used directly as yeast extract substitute as they did not show any inhibition on
growth of both tested strains. Autolysis and acid hydrolysis were investigated as means to further release the
protein from the spent brewer’s yeast cells. Acid hydrolysis by autoclaving at pH 2.0, 121 °C and 15 minutes
was chosen. The condition resulted in the total protein concentration of 17 g/l as compared to 9 g/l if no
hydrolysis was performed. Pretreating the spent yeast cells by homogenization did not further enhance the
protein released.
When producing ethanol in flask scale using sweet sorghum grain hydrolysate (syrup) supplemented with
spent cells hydrolysate at the protein concentration approximately equal to 3 g/l yeast extract, both tested
stain produced similar ethanol concentration of 46-47 g/l. Similar cultivation in the bioreactor resulted in
ethanol concentration of 31 g/l from S. cerevisiae after 35 h and 41 g/l after 27 h of Z. mobiliscultivation.