ABSTRACT
Sweet sorghum has been of particular interest for biological transformation into ethanol for
use as fuel or fuel additive because its stalks contain high fermentable sugar and it has short-life cycle
for 100-120 days. The aim of this research was to increase in the efficiency of ethanol production
from sweet sorghum stem juice (SSJ) cv. KKU 40 by Saccharomyces cerevisiae NP 01. This research
was divided into two parts. Part I was to increase ethanol production by continuous fermentation
using a cascade system and Part II was to increase ethanol production using VHG technology.
In Part I, when the SSJ containing total sugar of 240 g l-1
was use as a substrate for ethanol
production by various systems at the initial yeast cell concentration of 1108
cells ml-1
, the results
showed that the cascade system composed of a 2-litre continuous stirred tank bioreactor (CSTR) and a
three-glass tubular bioreactor in series with a total working volume of 3320 ml improved ethanol
production efficiency. At the dilution rate of 0.007 h-1
, ethanol concentration of the cascade system
was 51-60 and 5 % higher than that of the CSTR system and batch system, respectively. The
concentration, productivity and yield efficiency of ethanol using the cascade system were 107.17 g l-1
,
0.75 g l-1
h
-1
and 94.45% of the theoretical yield, respectively.
In Part II, the results showed that dried spent brewer’s yeast at 8 g l-1
could be used as a
nutrient supplement instead of yeast extract for ethanol production under VHG condition. When the
SSJ containing total sugar of 280 g l-1
and 8 g l-1
of dried spent brewer’s yeast was used for ethanol
batch fermentation under at the initial yeast cell concentration of 1108
cells ml-1
in a 5-litre
bioreactor, 6 % of ethanol concentration was increased compared to using the control SSJ (without
dried spent yeast). The concentration, productivity and yield efficiency of ethanol from the SSJ
supplemented with 8 g l-1
of dried spent brewer’s yeas were 104.54 g l-1
, 2.18 g l-1
h
-1
and 77.78% of
the theoretical yield, respectively at the fermentation time of 48 h. The results also showed that the
initial yeast cell concentration of 5107
cells ml-1
was suitable for ethanol production from the SSJ
under VHG condition in a 50-litre bioreactor. Ethanol concentration at the initial yeast cell
concentration of 5107
and 1108
cells ml-1
was not different but the fermentation time at the lower
cell concentration was 12 h longer. The cost of sweet sorghum for ethanol production (raw material
cost only) was 11 baht per litre.