4.5.2 Effect of glucose on ethanol production
Since the previous experiment implied that 10 g/L (NH4)2SO4 had the
efficiency to ethanol production. The optimization of glucose concentration was
observed by various the concentration of glucose from 0 to 25% (v/v) in M9 minimal
medium, pH 5.5 and 10 g/L (NH4)2SO4 as a nitrogen source. The effect of glucose
concentration on ethanol production and growth of yeast is presented in Figure 4.14. It
could be seen that ethanol production was slowly increased with the period of
incubation. The ethanol production in the fermentation broth was increased with the
5% (v/v) glucose but decreased beyond that. The final glucose utilization in the
fermentation broth was found to be used up at the glucose concentration equal to or
below 5% (v/v), but above the glucose concentration of 5% (v/v), the final glucose
utilization became quite appreciable. The growth of yeast and its maximum ethanol
concentration show in Figure 4.15. The maximum specific growth rate and maximum
ethanol concentration were increasing with an increase of glucose concentration for
5% (v/v). A reduction of ethanol production and growth of yeast were decreased when
glucose concentration was greater than 5% (v/v).
64
0
20
40
60
80
100
120
0 24 48 72 96 120 144 168 192 216 240 264 288 312 336 360 time (hours)
% glucose utilization
1% glucose 5% glucose 10% glucose 15% glucose 20% glucose 25% glucose(a)
It is apparent that high glucose concentration can be efficiently converted into
ethanol by the thermotolerant yeast strain S1. The results also indicate to a certain
extent the inhibitory effects of high glucose concentration on the ethanol production.
Glucose inhibition is related principally to osmotic effects (Bajpai and Margaritis,
1987). As pointed out by Ghose and Tyagi (1979), a high concentration of glucose in
the medium inhibits the growth of yeast cells and the production of ethanol, and the
inhibitory effect is attributed to high osmotic pressure. This inhibitory effect is one of
the major problems for ethanol production.