12 Fermentation Process
Fermentation is the final step in the ethanol production. A variety of microorganisms
like bacteria, fungi, and yeast can ferment the monomeric sugars to ethanol. In
general, fermentation is carried out under anaerobic conditions leading to the
glycosylation of one molecule of glucose into two moles of ethanol and two moles
of carbon dioxide as shown in Eqs. (3) and (4).
3C 5 H 10 O 5 ! 5C 2 H 5 OH þ 5CO 2 þ energy stored as ATP
ð
Þ
ð 3Þ
C 6 H 12 O 6 ! 2C 2 H 5 OH þ 2CO 2 þ energy stored as ATP
ð
Þ
ð 4Þ
Saccharomyces cerevisiae is the most commonly used yeast for fermentation, and
it is substrate specific. The efficiency of the fermentation depends on the concentration and nature of the substrate, the methods followed for the pretreatment and
hydrolysis, and the nature of the organism. It is necessary to maintain proper process
conditions such as temperature and pH for the optimal yeast growth. Yeast shows
tolerance to high sugar concentrations, is resistant to adverse conditions generated
by the presence of ethanol, and is stable at higher temperature [41].
S. cerevisiae can ferment only hexose; co-culturing with other microorganism
having the capability to ferment pentose can increase the yield of ethanol. Fermentation of SCT-derived fermentable sugars obtained after dilute acid pretreatment and
enzymatic saccharification, with 18 h-old culture of S. cerevisiae incubated at
28 Æ 2
C for 72 h, yielded high amount of ethanol of 11.365 g/L [8].
Alkali (NaOH) followed by dilute acid (H 2 SO 4 )-pretreated SCT yielded 48.17 g/
L of ethanol by using 10
7 cells/mL of S. cerevisiae TISTR 5596 strain [1]. Fermentation of the non-detoxified hydrolyzate obtained from the sequential pretreated SCT
using S. cerevisiae produced 31.928 g of bioethanol/g of dry biomass [16]. Fermentation of liquid hydrolyzate obtained after enzymatic-saccharified SCT using
S. cerevisiae yielded 27.2 g/L of ethanol [3]. Fermentation of liquid hydrolyzate
obtained from separate hydrolysis (SHF) and simultaneous saccharification (SSF) of
pretreated lignocellulosic mixture revealed higher ethanol productivity of about
1.396 g/L/h in SSF [17].
13 Conclusion
From the reported studies, it is evident that different pretreatment technologies have
been applied to SCT for efficient delignification and hemicellulose solubilization.
The application of combined pretreatment technologies needs attention in order to
convert the SCT into bioethanol. Also, the developed pretreatment should be
effective in delignification with low consumption of energy. From various reports,
it was proved that hydrolysis using enzyme efficiently converts the delignified SCT
into fermentable sugars. However, most of the SCT-based reports utilized the yeast
102
S. Niju and M. Swathika
Fermentation is the final step in the ethanol production. A variety of microorganisms
like bacteria, fungi, and yeast can ferment the monomeric sugars to ethanol. In
general, fermentation is carried out under anaerobic conditions leading to the
glycosylation of one molecule of glucose into two moles of ethanol and two moles
of carbon dioxide as shown in Eqs. (3) and (4).
3C 5 H 10 O 5 ! 5C 2 H 5 OH þ 5CO 2 þ energy stored as ATP
ð
Þ
ð 3Þ
C 6 H 12 O 6 ! 2C 2 H 5 OH þ 2CO 2 þ energy stored as ATP
ð
Þ
ð 4Þ
Saccharomyces cerevisiae is the most commonly used yeast for fermentation, and
it is substrate specific. The efficiency of the fermentation depends on the concentration and nature of the substrate, the methods followed for the pretreatment and
hydrolysis, and the nature of the organism. It is necessary to maintain proper process
conditions such as temperature and pH for the optimal yeast growth. Yeast shows
tolerance to high sugar concentrations, is resistant to adverse conditions generated
by the presence of ethanol, and is stable at higher temperature [41].
S. cerevisiae can ferment only hexose; co-culturing with other microorganism
having the capability to ferment pentose can increase the yield of ethanol. Fermentation of SCT-derived fermentable sugars obtained after dilute acid pretreatment and
enzymatic saccharification, with 18 h-old culture of S. cerevisiae incubated at
28 Æ 2
C for 72 h, yielded high amount of ethanol of 11.365 g/L [8].
Alkali (NaOH) followed by dilute acid (H 2 SO 4 )-pretreated SCT yielded 48.17 g/
L of ethanol by using 10
7 cells/mL of S. cerevisiae TISTR 5596 strain [1]. Fermentation of the non-detoxified hydrolyzate obtained from the sequential pretreated SCT
using S. cerevisiae produced 31.928 g of bioethanol/g of dry biomass [16]. Fermentation of liquid hydrolyzate obtained after enzymatic-saccharified SCT using
S. cerevisiae yielded 27.2 g/L of ethanol [3]. Fermentation of liquid hydrolyzate
obtained from separate hydrolysis (SHF) and simultaneous saccharification (SSF) of
pretreated lignocellulosic mixture revealed higher ethanol productivity of about
1.396 g/L/h in SSF [17].
13 Conclusion
From the reported studies, it is evident that different pretreatment technologies have
been applied to SCT for efficient delignification and hemicellulose solubilization.
The application of combined pretreatment technologies needs attention in order to
convert the SCT into bioethanol. Also, the developed pretreatment should be
effective in delignification with low consumption of energy. From various reports,
it was proved that hydrolysis using enzyme efficiently converts the delignified SCT
into fermentable sugars. However, most of the SCT-based reports utilized the yeast
102
S. Niju and M. Swathika