can negatively affect the fermentation due to osmotic pressure on cells affecting cell
viability (Szymanowska-Powalowska et al. 2012). Therefore, there is a need to
optimize the initial substrate concentration for efficient ethanol generation.
7.3.6.2 Microbial Load Concentration Affecting Bioethanol Production
Studies suggests that microbial load or initial inoculum concentration of fermenting
microorganisms does not significantly affect the final concentration of bioethanol but
can enhance consumption rate of sugar and fermentation time (Laopaiboon et al.
2007). The ethanol production is enhanced when initial inoculum concentration is
increased from 10
4 to 10
7 cells/mL. However, further increase in cell concentration
to 10
8 has no effect (Zabed et al. 2014). The increase in cell concentration at a certain
level can speed up the growth of cells, which results in the decrease in fermentation
time. Mojović et al. (2006) demonstrated that during fermentation of corn meal
hydrolysate, increase in initial inoculum concentration from 1 to 2 % reduced
fermentation time to 32 h from 48 h. Similarly, increase in yeast concentration
from 3 to 6% can lead to decrement in fermentation time to 48 h from 72 h (Breisha
2010). Microbial load often increases by contamination of broth with other microbes
which can compete with yeast for the substrate and thus negatively affect the ethanol
generation. Thus, there is need to regulate process for preventing contamination by
properly maintaining aseptic condition and use of antibiotics during fermentation
process (Szymanowska-Powałowska et al. 2014).
7.3.6.3 Accumulation of By-products Affecting Bioethanol Production
The high ethanol concentration has negative effect on the ethanologenic strain due to
end product inhibition, and dehydrating condition arises due to the presence of
ethanol. Therefore, continuous recovery of ethanol during fermentation and application of alcohol-tolerant strains can minimize the effect of by-product accumulation.
Metabolic activity of yeast and the contaminating bacteria can lead to generation of
soluble inhibitory by-products such as lactic acid and acetic acid (Graves et al.
2006). Glycerol is a by-product of ethanol generation process by yeast or even by
contamination of bacteria (Sarris and Papanikolaou 2016). The lactic or acetic acid is
produced by contaminating bacteria. They cause accumulation of these by-products,
which are undesirable as it can negatively affect the yeast growth and ethanol yield
(Białas et al. 2010).
7.3.7 Bioethanol Production
7.3.7.1 First-Generation Feedstock-Based Bioethanol
7.3.7.1.1 Bioethanol Production Using Starch-Based Feedstock
The starch-based bioethanol is 60% of the total bioethanol production using firstgeneration feedstock with sugar-based feedstock contributing to 40% ethanol
(Mussatto et al. 2010; Johnston and McAloon 2014). Starch-based bioethanol
requires an additional step of conversion of starch to reducing sugar through utilizing
7 Bioethanol Production: Generation-Based Comparative Status Measurements
177
viability (Szymanowska-Powalowska et al. 2012). Therefore, there is a need to
optimize the initial substrate concentration for efficient ethanol generation.
7.3.6.2 Microbial Load Concentration Affecting Bioethanol Production
Studies suggests that microbial load or initial inoculum concentration of fermenting
microorganisms does not significantly affect the final concentration of bioethanol but
can enhance consumption rate of sugar and fermentation time (Laopaiboon et al.
2007). The ethanol production is enhanced when initial inoculum concentration is
increased from 10
4 to 10
7 cells/mL. However, further increase in cell concentration
to 10
8 has no effect (Zabed et al. 2014). The increase in cell concentration at a certain
level can speed up the growth of cells, which results in the decrease in fermentation
time. Mojović et al. (2006) demonstrated that during fermentation of corn meal
hydrolysate, increase in initial inoculum concentration from 1 to 2 % reduced
fermentation time to 32 h from 48 h. Similarly, increase in yeast concentration
from 3 to 6% can lead to decrement in fermentation time to 48 h from 72 h (Breisha
2010). Microbial load often increases by contamination of broth with other microbes
which can compete with yeast for the substrate and thus negatively affect the ethanol
generation. Thus, there is need to regulate process for preventing contamination by
properly maintaining aseptic condition and use of antibiotics during fermentation
process (Szymanowska-Powałowska et al. 2014).
7.3.6.3 Accumulation of By-products Affecting Bioethanol Production
The high ethanol concentration has negative effect on the ethanologenic strain due to
end product inhibition, and dehydrating condition arises due to the presence of
ethanol. Therefore, continuous recovery of ethanol during fermentation and application of alcohol-tolerant strains can minimize the effect of by-product accumulation.
Metabolic activity of yeast and the contaminating bacteria can lead to generation of
soluble inhibitory by-products such as lactic acid and acetic acid (Graves et al.
2006). Glycerol is a by-product of ethanol generation process by yeast or even by
contamination of bacteria (Sarris and Papanikolaou 2016). The lactic or acetic acid is
produced by contaminating bacteria. They cause accumulation of these by-products,
which are undesirable as it can negatively affect the yeast growth and ethanol yield
(Białas et al. 2010).
7.3.7 Bioethanol Production
7.3.7.1 First-Generation Feedstock-Based Bioethanol
7.3.7.1.1 Bioethanol Production Using Starch-Based Feedstock
The starch-based bioethanol is 60% of the total bioethanol production using firstgeneration feedstock with sugar-based feedstock contributing to 40% ethanol
(Mussatto et al. 2010; Johnston and McAloon 2014). Starch-based bioethanol
requires an additional step of conversion of starch to reducing sugar through utilizing
7 Bioethanol Production: Generation-Based Comparative Status Measurements
177