of S. cerevisiae PE-2. As in the case of acid inhibition, these phenolics also are not
much harmless in lower concentrations, but when their concentration rise, inhibitors
need to be removed (Lyra Colombi et al. 2018). Toxicity due to vanillin is the
leading cause of cost reduction of bioethanol production (Ito et al. 2020). Moreover,
phenolics mostly do not decrease the yield of ethanol, but can reduce the rate at
which ethanol is produced. When they affect the activity and integrity of cell
membranes then they can also reduce the yields and growth rates. The aspect of
microorganism’s resistance to such inhibitors is dependent on the compositions of:
specific fatty acids, membrane composition, headgroups of phospholipids, and
content of proteins (Heipieper et al. 1994; Wikandari et al. 2019).
Other effects of phenolic inhibitors include their negative effects on cellulases.
The observation was made on the basis of the presence and absence of vanillin in the
cellulolytic solution. With vanillin, the cellulose was converted by a value of 26%
but in absence of vanillin it was 53% (Qin et al. 2016). Other phenolic compounds
like ferulic acid and p-coumaric acid also showed reduction of glucose production
from glucose by 16% and 30% respectively. Also, those phenolic compounds that
had been taken from the pretreated biomass had their effects on the functioning of
enzymes. Adsorption of cellulases to hydroxyl groups and derivatives of lignin had
their contribution to inhibitory effects (Kim 2018).
As the phenolic compounds have lower molecular weight so they can easily enter
the cell membranes of microbes by disturbing their internal structures, also they are
much stronger inhibitors as compared to other inhibitory compounds, e.g., weak
acids, furans, other byproducts (Jönsson and Martín 2016; Klinke et al. 2004). Ezeji
et al. (2007) indicated that p-coumaric acid and ferulic acids are the most harmful of
all the phenolic inhibitors, when checked with Clostridium beijerinckii (BA101)
strain, the growth of this strain of bacteria was reduced by 74% in the presence of
1 g/L of these two inhibitory compounds. In some other research, it was found out
that these two inhibitors reduced the growth of saccharomyces cerevisiae by 80%
(Adeboye et al. 2015). The fluidity of the membranes of cells can also be affected by
these phenolic inhibitors by decreasing the levels of potassium. Phenolic inhibitors
also affect the stability of the cell membranes that may result in DNA breakage that
will affect RNA and hence protein production, accumulation of carbohydrates, and
disturbance in growth of cells (Kim 2018).
There is not much information regarding the nature of inhibitory mechanisms by
phenolic compounds. It can be suggested that a multiple site inhibition mechanism
may be there, in which many aromatic molecules would be joined together and
cooperate with the enzyme to bring about not only inhibition but also inactivation.
Hemicellulasses and cellulases both are equally influenced by phenols derived from
lignin (dos Santos et al. 2019).
Table 5.3 dos Santos et al. (2019) represented the summary of inhibition mechanisms of furfural, weak acids, and phenolic compounds.
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FaizaKausar et al.
much harmless in lower concentrations, but when their concentration rise, inhibitors
need to be removed (Lyra Colombi et al. 2018). Toxicity due to vanillin is the
leading cause of cost reduction of bioethanol production (Ito et al. 2020). Moreover,
phenolics mostly do not decrease the yield of ethanol, but can reduce the rate at
which ethanol is produced. When they affect the activity and integrity of cell
membranes then they can also reduce the yields and growth rates. The aspect of
microorganism’s resistance to such inhibitors is dependent on the compositions of:
specific fatty acids, membrane composition, headgroups of phospholipids, and
content of proteins (Heipieper et al. 1994; Wikandari et al. 2019).
Other effects of phenolic inhibitors include their negative effects on cellulases.
The observation was made on the basis of the presence and absence of vanillin in the
cellulolytic solution. With vanillin, the cellulose was converted by a value of 26%
but in absence of vanillin it was 53% (Qin et al. 2016). Other phenolic compounds
like ferulic acid and p-coumaric acid also showed reduction of glucose production
from glucose by 16% and 30% respectively. Also, those phenolic compounds that
had been taken from the pretreated biomass had their effects on the functioning of
enzymes. Adsorption of cellulases to hydroxyl groups and derivatives of lignin had
their contribution to inhibitory effects (Kim 2018).
As the phenolic compounds have lower molecular weight so they can easily enter
the cell membranes of microbes by disturbing their internal structures, also they are
much stronger inhibitors as compared to other inhibitory compounds, e.g., weak
acids, furans, other byproducts (Jönsson and Martín 2016; Klinke et al. 2004). Ezeji
et al. (2007) indicated that p-coumaric acid and ferulic acids are the most harmful of
all the phenolic inhibitors, when checked with Clostridium beijerinckii (BA101)
strain, the growth of this strain of bacteria was reduced by 74% in the presence of
1 g/L of these two inhibitory compounds. In some other research, it was found out
that these two inhibitors reduced the growth of saccharomyces cerevisiae by 80%
(Adeboye et al. 2015). The fluidity of the membranes of cells can also be affected by
these phenolic inhibitors by decreasing the levels of potassium. Phenolic inhibitors
also affect the stability of the cell membranes that may result in DNA breakage that
will affect RNA and hence protein production, accumulation of carbohydrates, and
disturbance in growth of cells (Kim 2018).
There is not much information regarding the nature of inhibitory mechanisms by
phenolic compounds. It can be suggested that a multiple site inhibition mechanism
may be there, in which many aromatic molecules would be joined together and
cooperate with the enzyme to bring about not only inhibition but also inactivation.
Hemicellulasses and cellulases both are equally influenced by phenols derived from
lignin (dos Santos et al. 2019).
Table 5.3 dos Santos et al. (2019) represented the summary of inhibition mechanisms of furfural, weak acids, and phenolic compounds.
140
FaizaKausar et al.