5.1.1.4 Biological Pretreatment
Biological pretreatment makes use of white, soft, and brown rot fungi for degrading
the hemicelluloses and lignin of lignocellulosic biomasses (Saritha and Arora 2012;
Sindhu et al. 2016). Soft rot fungi and white fungi target the lignin and cellulose
while brown rots target only cellulose (Cheng and Timilsina 2011). Basically, these
fungi use their enzymes (lignases), and break lignins with these enzymes. The
process parameters of the method besides composition of biomass and nature of
biomass including moisture content, duration of incubation, rate of aeration, pH,
temperature and the most important, type of microbeare effective in pretreatment
(Sindhu et al. 2016). Sindhu et al. (2016) have explained the review of features of
biological pretreatment like enzymes involved and parameters and also the future
prospects. The duration of incubation for delignification is longer in the biological
pretreatment method, for this purpose greater space is needed which is a limitation
for use in industries. By using a suitable microbial selection, this factor can be
minimized (Sindhu et al. 2016). The second limitation of this strategy is that the rate
of hydrolysis is slower in comparison to other techniques (Saritha and Arora 2012).
When the sugar concentrations will be lower, then apparently the yield of ethanol
will also be lower. The inhibitors are also formed that need to be removed or
detoxified (Arora et al. 2016). Shirkavand et al. (2016) have suggested to make an
effective and efficient pretreatment strategy, the idea was to make a combined
pretreatment process. Combined pretreatment methods use the combinations of
many pretreatment methods which maximize the use of LC material at their best
(Sun et al. 2016).
5.1.1.5 Combined Pretreatments
Use of oxidative delignification, alkaline method, and biological pretreatment results
in the removal of lignin, while alkaline, acidic hydrothermal, and steam explosion
methods result in removing hemicelluloses from LC materials. The surface area
accessible to enzymes is increased by ammonia fiber explosion method and steam
explosion. The combination of these pretreatment strategies can be effective in
improving hemicellulose and lignin recovery and digestibility of LC materials. So
multiple combinations of pretreatment methods, e.g., ionic liquid method with
supercritical carbon dioxide method, mild acid with the biological method
(Ma et al. 2010), biological combined with mild chemical or physical methods,
alkali combined with hydrothermal (Yu et al. 2009), alkaline peroxide with hydrothermal or steam explosion method (Chen et al. 2008; Cuevas et al. 2014), alkali
pretreatment with dilute acid pretreatment (Lee et al. 2015), have been made to
pretreat multiple LC biomasses.
Cuevas et al. (2014) investigated the effects of alkaline peroxide pretreatment
combined with hydrothermal pretreatment method on prunings of almond trees; the
main things were changes in components and enzymatic digestibility. Sixty percent
5 Challenges in Bioethanol Production: Effect of Inhibitory Compounds
131
Biological pretreatment makes use of white, soft, and brown rot fungi for degrading
the hemicelluloses and lignin of lignocellulosic biomasses (Saritha and Arora 2012;
Sindhu et al. 2016). Soft rot fungi and white fungi target the lignin and cellulose
while brown rots target only cellulose (Cheng and Timilsina 2011). Basically, these
fungi use their enzymes (lignases), and break lignins with these enzymes. The
process parameters of the method besides composition of biomass and nature of
biomass including moisture content, duration of incubation, rate of aeration, pH,
temperature and the most important, type of microbeare effective in pretreatment
(Sindhu et al. 2016). Sindhu et al. (2016) have explained the review of features of
biological pretreatment like enzymes involved and parameters and also the future
prospects. The duration of incubation for delignification is longer in the biological
pretreatment method, for this purpose greater space is needed which is a limitation
for use in industries. By using a suitable microbial selection, this factor can be
minimized (Sindhu et al. 2016). The second limitation of this strategy is that the rate
of hydrolysis is slower in comparison to other techniques (Saritha and Arora 2012).
When the sugar concentrations will be lower, then apparently the yield of ethanol
will also be lower. The inhibitors are also formed that need to be removed or
detoxified (Arora et al. 2016). Shirkavand et al. (2016) have suggested to make an
effective and efficient pretreatment strategy, the idea was to make a combined
pretreatment process. Combined pretreatment methods use the combinations of
many pretreatment methods which maximize the use of LC material at their best
(Sun et al. 2016).
5.1.1.5 Combined Pretreatments
Use of oxidative delignification, alkaline method, and biological pretreatment results
in the removal of lignin, while alkaline, acidic hydrothermal, and steam explosion
methods result in removing hemicelluloses from LC materials. The surface area
accessible to enzymes is increased by ammonia fiber explosion method and steam
explosion. The combination of these pretreatment strategies can be effective in
improving hemicellulose and lignin recovery and digestibility of LC materials. So
multiple combinations of pretreatment methods, e.g., ionic liquid method with
supercritical carbon dioxide method, mild acid with the biological method
(Ma et al. 2010), biological combined with mild chemical or physical methods,
alkali combined with hydrothermal (Yu et al. 2009), alkaline peroxide with hydrothermal or steam explosion method (Chen et al. 2008; Cuevas et al. 2014), alkali
pretreatment with dilute acid pretreatment (Lee et al. 2015), have been made to
pretreat multiple LC biomasses.
Cuevas et al. (2014) investigated the effects of alkaline peroxide pretreatment
combined with hydrothermal pretreatment method on prunings of almond trees; the
main things were changes in components and enzymatic digestibility. Sixty percent
5 Challenges in Bioethanol Production: Effect of Inhibitory Compounds
131