with small sizes are more preferred due to increased digestibility and total yield,
although the use of small-size particles is difficult in the downstream processing
(Bolado-Rodríguez et al. 2016). On the other hand, the small size of particles affects
the efficiency of the pretreatment as it affects the proper microbial growth and
metabolism by reducing the aeration rate (Sharma et al. 2019), whereas larger
particle size affects the pretreatment process by reducing the penetration of microorganisms into the substrates and reducing the uniform air diffusion. Similarly, time
is another important factor which varies according to the microorganism and microbial enzymes. Taniguchi et al. (2005) reported highest sugar yield with rice straw
after hydrolysis using P. ostreatus when pretreated for 60 days (Taniguchi et al.
2005) whereas Salvachúa et al. (2011) reported less sugar concentration in wheat
bran pre-treated with P. chrysosporium-after 14 days. Further, an increased sugar
yield was reported for wood chips pretreatment by T. versicolor (Hwang et al. 2008).
Another important factor required for the treatment of the biomass is moisture
content as it is required in specific amount for proper microbial growth and biodegradation (Gervais and Molin 2003), although this also varies on the basis of type of
strain and biomass (Mustafa et al. 2016). Physical parameter such as temperature has
also been found to be another important parameter in enzymatic pretreatment
process which is necessary for the optimum microbial growth and cells’ metabolic
activities. Based on various microorganisms, the temperature optima also varied
from 25 to 30
C. Fungi from ascomycetes group can grow at a higher temperature
nearly up to 39
C, whereas, in the case of basidiomycetes, the required temperature
optima is 15 and 35
C (Sindhu et al. 2016). This is because of the difference in the
physiology of fungus substrate type and microbial strains (Isroi et al. 2011). The
WRF metabolism in solid-state system generates heat, which eventually enhances
the bioreactors’ gradient temperature (Wan and Li 2012), and plays as an important
challenge for the researchers while designing the bioreactor for the solid-state
pretreatment application in large scale. Similarly, pH in culture medium also affects
the microbial growth, enzyme secretion and hydrolysis (Sharma et al. 2019).
7.8 Advantages of Xylanases from Thermophilic
Microorganisms in Biorefinery
Various thermophilic microorganisms have been reported for the production of
different enzymes such as hemicellulases, amylases, cellulases, phosphatases, proteases, laccases, lipases, etc., which have various applications in different industries
like food, textile and detergent, dairy, pharmaceutical and others (Singh 2016). The
similarity of thermophilic microorganisms in their phylogenetic analysis and their
enzymes showed common origin with other mesophiles (Zeldes et al. 2015). Thus,
cellulases and xylanases were obtained from thermophilic origin, and their mode of
action was found to be similar except only with some specific features which indicate
their advantage at various industries. Thermophiles are found to be a good source of
7 Xylanases: A Helping Module for the Enzyme Biorefinery Platform
169
although the use of small-size particles is difficult in the downstream processing
(Bolado-Rodríguez et al. 2016). On the other hand, the small size of particles affects
the efficiency of the pretreatment as it affects the proper microbial growth and
metabolism by reducing the aeration rate (Sharma et al. 2019), whereas larger
particle size affects the pretreatment process by reducing the penetration of microorganisms into the substrates and reducing the uniform air diffusion. Similarly, time
is another important factor which varies according to the microorganism and microbial enzymes. Taniguchi et al. (2005) reported highest sugar yield with rice straw
after hydrolysis using P. ostreatus when pretreated for 60 days (Taniguchi et al.
2005) whereas Salvachúa et al. (2011) reported less sugar concentration in wheat
bran pre-treated with P. chrysosporium-after 14 days. Further, an increased sugar
yield was reported for wood chips pretreatment by T. versicolor (Hwang et al. 2008).
Another important factor required for the treatment of the biomass is moisture
content as it is required in specific amount for proper microbial growth and biodegradation (Gervais and Molin 2003), although this also varies on the basis of type of
strain and biomass (Mustafa et al. 2016). Physical parameter such as temperature has
also been found to be another important parameter in enzymatic pretreatment
process which is necessary for the optimum microbial growth and cells’ metabolic
activities. Based on various microorganisms, the temperature optima also varied
from 25 to 30
C. Fungi from ascomycetes group can grow at a higher temperature
nearly up to 39
C, whereas, in the case of basidiomycetes, the required temperature
optima is 15 and 35
C (Sindhu et al. 2016). This is because of the difference in the
physiology of fungus substrate type and microbial strains (Isroi et al. 2011). The
WRF metabolism in solid-state system generates heat, which eventually enhances
the bioreactors’ gradient temperature (Wan and Li 2012), and plays as an important
challenge for the researchers while designing the bioreactor for the solid-state
pretreatment application in large scale. Similarly, pH in culture medium also affects
the microbial growth, enzyme secretion and hydrolysis (Sharma et al. 2019).
7.8 Advantages of Xylanases from Thermophilic
Microorganisms in Biorefinery
Various thermophilic microorganisms have been reported for the production of
different enzymes such as hemicellulases, amylases, cellulases, phosphatases, proteases, laccases, lipases, etc., which have various applications in different industries
like food, textile and detergent, dairy, pharmaceutical and others (Singh 2016). The
similarity of thermophilic microorganisms in their phylogenetic analysis and their
enzymes showed common origin with other mesophiles (Zeldes et al. 2015). Thus,
cellulases and xylanases were obtained from thermophilic origin, and their mode of
action was found to be similar except only with some specific features which indicate
their advantage at various industries. Thermophiles are found to be a good source of
7 Xylanases: A Helping Module for the Enzyme Biorefinery Platform
169