has a high polymerization degree, then a bigger and stronger structure gets
interrupting any enzymatic action (Zhao et al. 2012).
Accessibility is a special variable among factors that influence the biomass
conversion into biofuels. It is very important due to its correlation to glucose yield
(Crowe et al. 2017). To put it in a simple way, accessibility shows how much
cellulose content is accessible to enzymatic hydrolysis. Cellulose attributes can
affect biomass quality, but hemicelluloses and lignin content add heterogeneity to
material structure and therefore they are taken into consideration too (Chandra et al.
2008). A strong and organized microfibril structure (surrounded by hemicelluloses
and lignin) produces a protecting barrier in cellulose (Arantes and Saddler 2010).
The exposed surface areas of cellulose are commonly divided into internal and
external surfaces. The external surface has bigger pores and increases in smaller
particle sizes. The internal surface, on the other hand, has smaller pores and seems to
be related to how effective enzymatic hydrolysis can be (Cosgrove 2005). It happens
due to pore size distribution, key factor that suggest more importance than external
surface area (Wang et al. 2018). Some enzymes can arrive to internal surface by
entering the bigger pores on external surface area, but they can’t reach the internal
surface, which has smaller pores (Harmoko et al. 2016). Pores have to be bigger than
5.1 nm to allow the enzymes to reach the lignocellulosic material, demanding
alterations in the biomass structure to increase pore size (Chandra et al. 2007).
Adding to this problem, most studies dry and pretreat their materials, making smaller
pore sizes (Luo and Zhu 2011).
Cellulose with low polymerization degree offers a larger number of settlement
sites for enzyme attack, collaborating substantially with hydrolysis of cellulose
chains. When materials are milling, it speeds up the process involving changes of
the degree of polymerization, porosity, pore size distribution, and crystallinity (Zhao
et al. 2012). Also worth noting, depolymerization is a mechanism that breaks
polysaccharides into monomers (Goufo and Mugisha 2018). This is important
because the depolymerized cellulose chains are formed in one stage of the hydrolysis. Cellulases are enzymes responsible for depolymerization, as they convert
polysaccharides (cellulose) into monomers (glucose) (Yücel and Göycıncık 2015).
This system is composed of three enzymes acting in a synergic way, meaning their
effects combined are stronger than the effects of them working on their own (Hideno
et al. 2009). Exposing cellulose by removing hemicelluloses and lignin, plus altering
the material, results in more efficient enzyme action, enhancing glucose yield
(Brienzo et al. 2017).
Particle size can also be considered to assess biomass quality (Chandra et al.
2008), as it influences in biomass conversion. The biomass particles have different
sizes and shapes, often forming agglomerates that make measuring their exact sizes a
difficult endeavor (Karimi and Taherzadeh 2016). It is assumed that smaller particles
provide a larger exposed surface area, increasing cellulose accessibility, which
consequently improves glucose yield (Chandra et al. 2008).
The hemicellulose presence interferes with cellulose accessibility, messing with
pore distribution and pretreatments and enzymatic hydrolysis efficiencies (Zhao
et al. 2017). Hemicelluloses occupy the spaces among and around cellulose fibers
2 Biofuels Generation Based on Technical Process and Biomass Quality
47
interrupting any enzymatic action (Zhao et al. 2012).
Accessibility is a special variable among factors that influence the biomass
conversion into biofuels. It is very important due to its correlation to glucose yield
(Crowe et al. 2017). To put it in a simple way, accessibility shows how much
cellulose content is accessible to enzymatic hydrolysis. Cellulose attributes can
affect biomass quality, but hemicelluloses and lignin content add heterogeneity to
material structure and therefore they are taken into consideration too (Chandra et al.
2008). A strong and organized microfibril structure (surrounded by hemicelluloses
and lignin) produces a protecting barrier in cellulose (Arantes and Saddler 2010).
The exposed surface areas of cellulose are commonly divided into internal and
external surfaces. The external surface has bigger pores and increases in smaller
particle sizes. The internal surface, on the other hand, has smaller pores and seems to
be related to how effective enzymatic hydrolysis can be (Cosgrove 2005). It happens
due to pore size distribution, key factor that suggest more importance than external
surface area (Wang et al. 2018). Some enzymes can arrive to internal surface by
entering the bigger pores on external surface area, but they can’t reach the internal
surface, which has smaller pores (Harmoko et al. 2016). Pores have to be bigger than
5.1 nm to allow the enzymes to reach the lignocellulosic material, demanding
alterations in the biomass structure to increase pore size (Chandra et al. 2007).
Adding to this problem, most studies dry and pretreat their materials, making smaller
pore sizes (Luo and Zhu 2011).
Cellulose with low polymerization degree offers a larger number of settlement
sites for enzyme attack, collaborating substantially with hydrolysis of cellulose
chains. When materials are milling, it speeds up the process involving changes of
the degree of polymerization, porosity, pore size distribution, and crystallinity (Zhao
et al. 2012). Also worth noting, depolymerization is a mechanism that breaks
polysaccharides into monomers (Goufo and Mugisha 2018). This is important
because the depolymerized cellulose chains are formed in one stage of the hydrolysis. Cellulases are enzymes responsible for depolymerization, as they convert
polysaccharides (cellulose) into monomers (glucose) (Yücel and Göycıncık 2015).
This system is composed of three enzymes acting in a synergic way, meaning their
effects combined are stronger than the effects of them working on their own (Hideno
et al. 2009). Exposing cellulose by removing hemicelluloses and lignin, plus altering
the material, results in more efficient enzyme action, enhancing glucose yield
(Brienzo et al. 2017).
Particle size can also be considered to assess biomass quality (Chandra et al.
2008), as it influences in biomass conversion. The biomass particles have different
sizes and shapes, often forming agglomerates that make measuring their exact sizes a
difficult endeavor (Karimi and Taherzadeh 2016). It is assumed that smaller particles
provide a larger exposed surface area, increasing cellulose accessibility, which
consequently improves glucose yield (Chandra et al. 2008).
The hemicellulose presence interferes with cellulose accessibility, messing with
pore distribution and pretreatments and enzymatic hydrolysis efficiencies (Zhao
et al. 2017). Hemicelluloses occupy the spaces among and around cellulose fibers
2 Biofuels Generation Based on Technical Process and Biomass Quality
47