3.2 Biochemical Conversion Technologies
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– Sonication (ultrasound) which ruptures the hydrogen bonds in lignocellulosic
structures and reduces the fermentation time;
– Bead beating that breaks the recalcitrant cell walls of biomass;
– High-pressure homogenization in which cell disruption occurs through highpressure impingement of accelerated cellular jet on the stationary valve surface
using the pressure-drop-induced shear stress on the cell suspension;
– Spray drying with gamma radiation which ruptures the β-1,4 glycosidic bonds
leading to the degradation of cellulosic biomass, an increase in the specific area
and solubility of the biomass as well as a decrease in the mechanical strength.
It has been shown that the delignification and depolymerization that take place
during the physical pre-treatments can considerably limit the side products generation during hydrolysis and fermentation. However, physical treatments are usually
of higher costs due to higher energy consumption compared to chemical methods
[98, 101, 104].
3.2.3.4 Biological Methods
Biological treatments mostly include enzymatic treatments, which involve the addition of commercial enzymes such as cellulase, amylase and agarase to the reaction
environment. Compared to chemical agents such as acids or bases, enzymes can
operate at mild reaction conditions and do not generate by-products. Harsh conditions such as alkali pre-treatment can result in the degradation of soluble sugars and
form various types of inhibitors such as furfurals. Enzymatic hydrolysis is the most
common pre-treatment method for bioethanol production from food waste [105]. It
also displays a high reaction and substrate specificity [106].
3.2.3.5 Physiochemical Pre-treatments
These methods include pre-treatments with hot water (steam explosion), carbon
dioxide explosion and Ammonia Fiber Explosion (AFEX). In hot water pretreatment, biomass is exposed to saturated steam at 160–240 °C under pressure
7–48 bar, which results in hydrolysis of biomass components such as hemicellulose and lignin. Hot water pre-treatments reduce the usage of chemical reagents
and are rather economic and highly efficient processes. The carbon dioxide explosion is similar to steam explosion and uses supercritical CO 2 . It increases the efficiency of cellulose hydrolysis to glucose and makes the biomass more vulnerable
to hydrolytic enzymes. Pre-treatments with ammonia include the AFEX and ARP
(ammonia recovery process). The former method consists of keeping the biomass
suspension in anhydrous ammonia at 60–90 °C under pressure above 30 bar and the
latter is the percolation of the ammonia solution through a packed bed. Both methods
result in the hydrolysis of biomass components and a decrease in crystallinity. They
87
– Sonication (ultrasound) which ruptures the hydrogen bonds in lignocellulosic
structures and reduces the fermentation time;
– Bead beating that breaks the recalcitrant cell walls of biomass;
– High-pressure homogenization in which cell disruption occurs through highpressure impingement of accelerated cellular jet on the stationary valve surface
using the pressure-drop-induced shear stress on the cell suspension;
– Spray drying with gamma radiation which ruptures the β-1,4 glycosidic bonds
leading to the degradation of cellulosic biomass, an increase in the specific area
and solubility of the biomass as well as a decrease in the mechanical strength.
It has been shown that the delignification and depolymerization that take place
during the physical pre-treatments can considerably limit the side products generation during hydrolysis and fermentation. However, physical treatments are usually
of higher costs due to higher energy consumption compared to chemical methods
[98, 101, 104].
3.2.3.4 Biological Methods
Biological treatments mostly include enzymatic treatments, which involve the addition of commercial enzymes such as cellulase, amylase and agarase to the reaction
environment. Compared to chemical agents such as acids or bases, enzymes can
operate at mild reaction conditions and do not generate by-products. Harsh conditions such as alkali pre-treatment can result in the degradation of soluble sugars and
form various types of inhibitors such as furfurals. Enzymatic hydrolysis is the most
common pre-treatment method for bioethanol production from food waste [105]. It
also displays a high reaction and substrate specificity [106].
3.2.3.5 Physiochemical Pre-treatments
These methods include pre-treatments with hot water (steam explosion), carbon
dioxide explosion and Ammonia Fiber Explosion (AFEX). In hot water pretreatment, biomass is exposed to saturated steam at 160–240 °C under pressure
7–48 bar, which results in hydrolysis of biomass components such as hemicellulose and lignin. Hot water pre-treatments reduce the usage of chemical reagents
and are rather economic and highly efficient processes. The carbon dioxide explosion is similar to steam explosion and uses supercritical CO 2 . It increases the efficiency of cellulose hydrolysis to glucose and makes the biomass more vulnerable
to hydrolytic enzymes. Pre-treatments with ammonia include the AFEX and ARP
(ammonia recovery process). The former method consists of keeping the biomass
suspension in anhydrous ammonia at 60–90 °C under pressure above 30 bar and the
latter is the percolation of the ammonia solution through a packed bed. Both methods
result in the hydrolysis of biomass components and a decrease in crystallinity. They