Page 44 - Lignocellulosic Biomass to Liquid Biofuels

Pretreatment of lignocellulosic biomass for efficient enzymatic saccharification of cellulose 23

on wood physiochemical structure changes. He et al. studied the effect of
ultrasound in pretreating eucalyptus wood with 300 W and frequency of
28 kHz. The results showed that ultrasound was effective in modifying
the physiochemical structure of eucalyptus wood [24]. Besides, ultrasound
has been found to be applied in some other bioprocessing of biomass,
such as anaerobic digestion of microalgal biomass for improving methane
yield [25], ultrasound-assisted alkali pretreatment for bioethanol and xyla-
nases production from chili residue [26], and so on.
Microwave pretreatment relies on the creation of heat by direct inter-
action between a heated object and an applied electromagnetic field [27].
Therefore the heating efficiency of microwave is higher with easier opera-
tion than the conventional conduction/convection heating (such as air
bath heating, water bath heating, oil bath heating, and oven heating)
[28,29]. However, microwave pretreatment is usually used as a heating
method for other pretreatment, especially for chemical pretreatments.
Microwave irradiation with water to pretreat switchgrass obtained 53%
higher total sugar than that treated with conventional heating pretreat-
ment [29]. As found by Zhao et al. [30], alkali pretreatment combined
with microwave irradiation of rice hulls could significantly increase the
production of cellulase and reducing sugars by Trichoderma sp. 3.2942.
The maximum filter paper activity, carboxymethylcellulose, and reducing
sugar content (RSC) were increased by 35.2%, 21.4%, and 13%, respec-
tively, compared with those of untreated rice hulls, which was primarily
due to the rupture of the rigid structure of rice hulls by the pretreatment.
Jin et al. studied microwave-Ca(OH) 2 pretreatment for catalpa sawdust
and found that the optimal condition was with 40 mesh particle size,
2.25% (w/v) Ca(OH) 2 dosage, 400 W microwave power, and 6 min pre-
treatment time. When the pretreated substrates were enzymatically hydro-
lyzed with 175 FPU/g enzyme loading for 96 h, the reducing sugar yield
reached 402.73 mg/g, being increased by 682.15% compared with that of
raw catalpa sawdust [27].
EB pretreatment relies on high energy electrons to create reactive radi-
cal species within the biomass and the secondary reactions of these radicals
typically lead to a series of bond reactions within cell wall polymers [31].
EB has been found to have the function of reducing the DP of cellulose
and causing alteration of lignin hemicellulose matrix [26]. EB pretreat-
ment also has been verified to show beneficial effects on the subsequent
treatment including acetic acid pretreatment, SE pretreatment, hot water
extraction, etc. When switchgrass was pretreated by EB irradiation, it was