First Principles of Pretreatment and Cracking Biomass to Fundamental Building Blocks 205
fermentation used at 190∘C and 10 min resulted in the best overall yield for glucose as
well as for xylose; 100 g wheat straw yielded 42.6 g glucose and 22 g xylose.
6.7.3 High-Energy Irradiations
The irradiation is a physical process. In this procedure, energy is transferred to material
without needing molecules. It is essential that the energy does not cause radioactivity
in the material. The irradiation dose is defined as the amount of energy absorbed per
unit of mass. The dose of 1 Gy is the absorption of one joule of energy per gram of
material. Irradiation process is selective and has high efficiency. The high yield is related
to rotational electrons of atoms or food molecules or polluting compositions. When
activated rotational electrons leave atoms, chemical changes are created in atoms
or molecules, which is known as ionization. In this process, cation (an atom with a
positive electrical charge) or an ion with negative electrical charge (anion) is formed.
The ionization process constructs active atoms or molecules named free radicals. As the
required energy for ionization is relatively low, foods do not change visibly by radiation
process. A large portion of absorbed energy is used to produce free radicals and induce
chemical reaction between radicals or between radicals and other molecules. A little
part of absorbed energy is converted in the form of thermal energy. Consequently,
food materials are processed with a little thermal energy and so sensory attributes and
nutritional value of foods will conserved. The ionizing radiation is introduced as one of
the suitable methods to conserve food and its safety. The application of this method for
lignocellulosic biomass pretreatment has been small compared to chemical, thermal,
and mechanical pretreatment. Recent methods are expensive and use high energy and
produce fermentation inhibitors. To solve this problem, irradiation pretreatment can
be used for its various advantages, for example, it does not produce inhibitors and is
economic. High-energy ionizing radiation includes
•γ-irradiation produced from Co-60 or Cs-137;
- X-irradiation produced from machinery sources;
- electron beam irradiation.
All the abovementioned energy sources can have similar effects on food materials,
but the main difference is their variable penetration rates. Theγ- and X-irradiation can
penetrate highly, and they can be used to treat foods as bulk, whereas electron beam
irradiation is suitable for surface treatment or to treat thin boxes and packages.
The application of irradiation technology to pretreatment of lignocellulosic biomass
was first described by Aokiet al. (1977). They showed that irradiation pretreatment by
γ-irradiation resulted in the physical change of biomass. Other researchers reported the
effect of irradiation degradation of various lignocellulosic materials such as sugarcane
bagasse (Hanet al., 1981), rice straw (Kumakura and Kaetsu, 1979, 1984), corn cob, and
peanut husk (Chosduet al., 1993) for increasing sugar yield. The irradiation-induced
reactions in the macromolecules of the cellulose materials are known to be initiated
through rapid localization of the absorbed energy within the molecules to produce
long- and short-lived radicals, which caused the secondary degradation of materials
through chemical reactions such as chain scission, cross-linking, and so on (Khanet al.,
2006). The efficiency of these two types of reactions depends mainly on the polymer
structure and radiation dose (Charlesby, 1981). To reduce the required irradiation dose
