206 Introduction to Renewable Biomaterials
and acceleration of the degradation of lignocellulosic materials for economic reasons,
irradiation pretreatment of rice straw with the addition of low concentrations of NaOH
was studied (Lu and Kumakura, 1993).
High degradation of lignocellulosic biomass by a relatively small irradiation dose is
attributed to the weakness of the chemical links between lignin, hemicelluloses, and
cellulose units as well as the change of compositions through alkali swelling. After the
termination of the irradiation process, produced radicals in amorphous regions will
be extinct quickly, while others that are trapped in the crystalline and semicrystalline
regions of the cellulose structure can decay at a certain period and cause a further
degradation of lignocellulosic biomass. The latter phenomenon is called an after effect
and was first described by Glegg and Kertesz (1956) in purified wood cellulose (the
viscosity of cellulose described at least 2 weeks after irradiation). A similar result was
reported by Gonget al. (1998) on cotton cellulose. Further researches on the postirradi-
ation degradation were under taken by the analysis of radicals, and a significant decline
of cellulosic radical was observed with extended storage time after irradiation and the
rate was dependent on temperature, humidity, presence of oxygen, and so on (Polovka
et al., 2006; Bayram and Delincée, 2004). The presence of lignin, which is a polyphenolic
material, will affect the overall radiochemical events. Phenoxy radicals appeared to
be important radical intermediates that ultimately transformed intoo-quinonoid
structures in lignin. The presence of quinine-type radicals in irradiated lignocellulosic
materials also was reported by Simkovicet al. (1986). As the great influence of after
effect on the irradiation degradation of lignocellulosic materials, some studies were car-
ried out to accelerate the decaying rate of radicals after the termination of irradiation.
The radiation degradation of straw was enhanced by heating after irradiation, which
attributed to the energetic excitation of radicals (Kumakura and Kaetsu, 1984). The
radiation pretreatment of the rice straw was also enhanced by heating after irradiation
(Lu and Kumakura, 1993). However, the after effect of irradiation pretreatment on
enzymatic hydrolysis of lignocellulosic biomass has not been reported. Chunpinget al.
(2008) evaluated the effect of irradiation pretreatment of wheat straw under different
doses of CO-60훿radiation. The weight loss and the fragility of wheat straw after irra-
diation, the combination effect of irradiation and mechanical crushing on enzymatic
hydrolysis of wheat straw as well as the after effect of irradiation were examined. Results
showed that irradiation pretreatment can cause significant breakdown of the structure
of wheat straw. The weight loss of wheat straw increased, and the size distribution
after crushing moves to fine particle at elevated irradiation doses. The glucose yield of
enzymatic hydrolysis of wheat straw increased with increasing irradiation doses and
achieved the maximum (13.40%) at 500 kGy. A synergistic effect was observed between
doses of 500 kGy with powder size of 140 mesh number. The irradiation after effect had
an important effect on enzymatic hydrolysis of wheat straw. The after effect of 400 kGy
(at twenty-second day) was 20% of the original effect in glucose production while the
after effect of 50, 100, 200 (at ninth day), and 300 kGy (at twentieth day) accounted
for 12.9%, 14.9%, 8.9%, and 9.1%, respectively, for reducing sugar production. Stiku
et al. (2008) investigated the effect of high dose irradiation as a pretreatment method
on two common lignocellulosic materials (hardwood and softwood) by assessing the
potential of cellulose enzyme derived fromAspergillus flavusLinn isolate NSPR 101 to
hydrolyze the biomass materials. The irradiation strongly affected the materials, causing
the enzymatic hydrolysis to increase by more than threefold. Maximum digestibility
