Sunflower - Bioenergy Crop Plants - page 720

etc. and chemical methods using sodium hydroxide (NaOH), hydrochloric acid, sulfuric acid, sulfur
dioxide, alkaline peroxide, phosphoric acid, ammonia, organic solvents (ethylenediamine),
supercritical carbon dioxide, etc. (Kosaric et al. 1980, Weil et al. 1994; Zhang et al. 1995) . Keller
et al. (2003) have even used biological methods for pretreatment of biomass through thermochemical
pretreatment methods and have reported these to be promising (Sheehan et al. 2003).
30.4.1 m Echanical p rEtrEatmEnt /m illing
Milling of lignocellulosic material is a popular method for increasing cellulose digestibility. The
material to be treated is subjected to shearing and compressive forces generated by the mill for a
specific period of time. Mechanically-based pretreatment technologies aim at reducing the size
of biomass to below #20 sieves and to biomass that shows the best mechanical performance (de
Sousa et al. 2004). Mechanical pretreatment technologies increase the digestibility of cellulose and
hemicellulose in the lignocellulosic biomass, resulting in substantial lignin depolymerization via
the cleavage of uncondensed aryl ether linkages (Inoeu et al. 2008). Solubility and fermentation
efficiency of these residues is also substantially increased by this pretreatment, leading to value-
added utilization of these residues (Qi et al. 2005). The use of mechanical chopping, hammer
milling, grind milling, roll milling, vibratory milling, and ball milling are some of the successful
low-cost pretreatment strategies (Mtui 2009).
Grinding of lignocellulosic material to a very small particle size makes it susceptible to enzymatic
hydrolysis, thus improving digestibility of cellulose and hemicellulose to glycans and xylans,
respectively (Chahal 1982). Tewari et al. (1987) studied the effect of grinding on the enzymatic
hydrolysis of wheat straw, bagasse, corn cob, and groundnut shell. A mesh size of 40 was found to
be the best, except in sugarcane bagasse, in which a mesh size of 20 gave the maximal enzymatic
attack (de Sousa et al. 2004). In our laboratory, sunflower stalks ground to a size of 40 mesh have
been found to be optimal for biomass digestion (Sharma et al. 2002b). Vaithanomsalt et al. (2009)
and Okur and Saracoglu (2006) reported a 40-mesh size for sunflower stalks and a 0.71- to 1.0-mm
size for sunflower hulls in their studies, respectively, for optimized pretreatment. Yuldashev et al.
(1992) and Alvo and Belkacemi (1997) reported milling as the sole pretreatment sufficient for
further optimized saccharification.
30.4.2 p hySicochEmical p rEtrEatmEnt
Elevated temperatures and irradiation are the most successful physical treatments in the processing
of lignocellulosic biomass. However, combined physical and chemical methods prove more
important in the breaking up of the crystalline biomass structure, providing an improved accessibility
of cellulose for enzymatic hydrolysis (Hendriks and Zeeman 2009). In addition to steam hydrolysis,
liquid hot water, ammonia fiber explosion, and carbon dioxide explosion are other such technologies
in which biomass is treated with high pressure, followed by sudden depressurization (Sun and
Cheng 2002). The changes in the steam-treated biomass depend on the temperature, pressure, and
time of exposure to steam. The organic acids derived from acetylated polysaccharides hydrolyze
the hemicellulose to soluble sugars. Secondary reactions that occur under more drastic conditions
result in the formation of furfurals, hydroxymethyl furfurals, and their precursors by dehydration
of pentoses and hexoses (Chahal 1982; Almeida et al. 2007). It has been suggested that these
compounds can reduce enzymatic and biological activity and even cause DNA breakdown (Endo
et al. 2008). The addition of sulfuric acid, sulfur dioxide, or carbon dioxide can further improve
upon the enzymatic hydrolysis, decrease the production of inhibitory compounds, and lead to more
complete liquefaction of hemicellulose, glucan, xylan, etc. (Sun and Cheng 2002). The Stake and
Iotech processes that are commercially operating in Canada involve the steam-heating of wood
biomass chips to approximately 180-200°C for 5-30 min in a continuous operation and to higher
temperatures (245°C) for a shorter time (0.5-2 min) in batch mode, respectively (Wayman 1980).