202 Introduction to Renewable Biomaterials
and also xylose is released before glucose. They stated that the optimal condition to
produce maximum holocellulose (xylose+glucose) is 500–550 psi pressure and 40 s
process time. Later, many studies were conducted by other researchers about the effect
of steam-explosion process on different biomass. Shultzet al. (1984) investigated the
effect of steam-explosion pretreatment on different biomass such as pretreatment
of mixture of wood, husk, corn stem, and bagasse. They showed that pretreatment
(temperature 240–250 and time 1 min) increased the rate of enzymatic hydrolysis.
Steam explosion is one of the most common methods for the pretreatment of
lignocellulosic materials. In this method, biomass is treated with high-pressure
saturated steam, and then the pressure is suddenly reduced to air pressure, which
makes the materials undergo an explosive decompression. Steam explosion is typically
initiated at a temperature of 160–260∘C (corresponding pressure, 0.69–4.83 MPa)
for several seconds to a few minutes before the material is exposed to atmospheric
pressure (Sun and Cheng, 2002). In this method, no chemicals are used. During this
pretreatment process, degradation compounds of pentoses and hexoses primarily
furfural and 5-hydroxymethyl furfural (5-HMF) are formed (Del Campoet al., 2006).
The biomass/steam mixture is held for a period of time to promote hemicelluloses
hydrolysis, and the process is terminated by an explosive decompression. The process
causes hemicellulose degradation and lignin transformation due to high temperature,
thus increasing the potential of cellulose hydrolysis. Hemicellulose is thought to be
hydrolyzed by acids during steam-explosion pretreatment. Grouset al. (1986) reported
that 90% efficiency of enzymatic hydrolysis was achieved in 24 h for wood chips
pretreated by steam explosion, compared to only 15% hydrolysis yield of untreated
wood chips. Removal of hemicelluloses from microfibrils is believed to expose the
cellulose microfibrils to cellulose enzyme. Lignin is only to some extent removed
from the material during the pretreatment but slightly redistributed on the cellulose
fiber surfaces due to melting and depolymerization reaction that can be avoided by
the quick release of biomass into atmospheric pressure. Hemicelluloses and lignin
depolymerization increase the pore volume of the pretreated material (Liet al., 2007).
Depending on the severity of the steam-explosion pretreatment, conversion of the
cellulose to glucose will also take place. Addition or immersion of the material with
H 2 SO 4 or SO 2 (typically 0.3–3% w/w) prior to pretreatment can decrease the time and
temperature of pretreatment and at the same time increase the recovery of sugars,
reduce the formation of inhibitors, and improve the enzymatic hydrolysis (Ballesteros
et al., 2006). For pretreatment of softwoods, the addition of an acid (H 2 SO 4 or SO 2 )is
necessary to make the substrate accessible for enzymes (Jorgensenet al., 2007; Stenberg
et al., 1998). Steam provides an effective vehicle to rapidly heat cellulose to the target
temperature without excessive dilution of the resulting sugars. Rapid pressure release
reduces the temperature and quenches the reaction at the end of the pretreatment.
The rapid thermal expansion opens up the particulate structure of the biomass and
enhances the digestibility of cellulose in the pretreated state (Jorgensenet al., 2007).
The important variables in steam-explosion pretreatment are the time and temperature
of pretreatment, particle size, and moisture content of biomass. Good hemicelluloses
solubilization and hydrolysis can be achieved by either high temperature with short
residence time process (27∘C for 1 min) or low temperature with long residence time
process, for example, 190∘C for 10 min (Duff and Murray, 1996). The advantages
of steam-explosion pretreatment include the low energy requirement compared to
