Fuels and Chemicals from Hemicellulose Sugars - Biotechnology in China: Biofuels and Bioenergy - page 202

Fig. 1 Schematic structure of corn fiber heteroxylan (modified from [ 11 , 16 ])

In general, hemicellulose can be hydrolyzed to its sugar constituents by
chemical or enzymatic processes. The chemical hydrolysis of hemicelluloses is
much easier to accomplish than the hydrolysis of cellulose, due to the heteroge-
neous structure and composition of hemicelluloses and its low degree of
polymerization and low crystallinity [ 17 ]. Generally, the first step of hemicellu-
lose-based fuel or chemical production is lignocellulose pretreatment, and different
pretreatment methods have been developed. In most cases, hemicelluloses are
commonly removed during the initial stage of biomass processing, aiming to
reduce structural constraints for further enzymatic cellulose hydrolysis (Fig. 2 )
[ 18 , 19 ]. The main process options for the selective fractionation of hemicelluloses
from biomass include the use of acids, water (liquid or steam), organic solvents,
and alkaline agents (Table 2 )[ 20 ]. The last two are not selective towards hemi-
cellulose as they also remove lignin, which in turn can hinder the fermentation or
bioconversion process of the hemicellulose sugars. Therefore, acid/water/steam
pretreatments are the most commonly applied technologies, yielding a selective
solubilization of hemicelluloses and producing hemicellulose-rich liquids totally
or partially hydrolyzed to oligomeric and monomeric sugars and cellulose-

enriched solids for further bioprocessing.
Many acids are known to be good hydrolytic agents. The common method of
acid hydrolysis uses dilute acid [ 21 ]. Dilute sulfuric acid hydrolysis has proved to
be a favorable method for pretreatment before enzymatic hydrolysis and also for
the conversion of hemicellulose to sugars [ 22 - 24 ]. One of the earliest commercial
hydrolysis processes was a dilute sulfuric acid process carried out at a relatively
low temperature for a prolonged period of time. Recently, a great deal of research
has examined the dilute acid hydrolysis of woods and agricultural residues to
produce sugars. During acid hydrolysis of hemicellulose, pentoses are degraded
rapidly to furfural and condensation byproducts [ 21 ]. These compounds may also
inhibit the later fermentation processes, leading to lower yield and productivity,
and a prior detoxification treatment might therefore be required. In order to prevent
the decomposition of sugars, especially pentose, a more dilute acid, a shorter
reaction time, a lower temperature, and the rapid removal of hydrolytic agents are
required.