52 Introduction to Renewable Biomaterials
Due to severe treatments caused by acidic and partly applied heated extraction
processes, hemicellulose degrades by losing its chain length, and consequently, exhibits
high polydispersity. Steam and microwave treatments are combined with chemicals to
dissolve the hemicellulose; however, due to the complexity of those methods, they have
been applied mainly on a small scale for hemicellulose extraction. In steam treatment,
ester bonds in the hemicelluloses will be cleaved and then the wood is treated with
steam, resulting in the formation of acetic acid. This will lower the pH and, thus,
induce autohydrolysis of the glycosidic bonds in the hemicelluloses. Such a process
will generate a low-molecular-weight, water-soluble hemicellulose. From this method,
one can get a low percentage yield of hemicellulose with significant contamination of
dissolved cellulose and lignin and degradation products of the hemicellulose.
An ionic liquid/cosolvent extraction system has been used by Froschauer and
coworkers to separate hemicellulose and cellulose from wood pulp. In this study,
hemicellulose-rich birch kraft pulp was selectively separated, with high levels
of purity, into pure cellulose and hemicellulose fractions by using mixtures of
cosolvents (water, ethanol, or acetone) and the cellulose-dissolving ionic liquid
1-ethyl-3-methylimidazolium acetate (EMIM OAc) with reaction conditions of 60∘C
for 3 h under stirring. This process was used to generate dissolving pulp that met
the manufacture of revived cellulose products and cellulose derivatives because of its
extraordinary cellulose content, which was assumed to be more than 90%, as well as the
product’s high brightness and uniform molecular-weight distribution.
Alkaline extraction is well studied and is known as a strong and efficient method
for hemicellulose extraction. Indeed, alkaline treatment can cleave the ester linkage
between ferulic acid of lignin and the glucan and arabinan residues of hemicellulose
in the cell wall, thus releasing oligo- and polymeric hemicellulose rather than sugars,
as obtained from dilute-acid extraction or hot-water extraction. The extraction of
hemicelluloses from various biomasses is expected to be closely related to the amount
of LCCs. The LCCs entail covalent linkages between lignin and carbohydrates, mainly
hemicelluloses. The major types of LCCs include phenyl glycoside, benzyl ether, and
benzyl ester types of linkages. Different biomasses have different frequencies of LCC,
for example, the amounts of ether and ester LCC linkages in pine and aspen cellulolytic
enzyme lignin (CEL) have been reported to be about 2.2–2.5 and 0.3–0.6 per 100
monomeric lignin units, respectively.
The decision on which method to use for hemicellulose extraction is highly dependent
on the final application of the recovered hemicellulose. For example, if the extracted
hemicellulose is targeted for the production of bio-ethanol, then the monomeric form
of sugars is required, and thus, it may be preferred to use dilute acid or hot water.
Some other applications require a high-molecular-weight polymer (blended plastics,
hydrogels, and others). For those applications, it may be extremely important to use the
alkaline method for the isolation of hemicellulose from the biomass.
2.3.2.1 Structural Characterization of Hemicellulose
Great interest in converting lignocellulosic biomass into valuable, green fuels and
chemicals has challenged researchers to develop methods for determining the struc-
ture, accurate chemical composition, quantity, and potential uses of hemicellulose
in the lignocellulosic biomass [26]. Several analytical methods have been used to
characterize hemicellulose: high-performance liquid chromatography (HPLC), gas
