194 Introduction to Renewable Biomaterials
6.2.8 Hydrogen Bond
The existence of hydrogen bonds is distinguished between cellulose chains. The
hydrogen bond is formed between a hydrogen atom of one hydroxyl group of a glucose
monomer and the oxygen atom of a hydroxyl group of another glucose monomer in the
parallel polymer chain of cellulose. Cellulose fiber formation and its insolubility in water
is the result of hydrogen bonds. Also, hydrogen bond has been found in hemicellulose
polymers. The connection of hemicellulose to cellulose is not very strong, which is due
to the absence of primary alcohol functional groups outside the pyranoside ring that
restrict the capacity of hemicellulose to form hydrogen bonds.
Breakdown of hydrogen bonds can be done by applying high temperature or
by substituting the forming hydrogen bond molecules. Generally, there are two
procedures to cleave a hydrogen bond:
1- Recognizing groups that they can form hydrogen bonds of higher energy than the
ones formed in cellulose.
2- Varying and modifying the cellulose structure. This action can be achieved by
physical destruction of the cellulose or by chemically producing cellulose derivative
compound such as cellulose acetate (Bochek, 2003).
To produce the necessary carbon source for preparing media culture for fermentation
process, various resources, including simple sugars, starch, fats, and agricultural and
food wastes were used. The source of carbon is a large portion of fermentation process
costs. Thus, using lignocellulosic wastes is economical and environmentally friendly.
Agricultural wastes and crops are an attractive feedstock for use in food and bioenergy
production. However, in Asia, Canada, and Europe, rapeseed and other crops are
cultivated for the purpose of obtaining oil and foods for food and biodiesel resources.
In general, agricultural straw is composed of three main fractions, including about 34%
cellulose 19% hemicellulose, and 6.9% lignin. Due to their high content of fermentable
sugars (more than 60%), cereal straw has been suggested as a raw material for use as
a biofuel feedstock (Kyeonget al., 2011). In order to use this potential sugar source,
pretreatment must be used. In the following, a summary of researches done in the field
of lignocellulosic pretreatment is discussed. An initial pretreatment stage (acid and
alkali hydrolysis, steam explosion, wet oxidation, etc.) is needed to soften the material
and break down the biomass structure to make it more susceptible to enzymatic attack
before fermentation (Mtui, 2009). After initial pretreatments on lignocellulosic material
and access to cellulose, enzymatic hydrolysis must be applied. Cellulose enzymes such
as endoglucanase, exoglucanase, andβ-glucosidase are employed. Enzymatic hydrolysis
can be improved by optimization of substrate concentration, enzyme-to-substrate
ratio, and using surfactant and enzyme mixtures (Champagne, 2008).
6.3 Define Pretreatment
6.3.1 What Is the Purpose of Pretreatment?
The purpose of pretreatment is removal of lignin and hemicelluloses, reduction of cel-
lulose crystallinity, and increase of biomass porosity. The ideal pretreatment consists of
- avoiding the need for reducing the size of biomass particles;
