182 Introduction to Renewable Biomaterials
and conversion procedure, the amount of production yield of final products (sugars,
alcohol, organic acids,...), and fermentation inhibitors should be considered.
Forests and trees are one of the main resources of lignocellulosic materials. But their
production is a time consuming process, and irregular cutting of trees is forbidden in
some countries. Marine algae is another resource of cellulose, but the amount of their
cellulose content and dry matter is low; therefore it is not an economic source. The
agricultural plants are generated in large quantities each year, and so they are suitable
resources to produce cellulose. Besides this, the agricultural wastes such as bark and
pulp of fruits after processing can be used to generate fermentative product feedstock.
Eighty years ago, extensive researches were conducted to produce fermentative
products especially bioethanol. In advanced countries like America, Canada, Brazil,
Cuba, and European countries, bioethanol is used as a part of (20%) vehicle fuel.
Industrial processes for bioethanol production use sugar cane, corn, and cereal grain
as feedstocks, since the price of feedstock contributes more than 55% of the production
cost, free or low-cost feedstocks such as hemicellulosic biomass and agri-food wastes
canbeusedtoreducethecostoffermentationprocess(DelCampoet al., 2006). Here,
there are several questions related to lignocellulosic materials. First, how can the pote-
tial of lignocellulosic materials be used? How can these materials be processed? Will
this process be economic and efficient and will not produce fermentative inhibitors?
To answer these questions, the structure and chemical compositions of lignocellulosic
materials should be attended. As stated earlier, stem of plants and tree trunk are
composed of lignocellulosic materials. Cellulose is the main component of plant cell
wall. It is responsible for plant’s chemical and mechanical resistance (Ravenet al., 1992).
Hemicellulose is a copolymer with 5- or 6-carbon sugars. Lignin is an aromatic polymer
that forms a shield on plant cell wall. It is evaluated that 7.5× 1010 tons of cellulose
is consume and regenerated per year (Kirk-Otmer, 2001). Thus cellulose is the most
abundant polymer on the earth. There is a little protein, mineral, and so on in cellulosic
material structures. The variety of lignin, hemicelluloses, and cellulose depends on the
production source (hardwoods, softwoods, or grasses). Cellulose is a polymer of sugar
units. Hence, it is fermentable and can be converted by chemical treatment in the form
of different products such as carboxymethyl cellulose (CMC), hydroxypropyl methyl
cellulose (HPMC), and crystalline cellulose. These are hydrocolloid materials and act
as coating agent, thickening agent, and so on in different industries especially in the
food industry. Cellulose is found in crystalline and amorphous forms. Few chains of
polymer are connected together and form microfibril, and so connection of different
microfibrils together lead to the formation of fibers. By this way, crystalline structure
of cellulose is generated. To separate cellulose from lignocellulosic materials, first it
should be released because cellulose and hemicellulose connect together, and there
are cross-links between them. After that, crystalline structure of cellulose must be
converted to amorphous structure that is hydrolyzed better by enzymes (Mtui, 2009).
Before two decades, extensive researches have been conducted about the efficient con-
version of lignocellulosic materials to fermentable sugars. Based on these researches, we
can say the majority of the carbohydrates can be hydrolyzed to single sugars or monosac-
charides and fermented to ethanol. However, in the case of cellulose and hemicelluloses,
the necessary technology for this process differs from that which is employed by the
conventional starch-to-ethanol industry because of the complex chemical structure
of their compounds. An initial pretreatment stage (acid hydrolysis, steam explosion,
