The primary crops, currently being used or considered for ethanol production can be categorised by agricultural and process-related properties. The concentration of fermentable carbohydrates and other parameters in feedstocks, used for ethanol production are highly variable (see table 1).
Table 1: General ethanol feedstock consituents (wt %)
1 % w/w (as-is basis)
2 % w/w (dry basis)
Photosynthesising plants harness sun´s energy, in order to convert atmospheric carbon dioxide and water dextrose, resulting in the release of oxygen into the atmosphere.
Figure 1: Photosynthesis - conversion of CO2 in the presence of sun light to make sugar and oxygen.
Carbohydrates are polyhydroxy aldehydes, polyhydroxy ketones, or compounds that can be hydrolyzed from them. The smallest carbohydrates, those that cannot be hydrolyzed to smaller carbohydrate units, are called monosaccharides (e.g. glucose, fructose). Those, consisting of two monosaccharides, which may be the same or different compounds, are called disaccharides (e.g. lactose, maltose). Carbohydrates consisting of a few monosaccharides are called oligosaccharides (e.g. raffinose). Polysaccharides contain thousands of covalently linked monosaccharides. Among the most important polysaccharides in nature are starch (amylase and amylopectin), cellulose, and hemicellulose.
The two dominant simple sugars (monosaccharides) are the five-carbon sugar, D-xylose and the six-carbon sugar D-glucose (figure 2). D-glucose serves as readily available chemical energy and as a supply of carbon for producing more-complex materials (disaccharides, oligosaccharides, polysaccharides).
Figure 2: Primary plant monosaccharides
These two sugars, in combination with several other minor sugars, serve as building blocks for the production of more-complex carbohydrates. (e.g. sucrose, a disaccharide containing one molecule of D-glucose and one molecule of D-fructose; starch, cellulose and hemicellulose). These more-complex carbohydrates function as structural components and as long-term energy stores.
The plant´s primary method of storing energy for extended periods of time is starch production. There are two major types of starch, amylase and amylopectin, which differ in bond structure, reactivity and associated physical properties (figure 3).
Figure 3: A molecular representation of the two components of the starch molecule
Amylose is a linear dextrose polymer of α-1,4 bonds with a molecular weight ranging from 4,000 to 340,000. Amylose can be hydrolysed with acid or with enzymes.
Amylopectin is a nonlinear carbohydrate polymer, which contains millions of D-glucose units, linked by α-1,4 and α-1,6 bonds, resulting in a branched configuration (figure 3). Therefore the reactivity of the molecule as well as the physical properties of the associated starch slurries are changed.
Cellulose is the most abundant molecule in nature. It is also a D-glucose polymer, with an average molecular weight of 1.2 to 2.7 million. Cellulose has a dominant bond structure with ß-1,4 linkage, indtroducing significant changes in the reactivity of the molecule. Cellulose contains numerous weak hydrogen bonds additionally to ß-1,4 D-glucose bonds (figure 4). Therefore, the result is a crystalline, hydrophobic molecular structure.
Figure 4: A molecular representation of the structure of cellulose
In addition to cellulose, hemicellulose and lignin with associated acetyl-group cross-linkages are often found together, producing a very recalcitrant molecule, which is capable of withstanding microbioal attack.
Hemicellulose is a polysaccharide, containing from 500 to 3,000 sugar monomers, especially found as key component of plant cell-wall structures. Hemicellulose, the second most abundant carbohydrate in nature is a mixed polymer (in contrast to starch and cellulose) of five- and six-carbon sugars, with the dominant one being xylose. Hemicellulose has random amorphous structures, which can be hydrolysed by dilute acids, bases and appropriate enzymes. The constituent sugars of hemicellulose are not readily fermentable to ethanol by naturally occurring ethanologenic microbes.
Lignin is a complex chemical compound and an integral part of the secondary cell wall of plants. Structurally, lignin is a cross-linked racemic macromolecule with molecular masses in excess of 10,000 u. Additionally, it is one of the most abundant organic polymers on Earth. Lignin - a hydrophobic biopolymer, has no defined primary structure, because of its heterogeneity. The main function of lignin is the support through strengthening of wood (xylem cells), filling the spaces in the cell wall between cellulose, hemicellulose and pectin components. Lignin is indigestible by animal enzymes. Only some fungi and bacteria secrete ligninases, which can biodegrade the polymer. Some lignolytic enzymes are e.g. manganese peroxidise, lignin peroxidase and cellobiose dehydrogenase.