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In conventional ethanol production, starches and sugars are converted into ethanol in a few steps involving a series of enzymes. Thermostable α-amylase is used in the presence of water and heat to liquefy the starch, followed by glucoamylase to turn the liquefied starch to sugars. Subsequently, a biocatalyst is added in the form of yeast that ferments the sugars to ethanol.
The overall and simplified process to convert starch, via glucose, to ethanol and CO2, looks as follows:
This process is a mature technology that is carried out industrially. When lignocellulose is the substrate for ethanol production, there may be some drawbacks, however. These drawbacks relate to the complexity of the lignocellulosic cell wall components that make up the biomass.
Furthermore, hydrolysis of the hemicellulose portion of the lignocellulosic biomass generates significant amounts of pentose sugars and potential microbial inhibitors. The presence of these compounds exerts undue stress on the fermenting microorganism, leading to poor cell growth and low ethanol productivity.
To put the above in a shorter form, fermentation by common yeast is limited by the fact that yeast fungi basically can ferment only such sugars with 6 carbon atoms in them (hexose's) while many of the sugars present in biomass are pentose's, i.e. they contain 5 carbon atoms, or pentose's are formed as intermediate products.
With natural yeast the fermentation is actually limited only to glucose but starch as well as some other types of sugar can be enzymatically transformed into glucose and hence qualify as fermentable. Thus, only some materials, those where fermentable sugars are readily available for the enzymes and the yeast, are naturally suitable for alcohol fermentation while most are not. However, two of the main constituents in plant cell walls – cellulose and hemicellulose – are both basically built up by fermentable sugars. The problem is that they are partly crystalline and are embedded in lignin, so that the sugars are not accessible. Different pre-treatments including dilute acids, hot water or ammonia make it possible to break this structure, decrystallize the cellulose and make the sugars in cellulose and hemicellulose accessible for subsequent fermentation. These processes are still under development and so far they suffer from high costs as well as of some of the by-products being inhibitors to the fermentation process. There is also research on-going to genetically modify micro-organisms so as to render pre-treatment unnecessary.
The product from fermentation is a dilute alcohol that needs be concentrated through distillation to attain fuel quality.
The main hindrances for an increase in the use of ethanol today are:
- The price for ethanol. The price reflects mainly the cost for distillation and is not likely to drop within a foreseeable future.
- The raw material for ethanol. A large fraction of today’s ethanol production is based on dedicated crops (maize) grown on fertile land. An increasing worldwide awareness of the scarcity of such land has triggered a reaction to the use of ethanol as a fuel.
Due to the competition about fertile land, ethanol production should only be based on fermentable residuals or at least on non-edible biomass.