Wood and starch are both polysaccharides but in starch the glucose molecules are oriented in the same direction (making it easier for certain microorganisms to digest etc).
What is cellulose?
An insoluble substance that is the main constituent of plant cell walls and of vegetable fibers such as cotton, paper and wood.
Cellulose molecules consist of long chains of glucose molecules as do starch molecules, but have a different structural configuration.
How is cellulose treated to be used by yeast, to make bioethanol?
Most cellulases are complexes of three enzymes working together to hydrolyse cellulose.
First, an endoglucanase (an enzyme) breaks one of the chains within the cellulose crystal structure.
Then, an exoglucanase (another enzyme) attaches to one of the loose ends, pulls the cellulose chain out of the crystal structure, and works its way down the chain, breaking off units of cellobiose (two glucose units joined together).
Finally, a beta-glucosidase (enzyme that acts upon bonds linking two or glucose-substituted molecules) splits the cellobiose into two glucose molecules, which can then be fermented into ethanol.
What biological processes are involved in changing starch into bioethanol?
How is Ethanol Made
The modern ethanol production plants employ dry mill method, that involves use of yeast to convert sugar into ethanol.
The major steps in this process are as follows:
Milling: The entire corn kernel or feedstock or other starchy grain is passed through a hammer mill which grinds it into a course flour called meal.
Liquefaction: The meal is mixed with water and alpha-amylase enzyme to form a 'mash' which is passed through cookers where the starch is liquefied. Cookers with high temperature stage (120 - 150º Celsius) and a lower temperature holding period (95º Celsius) are used for liquefaction. These high temperatures reduce the bacteria levels in the mash.
Saccharification: At this stage the mash from the cookers is cooled down and the secondary enzyme (gluco-amylase) is added to turn the liquefied starch into fermentable sugars
Fermentation: The mash is cooled and transferred to fermenters where yeast is added and fermentation of sugar to ethanol and carbon dioxide (CO2) begins. This fermenting mash is allowed to pass through several fermenters until is it completely fermented and leaves the final tank. The entire fermentation process usually takes about 40 to 50 hours.
Distillation: The fermented mash is now referred to as beer. The beer is pumped to the continuous flow, multi-column distillation system where the alcohol is separated from the solids and water with the help of heat. In the final distillation column, the alcohol at the top of the final column leaves behind a highly concentrated ethanol.
Dehydration: At this stage, the alcohol in the top of the last column is then passed through a dehydration system where the left over water is removed. The alcohol product now called anhydrous ethanol (pure, without water)
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