Ethanol

(i) from starch and sugar

Alcohol is produced from most grains. In Canada and U.S.A., maize (corn) is the raw material. Although chemistry texts tell us that ethanol is made from ethene, much industrial ethanol is produced by fermentation. This section is based on information from the Chippewa Valley Ethanol Company

The process
The corn passes through hammer mills, which grind it into a fine powder called meal. The meal, mixed with water and alpha-amylase, passes through cookers, where the starch is liquefied. Continuous cookers with a high temperature stage (120-150°C) and a lower temperature holding period (95°C) are used.
The mash from the cookers is cooled and the secondary enzyme (gluco-amylase) added to convert the liquefied starch to fermentable sugars.

2(C₅H₁₀O₅)_n + nH₂O ® nC₁₂H₂₂O₁₁

This enzyme is active at 55°C.
Yeast is added to the mash to ferment the sugars to ethanol and carbon dioxide.

What 2 reactions are usually involved?

* one enzyme catalyses the hydrolysis of maltose to glucose,
* another enzyme catalyses the decomposition of glucose to ethanol and carbon dioxide

In a continuous process, the mash flows through several fermenters until all sugars have fermented.
The ethanol content of the mash is about 10%. The mash also contains water, non-fermentable solids, yeast cells. It is pumped to the continuous flow, multi-column distillation system, where ethanol is removed from the solids, water and other material.
The distillate (about 95% alcohol, 5% water) is transferred to purification columns.The other material travels to centrifuges.The water content must be removed before the ethanol is suitable as a fuel for internal combustion engines. Fusel oils (higher alcohols) are removed.

Industrial ethanol is denatured by the addition of propanol and butanol. In some countries, denatured ethanol is dyed.

The centrifuge separates solids and liquids. In the evaporation unit, liquid from the centrifuge (thin stillage)is separated into syrup which is added to the grain, and water which is returned to the mash bin.

There are two co-products created in the production of alcohol:
Carbon dioxide, given off in great quantities during fermentation, will be collected and cleaned of any residual alcohol, compressed and sold as an industrial commodity.
DDGS is recovered from the stilllage and will be sold as an animal feed ingredient to farmers.

There is a fungus in the Candida genus which directly ferments starch to ethanol.