Production of
Ethanol by
Fermenting Sugars
ETHANOL
One method of making ethanol is to use plant
material which contains starch.
The reaction is called fermentation where the
sugar is converted into an alcohol.
Fermentation is an enzyme-controlled
reaction that takes place inside living yeast
cells in which glucose is broken down into
alcohol and carbon dioxide with the release
of energy.
The carbohydrates in the food must be broken
up first to form sugars. This is done by
hydrolysing the carbohydrate.
Hydrolysis is a reaction in which water
molecules (‘hydro’=water) react with larger
molecules to split them (‘lysis’= split apart)
into two or more smaller molecules.
O
H
H
O
H
H
O
H
H
water molecules
individual glucose molecules
part of a starch molecule
The sugar is then converted into alcohol.
The equation for the reaction is :
glucose → ethanol + carbon dioxide
C6H12O6 → 2C2H5OH + 2CO2
Fermentation is an enzyme-controlled reaction
that takes place inside living yeast cells in
which glucose is broken down into alcohol and
carbon dioxide with the release of energy.
Reactions that are catalysed by enzymes
normally produce the best results at
temperatures around 40OC.
The reactions will be slower at lower
temperatures and at higher temperatures, over
70OC, the enzyme will stop working because
the high temperature denatures the enzyme.
The fermentation process takes place inside
living cells therefore if the yeast dies then
the reaction will stop.
When the alcohol concentration reaches about
14% the it begins to kill the yeast.
Fermentation reactions therefore have a
limit to the percentage of alcohol they can
produce.
Biological Action
Yeast:
•
•
•
•
•
Living organism
Microscopic fungus
Feeds on sugars
Is a biological catalyst
Therefore classed as an enzyme
Conditions for Fermentation
pH: prefer a neutral pH (or round about) if it
is too acidic or alkaline the yeast will die.
Temperature: Room temperature is ideal; too
cold and the yeast stops
working (goes into a “sleep”
state) and too warm and it dies.
Alcohol concentration: yeast does not like
alcohol, about 14%
alcohol the yeast dies
Distilling The Alcohol
The alcohol concentration can be increased by
separating the alcohol from the water, this
is carried out via a process called distillation.
The separation by distillation is an easy
process as ethanol boils at 78OC and water
boils at 100OC although both liquids will
evaporate to some extent at any temperature.