Rates of Reaction

Catalysts - Enzymes.

What is Fermentation?

Yeast is a microorganism containing an enzyme
which will convert a sugar (glucose) solution
into carbon dioxide and alcohol (ethanol).
This process is called fermentation.

The word equation for fermentation is

glucose + yeast  arrow  carbon dioxide + ethanol.

Carbon dioxide gas bubbles out of the solution
into the air leaving a mixture of ethanol and water.
Ethanol can be separated from the mixture by
fractional distillation. Fermentation must be carried
out in the absence of air to make alcohol.
If air is present, ethanoic acid is made instead of alcohol.
Fermentation works best at a neutral or slightly acidic pH.

What is a Batch Process?

In a batch process the reaction vessel must be
emptied and cleaned and then refilled with the
new starting materials. A batch process takes more time
and is more expensive than a continuous process.

Yeast is used in a batch process to make alcohol in beer
and wine. An enzyme in yeast acts on the natural sugar
in malt (to make beer) and grapes (to make wine).
When the alcohol concentration reaches about 10 - 14%,
the alcohol damages the yeast and fermentation stops.

Is Drinking Alcohol Harmful?

Different alcoholic drinks contain different amounts of
alcohol. Some people drink alcohol for enjoyment, some
drink to excess and some people become addicted to
alcohol. The harmful effects (physical and social) of drinking
excess alcohol are widespread and reach all parts of society.

How is Ethanol made on a Large Scale?

Ethanol is made from both renewable and non-renewable
resources. Ethanol can be made on a large scale for use as a
fuel or solvent by the hydration of ethene (non-renewable)
or by the fermentation (see above) of sugar cane (renewable).

What does Renewable mean?

Renewable means that the resource can be replaced.
For example more sugar cane can be grown for
fermentation. Compare this with using fossil fuels
which are the non-renewable source of ethene.


Yeast is used in the baking of bread.
The carbon dioxide produced causes the bread to rise
and fills the bread full of bubbles.
The alcohol evaporates during the baking process.

and Cheese.

Bacteria can be added to milk to make yoghurt.
An enzyme in the bacteria reacts with sugar in the milk
(called lactose) and converts it into lactic acid.
Lactic acid changes milk proteins and forms the thicker yoghurt.

Cheese is made by adding an enzyme called rennet
after bacteria have produced lactic acid in milk. Rennet makes
milk proteins turn solid and this is the basis for cheese.

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