http://www.gcsescience.com/rc17-fermentation-yeast-alcohol.htm

Fermentation.

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

The word equation for fermentation is

glucose + yeast 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.

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%,
the alcohol damages the yeast and fermentation stops.
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.

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.


Ethanol can be made on a large scale for use as a fuel or solvent
by the hydration of ethene or the fermentation of sugar cane.
Sugar cane is a renewable resource.
Renewable means that the resource can be replaced.
For example more sugar cane can be grown.
Compare this with the use of fossil fuels (the source of ethene)
which are a non-renewable resource.

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.


Yoghurt 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.