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Notes:

Some of you remember this from your chemistry, this is what happens during fermentation:

The Maltose gets inside the yeast's body where an enzyme first of all breaks it down into glucose.

While oxygen is available, further enzymes in the yeast combine the Glucose with oxygen to make water, carbon dioxide and lots of energy for the yeast cell in the from of Adenosine TriPhosphate, or ATP.

{C6H12O6 + 6 O2 +enzymes --> 6 H2O + 6 CO2 + 36 ATP}

The yeast reproduces rapidly. Since a packet of brewers yeast only contains about 10 grammes of little animals for a 23 litre brew, they need plenty of time to multiply, hence the aeration of the wort.

Once the air has run out, the reaction changes and will produce less energy for the yeast cell, but fortunately for us it now produces ethanol as a by-product.

{C6H12O6 + enzymes --> 2 C2H5OH +2 CO2 + 2 ATP + heat}

If the original gravity of the beer was 1044 and the final grvity 1010, the beer will contain 4.6% alcohol.