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Fermentation 101

Beer Smith Link

Fermentation, what is it?
It is the way yeast converts sugars to (ethyl) alcohol and carbon dioxide.

To begin fermentation, the cooled wort is transferred to a carboy or conical fermenter, then yeast is added, either before of after the wort is transferred.  If you are using ale yeast you will need to maintain a temperature around 70º F.  If a lager yeast is being used, a temperature of around 48º F should be maintained. Fermentation produces heat so maintaining the proper temperature is important.
Before fermentation the specific gravity of the wort is measured by a hydrometer. The specific gravity is measured again at the end of the fermentation cycle to determine how much alcohol is in the beer and to determine if fermentation is completed.  To measure the gravity, use the scale (see photo) to determine the specific gravity, usually expressed digitally as 1.049 (water alone is in theory 1.000).  

                                                                                                          Shown here is water at 1.000.

The reading after fermentation is compared and the alcohol content is determined by this formula:
Original gravity – final gravity (without the decimal points) ÷ 7.5

You may also use a brewing software such as BeerSmith2 to calculate alcohol content.

How does fermentation take place?

When the wort is put in the fermenter the concentrations of sugars are high.   Yeast is then added. The yeast digests the sugars and uses it for energy to reproduce. As it does, the sugars are broken down by a process called glycolysis. The product of glycolysis is 2 three-carbon sugars, called pyruvates, and adenosine triphosphate that supply food energy to the yeast and allows it to multiply. The 2 pyruvates then are converted by yeast into carbon dioxide and ethanol alcohol.  The fermenter is protected from outside air by an S type valve or air-lock which allows carbon dioxide to escape without allowing air back in.  This reduces the threat of contamination by airborne yeasts.

When fermentation is nearly complete, most of the yeast will settle to the bottom of the fermenter.  This can be harvested and saved then used in the next batch of beer.  Yeast can be reused a few times before it needs to be replaced. It will over time mutate and produce off flavors.  It also can be harvested to produce high cell counts for higher gravity beers.  See my article about reusing yeast.

Some of the yeast will stay in the beer unless filtered of pasteurized.  The way we carbonate beer naturally is by adding a small amount of sugar to reactivate the yeast just before bottling.  As the yeast metabolizes the small amount of sugar, it creates carbon dioxide gas again and we have natural of bottle carbonation.  Store the bottles at the same temperature as the initial fermentation took place.
The same can be accomplished in a keg by adding the same amount of sugar to the keg, transferring the beer to the keg and sealing it.  In a week or 2 you will have natural carbonation.

BB link

For more information, I strongly suggest this book for further study.

This photo is linked. 

Link to AIH