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In brewing, alcoholic fermentation is the conversion of sugar into carbon
dioxide gas (CO2) and ethyl alcohol. This process is carried out by yeeast enzymes. This is in fact a complex series of conversions that
bring about the conversion of sugar to CO2 and alcohol. Yeast is a member
of the plant family and in brewing we use the sugar fungi form of yeast.
These cell gain energy from the break down of the sugar. The by-product,
CO2, bubbles through the liquid and dissipates into the air. The other by-product alcohol,
remains in the liquid which is great for us but not for the yeast, as the
yeast dies when the alcohol exceeds its tolerance level.
Brewer's yeast tolerate up to about 5% alcohol. Beyond this alcohol level the yeast cannot continue fermentation. Wine yeast on the other hand tolerates up to about 12% alcohol. The level of alcohol tolerance by yeast varies from 5% to about 21% depending on yeast strain.
The fermentation process has other limits such as temperature. Greater than 27C kills the yeast less and than 15C results in yeast activity which is too slow.
The amount of sugar in the solution can be too much and this can prevent fermentation. Some recipes suggest adding the sugar in parts throughout fermentation rather that all at the beginning. This is especially true if the brew is aimed at producing a high level of alcohol. Some yeast strains have evolved to handle higher sugar levels. Yeast such as Tokay and Sauterne. The normal, home brewing, fermentation is in two parts.
Part 1
Aerobic (Oxygen is present)
This is the initial rapid process where the yeast is doubling its colony size every 4 hours.
(Usually 24-48 hours)Part 2
Anaerobic. (No oxygen present)
Slower activity and the yeast focuses on converting sugar to alcohol rather that increasing the number of yeast cells.
(This process can take from days to weeks depending on the yeast and the recipe)
Where can I buy yeast? Try your local homebrew shop
Overall chemistry of fermentation
The overall process of fermentation is to convert glucose sugar (C6H12O6) to alcohol (CH3CH2OH) and carbon dioxide gas (CO2). The reactions within the yeast to make this happen are very complex but the overall process is as follows:
C6H12O6 ====> 2(CH3CH2OH) + 2(CO2)
Sugar ====> Alcohol + Carbon dioxide gas
(Glucose) (Ethyl alcohol)
Note: The sugars used can be a range of fermentable sugars. These sugars are converted by enzymes to glucose which is then convered to alcohol and CO2
Why does yeast stop working at certain levels of alcohol?
The ability of yeast cells to convert sugar into Carbon dioxide and Alcohol is down to enzymes. Several enzymes are involved each does its step in the process. The final step is Zymase reduction which takes the end product of the other enzymes (acetaldehyde/glycerol), and turns this into good old ethyl alcohol. Sadly alcohol actually destroys enzymes and kills the yeast cell if in high concentrations. This happens at different levels for different strains of yeast.. Brewers yeast cannot withstand much beyond 5 or 6% Alcohol by volume. Wine yeast is more tollerant at a range of 10-15%
Specially cultured strains of yeast with the correct enviroment can withstand alcohol levels up to 21% alcohol.
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