Fermentation is the most important phase of the home brewing process.
It is all about providing the yeast with the best conditions possible to allow it to convert the sugars contained within the Wort into alcohol, carbon dioxide and desirable flavours and aromas. Two of the most important parameters for us as homebrewers are the dissolved oxygen content and temperature of the ferment.
Sufficient oxygen must be supplied early in the ferment to allow for growth of the yeast (the aerobic phase). Once this is finished, it is vital that no more oxygen is allowed to contact the Wort, and the air is 26% oxygen. So, do not be tempted to open the lid, even to have a look (or worse yet stir or test by dropping the hydrometer into the fermenter!). This is a source of oxygen and airborne contaminants.
‘Yeast doesn’t make alcohol when oxygen is available, it just makes more yeast and plenty of CO2. This can produce undesirable oxidation reactions which spoil beer reduce its keeping qualities and the extra growth results in the excessive production of esters.
Our job as brewers is to provide the yeast with the optimal conditions for it to produce the style of beer we require.
If fastest cell growth occurs at elevated temperatures, then why don’t we ferment at this temperature, decreasing the time it takes to make the beer? Why do we make yeast work slower? It is because what is best for the yeast, is not best for beer. As the cells grow and multiply, yeast produces many compounds. The most noticeable of which are esters. These can significantly impact the flavour and aroma of a brew. As the temperature of fermentation rises, more ester production occurs.
The optimum ale fermentation temperature has been found between a range of 18º-22ºC. This temperature range strikes the best balance between yeast growth and ester levels for most ale strains. The vast majority ale strains are unable to ferment or grow below 12° C, which is the most common temperature used in lager fermentation. This cooler environment greatly reduces the ester-forming ability of most lager strains, creating the clean flavour associated with this type of beer. With Ester levels kept low, this places the emphasis on malt and hop flavours.