Beer fermentation temperature is one of the most critical factors which affect the quality and consistency of the flavour of your homebrew.
Yeast works best within a specific temperature range. For most ale yeasts, this is between eighteen and twenty-two degrees Centigrade (65 to 72 degrees Fahrenheit).
At higher temperatures, fermentation takes place more quickly. At lower temperatures, it slows down and may even stop altogether.
Fermentation is exothermic, which means that it generates heat and increases the temperature of the fermenting wort. How much the temperature increases depends on how vigorously fermentation is taking place. During the early stages of fermentation, it’s common to see a temperature increase of around five degrees Centigrade (approximately ten degrees Fahrenheit).
Beer Fermentation Temperature
The ideal temperature which should be maintained during fermentation depends on the type of yeast being used and the flavour profile of the beer which you are brewing.
Generally speaking, you should pitch the yeast near the mid-point of the yeast’s ideal temperature range before allowing the temperature to drop by a couple of degrees during the next few hours. This will help the yeast to grow and get fermentation off to a good start.
During the next twenty-four hours, as fermentation gets more vigorous, the temperature of the wort naturally rises due to the heat given off by fermentation.
If the temperature of the wort gets too high at this stage, it will create more esters and fruity flavours. More fusel alcohols are generated at higher temperatures, which can give the beer a very harsh aroma, often compared to nail polish remover or paint thinners.
Conversely, a lower temperature will lead to a slower fermentation and possibly a cleaner, crisper taste.
As fermentation completes, it slows down, and the yeast starts to go dormant and sink to the bottom of the fermenting vessel.
By warming the wort by a couple of degrees at this stage, typically four to five days after fermentation started, you can ensure that the yeast remains in suspension and finishes off fermenting.
This ensures that all of the available sugar is consumed by the yeast and is especially important with some types of ale yeasts, which are known for flocculating early if the temperature drops even slightly. It also ensures that the yeast can clean up after itself, removing the unwanted by-products of fermentation.
Once fermentation has finished, you should allow the fermenter to cool back down to room temperature again.
Some homebrewers like to cold crash their beer at this stage, which means that they cool the fermenter down quickly to ensure that all the yeast flocculates and drops to the bottom of the fermenting vessel.
Why does fermentation increase with temperature?
Temperature affects the rate at which yeast cells multiply and consume sugars. This is especially true during the exponential growth phase of fermentation when the yeast is most active.
As the temperature rises, the yeast cells become more active and multiply more rapidly, and the growing cells consume more sugar. Fermentation increases producing alcohol and a foamy layer on top of the wort known as krausen.
The rate at which alcohol is created by the yeast is called the fermentation coefficient. The fermentation coefficient increases with temperature up to a maximum temperature of around forty degrees Centigrade (104 degrees Fahrenheit).
What is the best temperature for beer fermentation?
The best temperature for fermentation depends on the strain of yeast which is being used.
Most top-fermenting ale yeasts prefer temperatures between eighteen and twenty-two degrees Centigrade (64 to 72 degrees Fahrenheit).
Lager yeasts prefer lower temperatures typically between ten and fifteen degrees Centigrade (50 to 59 degrees Fahrenheit).
On the other hand, Mead and French Saison yeast strains can tolerate temperatures as high as thirty degrees Celsius (86 degrees Fahrenheit) and Kveik, a Norwegian brewer’s yeast can be used at temperatures as high as forty degrees Celsius Centigrade (104 degrees Fahrenheit).
I’ve included a list of the recommended temperature ranges for most commercially available brewers’ yeasts in this post.
What happens when fermentation temperature is too high?
As mentioned earlier, at higher temperatures, yeast grows more quickly, and fermentation becomes more vigorous.
This might sound like a good thing, but unfortunately, when yeast grows too quickly, it depletes the nutrients available in the wort and becomes stressed, producing unwanted by-products that spoil the beer’s flavour.
Also, when fermentation takes place at higher temperatures, a higher proportion of fusel alcohols are produced, commonly believed to be the main cause of hangovers.
There’s more information about what happens when you ferment beer too warm in this post.
