Fermentation temperature & diacetyl rest

Homebrewers often overlook the importance of correct fermentation temperature. There's no point investing time, money and effort making sure all other aspects of your brew are perfect if you then ferment it too cool or, in particular, too warm.

Controlling fermentation temperature

The ideal temperature for fermenting ales is about 18ºC to 20ºC (64ºF to 68ºF). If using a true lager yeast, a temperature of about 9ºC to 12ºC (45ºF to 55ºF) will produce a clean, crisp taste. There is no point using a lager yeast at higher temperatures because off flavours will be produced. Use an ale yeast instead, unless you are trying to produce a particular style of beer such as steam beer (California common), which are brewed with lager yeast at ale temperature. If you're a kit brewer, remember that most "lager" kits actually come with an ale yeast, which will stop working at low temperatures. If you are unsure whether the yeast with your can of concentrate is ale or lager, assume it is an ale yeast and brew it at ale temperature, or spend a couple of dollars on a lager yeast and ferment the beer cool.

As well as the right fermentation range, a constant temperature is also important for the yeast to do its work as effectively as possible. Wrapping the fermenter in a blanket or towel, or putting it in an enclosed space such as a cupboard or even a fridge that is turned off can help insulate against temperature fluctuations.

Depending on where you live you may be able to time your brewing to avoid the need for any artificial temperature control — lagers in the cool winter months and ales in the warmer spring and autumn.

However, most brewers will at times need to cool or maybe heat their wort as it ferments. Various methods can be used.


Many brewers invest in a fermentation fridge to ensure a correct and constant temerature during fermentation. Even on their warmest setting refrigerators are likely to be too cold even for lager brewing. You will need to hook up a temperature controller such as an STC-1000 to turn the fridge on and off to maintain a constant temperature. Some temperature controllers only turn on the fridge to cool the fermenting wort when the temperature rises to a set level (or turn on a heater if the temperature drops too low, but not both). However, controllers such as the STC 1000 can be hooked up to a heat pad or belt to warm up the wort in cold weather. Many brewers will say a fermentation fridge is one of the best investments in brewing equipment and biggest contibutors to making a quality beer.

Wrapping the fermenter in a wet towel and keeping the towel well soaked is an effective way to reduce the temperature of the wort up to several degrees. The evaporating water will cool the towel and the fermenter. Directing a fan at the fermenter will increase the rate of evaporation from the towel and keep the wort even cooler. A large block of ice on top of the fermenter will keep the towel wet constantly.

Sitting the fermenter in an ice bath is another method of cooling if the ambient temperature is extreme.


If the need arises to heat the wort, there are several options. One is with an aquarium immersion heater.The downside of an immersion heater is that it needs to be well sanitised because it is placed into the wort.

Other methods include a belt heater that is wrapped around the fermenter, placing the fermenter on a heating mat or using a “hot box”, which is a wooden or plastic box with a light globe in it. Heat belts and mats are available from most homebrew shops and you can easily make up a hot box. Just make sure the light globe is well clear of any wood or plastic that could melt or cause a fire. Also

Remember that if you do heat your wort it should be within the temperature range for the yeast you're using — ideally 18C to 20C for ales and 9C to 12C for lager yeast. Going higher will produce off flavours.

Diacetyl rest

As yeast ferments the wort, one of the hundreds of compounds it produces is diacetyl. Diacetyl imparts a flavour usually described as butterscotch. In most styles of beer, including lager, this is undesirable, although it is acceptable in low levels in certain beer. All yeast produces diacetyl, so it can't be prevented. However, at ale-brewing temperatures in the 18C to 20C range the yeast consumes the diacetyl it produces. But at low temperatures the yeast doesn't consume the diacetyl and it will remain in the beer unless removed. This is done by slowly raising the temperature of the beer as it nears the end of fermentation, in a process called diacetyl rest.

Step by step: Diacetyl rest for lagers

  1. Ferment the beer at lager temperatures (9C-12C).
  2. When the beer is within a few (usually two to five) gravity points of the estimated final gravity allow it to warm up to 18C to 21C.
  3. Keep the beer at this temperature for two days. During these two days the yeast will consume the diacetyl it produced earlier in the fermentation.
  4. Cool the beer back down to lagering temperature (as close to freezing as you can get it).
  5. Continue brewing as normal.


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