Fermentation 101: An Introduction To Fermenting Beer For Beginners

Fermentation 101: An Introduction To Fermenting Beer For Beginners

So, what is fermentation and why is it important?

Fermentation is… magic.


Well, maybe it is a bit more sciencey than that.

But anyone who has sprinkled yeast on wort and come back a few days later to find that it has become beer knows the truth.

Fermentation is magic. With maybe a bit of science.

Simply put, fermentation is the process by which yeast convert the sugars in the wort into alcohol and carbon dioxide.

Yeast want to ferment beer. Its literally what they live for. Truly it has been said that we make wort, but yeast make beer. If you get the two together its almost impossible to not end up with beer. 90% of enquiries from brewers worried about their fermentation are solved with a simple, “Don’t worry. Relax. Have a home brew. She’ll be right”.

However, there are definitely some steps we can take and things we can control that will allow you to make the best possible beer.


To ferment your wort you need a fermenter and some yeast.

Fermenters come in all shapes and sizes.

Most people are familiar with the old school standard HDPE fermenters – the white plastic tubs that have been around seemingly forever. Many, many homebrewers – myself included – started with one of these and they are great – cheap and completely functional.

The next step up, and thoroughly recommended if you are kegging your beer is a pressure fermenter.

The FermZilla range is brilliant for this at a great price point and with many advantages such as being transparent (so you can watch your fermentation in action) as well as being lightweight and easy to clean.

A lot of brewers like kegmenters – basically converted kegs that are used for fermentation.

Essentially, any sanitisable, airtight container of sufficient volume will do the job. You need to able to fit an airlock or spunding valve to allow CO2 to blow off.

Temperature Control

Yeast activity is exothermic – that is, yeast will actively produce heat . Most fermentation off flavours are a result of fermentation activity at too high a temperature. Thus, having the ability to control the temperature of fermentation is a huge advantage in brewing great beers. It also allows you to brew more complex or tricky styles – Belgian beers that like high temperatures, or lagers that need low temperatures, for example.

The ideal way to control fermentation temperature is with some kind of fermentation chamber. A re-purposed fridge with an external temperature controller and a heat source such as a heat wrap or a heat belt is a super inexpensive way to set up a chamber. There are also dedicated fermentation chambers that both heat and cool, and can be controlled and monitored remotely.

We have an entire ecosystem based around wifi and Bluetooth enabled fermentation devices – check the website for more details. The RAPT Fermentation Chamber in particular is probably the most powerful yet affordable deciated Fermentation Chamber on the market!

However you elect to do this, a really good rule of thumb is to keep the temperature controlled for the first 48 hours of fermentation. This is when the yeast multiply, and thus also when temperature related off flavours will develop. After the first 2-3 days temperature is typically less critical, but obviously too high or low a temperature (if outside the safe range for the yeast) can cause issues such as off flavours or stalled fermentation.

If you have no means of temperature control that’s OK! There’s lots of ways of keeping temperatures down – keeping the fermenter in a cool area or cellar, using wet towels or ice baths for example. Just remember that the first 24-48 hours are the most important. Seasonal brewing is also a thing – brew ales or Belgian styles with yeasts that like warmer temps in the hotter months, and save lagers for winter.

During fermentation you may notice there is a large head on the wort. This is the krausen and is typically a good sign that fermentation is underway and proceeding happily. Generally, this will collapse and drop out once fermentation is done.

When fermenting it is important to use an airlock or spunding valve (for pressure capable fermenters) during the process to allow CO2 to blow off. This can be a good indication that fermentation is proceeding, as well s a guide to when it is done. However, the only accurate way to be sure that fermentation is finished is by taking a gravity reading – if you have a stable reading over three days we can be pretty certain that fermentation has done.

The use of a wifi/Bluetooth capable floating hydrometer such as the Pill can actually take pretty much all the guesswork out of fermentation, with near realtime updates of gravity and temperature from within the fermenter itself – it’s like cheat mode for homebrewing!

If you can control the temperature, doing a cold crash after fermentation will help to drop yeast and trub out of the beer, leaving clear beer to be transferred to bottles or kegs for carbonation. Cold crashing is simply the process of dropping the temperature of the beer in the fermenter to between 2°C to -1°C for at least a couple of days. Rule of thumb – the colder the crash the less time needed.

Length Of Fermentation?

How long does fermentation take, I hear you ask?

Well, the answer is, of course, that it depends.

With adequate pitching rates, use of yeast nutrient and good temperature control, most ale yeast fermentations will be done in 5-7 days. Visual signs that fermentation are done include the krausen dropping, airlock activity slowing or stopping and the yeast starting to flocculate – that is, drop out of suspension so that it forms a pale tan coloured layer in the bottom of the fermenter.

While these are good indicators that fermentation is done, the only way to be absolutely sure is through gravity readings. You will need to take gravity readings over three days once you think fermentation is done. If A: the expected gravity (or near to it) has been reached, and B: the gravity remains completely stable over three days, then we can be sure that fermentation is done.

There is no problem with leaving the beer for a few days after fermentation has finished – this will cause no harm at all and many suggest that it can even be beneficial, depending on style. When fermenting lager yeasts (and ales, to a certain degree) it is quite important to raise the temperature of the ferment for the last couple of days for a diacetyl rest. Diacetyl and its precursor are produced by the yeast during fermentation, but will be absorbed during the tail end of fermentation. If your beer has a sweet, buttered popcorn or caramel flavour, or feels slick on the tongue, this can be due to diacetyl in the finished beer due to incomplete fermentation.

Once fermentation is complete, cold crashing is highly recommended for pretty much all beer styles. Simply drop the temperature of the fermenter to between -1°C – 2°C for a few days. This helps clear the beer, assists with flocculation and ensures that cleaner, clearer beer is transferred into the keg or bottle.


Once the beer has finished cold crashing, it is ready to be transferred – see this article for a guide to bottling, and this for kegging.

If you can both pressure ferment and keg your beer, then this is essentially the holy grail for homebrew, as it allows you to do a closed, O2 free pressure transfer from the fermenter to the keg, without needing to open either vessel. This is we are very strong advocates of the benefits of pressure brewing. If you are going to be kegging your beer, then fermenting in a pressure capable fermenter such as a 30L FermZilla All Rounder is an absolute no brainer.

If you have the ability to can the beer, then this article covers the ins and outs of canning.

Pressure fermentation is a subject in its own right – we have a pretty comprehensive overview on pressure fermenting here.

If bottling the beer, you will need to wait a couple of weeks for carbonation to occur from secondary fermentation. If kegging then you can pretty much drink it straight away, should you wish – see this article for a guide to fast or even really fast ways to carbonate kegged beer.

Note that not all beer styles respond well to being drunk immediately – they may taste a little ‘green’ straight after packaging and require a week or two in the keg (sometimes even a couple of days can make quite a difference).

Hopefully this helps to clear up some of the mysteries surrounding fermentation. It’s really quite simple – simple enough that anyone can quite easily turn their hand to making their own beer!


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