The Beer Fermentation Process

The Beer Fermentation Process

A Guide to Beer Fermentation

Fermentation is a process whereby yeast converts glucose in the wort to ethyl alcohol and carbon dioxide gas (CO2) to give beer its alcohol content and carbonation. The fermentation process starts when cooled wort is transferred to a fermenting vessel and yeast is added. 

How long do I ferment my beer?

Many new brewers wonder what the best timeframe is for fermenting their beer. The truth is that we have absolutely no control over our fermenting time. Once we pitch our yeast, it’s up to the yeast to do all the work!

We can, however, change certain conditions to shorten or lengthen this timeframe – like controlling fermentation temperature. This will depend on the yeast strain you’re using and what you’re looking for in the beer.

Controlling fermentation temperature

Temperature control is a super-important part of fermentation and can easily make some of the most significant changes in fermentation – both in quality of the finished product and the fermentation time.

Different temperatures can affect the fermentation ability of different yeasts. Typically the cooler the temperature, the slower the yeast will work, and the warmer the faster.

The general rule is the hotter the fermentation the more likely you are to get off-flavours and unwanted attributes in the beer. If using cooler temperatures, you can sometimes experience stalling, extended timeframes or challenges with getting to the normal level of fermentation. Yeast strains like Kviek are the exception along with certain styles like wheat beers and Saisons. 

The basic rule of thumb for fermentation temperatures
  • Lagers: 8-12 °C (and once you get more advanced you can look at diacetyl rests)
  • Ales: 18-22 °C

This rule can differ, as mentioned above, for styles such as wheat beers and Saisons. These are often fermented around 27 °C to get more phenolic and peppery notes that the styles are well known for.

The recent boom in the brewing world is Kviek. This ‘super yeast’ can easily ferment a beer at over 30 °C without attracting any of the typical off-flavours one would get with other strains, making it a great yeast for brewing in warmer climates!

How do I know when my beer is finished fermenting?

A mistake that new brewers often make is using the airlock on the fermenter to gauge progress. An airlock is exactly what it describes. It’s a device that ensures nothing gets into the fermenting beer and also lets the built-up CO2 escape.

While many of us are mesmerised by the airlock making that ‘gloop’ sound every few seconds, all this is telling us is that CO2 is escaping from the fermenter. If there is a non-perfect seal on the fermenter, this CO2 could be escaping and the airlock will stop bubbling.

At the end of the day, there is only one way to know if your beer has finished fermenting – by using a hydrometer or refractometer. These devices allow you to check the sugar levels in the wort/beer.

The general advice for knowing when your beer is completed and ready for packaging is to have a stable specific gravity (SG) reading over 2-3 days. This is to ensure fermentation has indeed completed.

What do I do when my beer is finished fermenting?

It is advisable to let the beer rest a few days post-fermentation. This will allow the beer to settle out and clear with the yeast flocculating at the bottom of the fermenter. If you’re able to reduce the temperature somewhat, we would advise in favour of this as it can assist in clearing the beer.

When fermentation is complete, you can choose to package immediately, look to age it longer or add something else such as fruit, oak or by itself in the case of some lagers. This all depends on the beer you have brewed. 

It was long-thought that you should ‘rack’ the beer after the primary fermentation into a secondary fermenter to get it off the yeast cake and allow it to better condition for packaging. These days, the risks of potential oxidation and contamination rarely outweigh the benefits of secondary fermentation. Secondary fermentation is generally recommended only when a secondary fermentation will actually occur – ie. bottle or keg carbonation.

What should my beer look like when fermenting?

It’s a bit like Schrodinger’s cat… You can open the top and observe what’s going on inside the fermenter but this may change the result, in many cases for the worse. Clear/see-through fermenters are great if you want to view the fermentation progress without having to open the fermenter and exposing the beer to air. 

What the beer looks like during fermentation though, depends on our little buddies – yeast. Here is a rundown of what they are doing once they are pitched into the wort:

Lag phase (0-15 Hours) | Who’s that? Who’s there?

In this phase, the yeast cells are waking up and are trying to figure out what’s going on.

As they awake and get ready to start the day, they look for their morning stimulants like oxygen, minerals, and amino acids. While they do this they start to realise all the food around them. We like to imagine them thinking “how am I going to eat all of this on my own?”

During this phase, there is no airlock activity and only a small amount of natural convection of the wort in the fermenter due to any remaining temperature stratification. 

Growth phase (4 hours – 4 Days) | Making friends and feeding

“Alright, I need some buddies for all this food!” The yeast starts replicating and getting to work on the sugars on the wort.

The Krausen – a foamy head of yeast proteins and sugars – starts to form and grow. Large amounts of CO2 are starting to produce and the airlock starts going mad. Additionally, because the yeast producing alcohol produces heat, the heat convection in the wort starts to increase and you start to get a gentle rolling over of the wort in the fermenter.

Most of the alcohol, flavour and aroma compounds are produced at this point.

Stationary Phase (3-10 days) | Let’s clean up this mess

All the easy sugars have now been eaten and the Krausen starts to turn from creamy white to yellow (from the precipitated malt and hop components) and brown (from the oxidised hop resins).

The yeast starts to absorb many of the compounds we consider off-flavours like higher alcohols, diacetyl, sulphur compounds and esters and reforms them into more alcohol and other ‘nicer’ esters.

At this point, the fermenting wort is called ‘green’ beer and has not reached the proper balance in flavours. The airlock and convection start to slow as the yeast settle into the long haul and go to sleep, dropping out of solution as the little remaining food disappears.

Death Phase (Several weeks) | My work here is done

Airlock activity may stop (or have the occasional bubble) and convection stops. The yeast is mostly asleep at this point and it hangs out at the bottom of the fermenter. The beer starts to clarify and the flavours in the beer mature.

Interested in learning more about what to do with your beer once it’s finished fermenting? Read our blogs on how to bottle beer or how to keg beer.

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