Understanding the brewing fermentation process

To make great beer, you need to have a working knowledge of the brewing fermentation cycle.

Let’s cover the three distinct stages of fermentation.

Stage one - waiting

Oxygenated wort is a rich environment for yeast (and other microbes) to reproduce.

During the lag time before fermentation actively starts, the yeast cells are rapidly multiplying and building in number.

They are consuming oxygen, which is a good thing for beer stability later on.

Foamy-coiny circles forming.

Foamy-coiny circles forming.

Your beer’s first sign of fermentation is fine white foam on the surface. This will first show as light circles the size of a small coin and progressively build in size and motion from there.

Assuming your yeast is properly rehydrated, healthy and working at the correct temperature, you should see signs of fermentation within 24 hours for ale yeasts (48 hours for cooler, slower lager yeasts). Any longer than this, something is amiss.

Check your brewing temperature. If this is not the cause, you may have pitched an unhealthy yeast. Rehydrate and pitch another yeast packet.

Stage two - primary fermentation

Brewing fermentation process - high krausen

Brewing fermentation process - high krausen

This is where it really gets exciting. The small coin-sized circles will grow to a thick creamy layer usually an inch thick. You can often see bubbles popping on the wort surface if using plastic wrap to cover your beer.

You can smell the aromas of malt, hops and yeast, which can attract a few comments from the household.

There will be a ring of dark resin on the top and side of your thick yeast layer. This resin is a combination of hop oils, protein and dead yeast cells. This is normal.

It is also a harsh-tasting combination and fortunately much will end up stuck to the top wall of your fermenter and not in your beer.

Some brewers advocate skimming off this resin from the top of the yeast for a cleaner beer. However, this also offers the downside of the potential for infection from contaminates.

The resin is also largely insoluble once formed, so not worth the risk either way.

You will also notice your beer will lighten in color – this is due to the pale yeast mixing through the wort and making it lighter.

Another clear sign that fermentation is active is a white layer on the bottom of your beer about a quarter of an inch thick. This is the yeast and a sign that it has multiplied exponentially.

During primary fermentation, your wort may rise in temperature. Active fermentation generates its own heat. This can be up to 40 °F (4°C) higher than the ambient temperature.

Primary fermentation will last four to six days for ales, or six to ten days for lagers.

Has my beer started fermenting?

Occasionally you may not get strong signs that your beer has started primary fermentation.

The clear cut way is to test regularly with a hydrometer to check that the gravity is dropping (see below).

Your testing sample will also be slightly carbonated.

Another sure sign is the faint dark ring around the top of your fermenter and a yeast layer on the bottom.

If you have these, I suggest leaving well enough alone and not interfering.

If your temperature is correct, your yeast will soon build momentum for a healthy fermentation.

How to use a hydrometer

To use a hydrometer correctly:

  • Fill the sample tube with wort to float the hydrometer.

  • Spin the hydrometer to dislodge any bubble (these will float the hydrometer and give a false reading).

  • Read the number at the wort’s surface. This is your beer’s gravity.

Note the wort rises to 1.040, however the horizontal line intersects 1.042 (true reading).

Note the wort rises to 1.040, however the horizontal line intersects 1.042 (true reading).

You may notice a sight curve as the wort rises up the side of the hydrometer.

This is not the reading point – look at where the horizontal straight line of the wort’s surface intersects.

Stage three - winding down and cleaning up

Towards the end of your fermentation, you will notice the top thick creamy layer start to subside. Your beer will darken as the pale yeast starts to drop from the solution.

Your wort’s gravity will remain stable for two consecutive days. This will vary depending on your ingredients and yeast strain. For a moderate strength beer (5% ABV) this will be around 1.008 -1.014.

Once you hit your final gravity, don’t be in a rush to bottle. After fermentation, the yeast will continue to work to clean up off-flavors and harsh by-products.

Leaving your beer in the fermenter for another seven days will improve the profile.

You can safely leave ales for up to two weeks in the fermenter (in total) and four weeks for lagers. After this, the yeast will start to consume itself and create bad flavors.

If you are adding hops to the wort, add them after primary fermentation has stopped.

During active fermentation the CO² bubbling action will strip out delicate hop aromas.

How to work out alcohol percentage

Quite importantly, you and people you share your beer with will want to know how alcoholic it is.

The most accurate way for homebrewers to measure alcohol by volume (ABV) is through a hydrometer.

If you know your original gravity (OG) and your final gravity (FG), you can work out how much sugar has been consumed and in turn the alcohol produced.

Fortunately the math is pretty straightforward:

ABV = (OG - FG) x 131 + 0.5 (from priming sugar)

If you are not using priming sugar (ie carbonating with CO² in a keg), remove the 0.5.

So to work out the alcohol in a beer with starting gravity of 1.044 and final gravity of 1.008:

= (1.044 - 1.008) x 131 + 0.5

= 0.036 x 131 + 0.5

= 4.7 + 0.5

= 5.2 % ABV

*Your OG and FG may vary slightly – use your numbers.


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Homebrewing With Kits—The Beginner’s Guide

To help you master homebrewing with kits, I’ve put together a guide based on my experiences over nearly two decades of homebrewing.

This is the information I wish I had when I first started. I hope it helps you make awesome homebrew.

Download your copy today.


Homebrewing Basics Series