Fermentation is a natural biological mechanism performed by microorganisms such as yeast. During the process of fermentation, yeast convert sugar into alcohol, carbon dioxide, and other chemical compounds. This process helps not only create but also determine the flavor of fermented food and beverages like bread and beer. That’s why learning to properly manage the fermentation process, which is really all about making  yeast happy and productive, is a critical component to making great beer.

In this article, we’ll cover the basics of what happens during fermentation. We’ll also talk about creating the optimum conditions for fermentation in order to make great beer. We’ll cover advanced topics such as basic lager fermentation, pressure lager fermentation, quick lager fermentation, high gravity beers, and problems such as fixing a stuck fermentation in separate articles.

Remember this is just a primer. For a deeper dive into the science of yeasts and fermentation, we highly recommend Yeast: The Practical Guide To Beer Fermentation by Chris White and Jamil Zainasheef.

Fermentation Equipment

After mashing, boiling, and cooling liquid wort to a temperature of about 65-75 Fahrenheit, it’s time to “pitch” yeast and kick off fermentation. However, before that last step, the wort will be transferred to some kind of container suitable for this part of the process. In this case, the technical name of what is needed is called a fermenter, which could take the form of a plastic bucket, a glass or plastic container called a “carboy,” or perhaps some other similar device.

While glass has the advantage of being harder to scratch, and potentially easier to sanitize, it’s harder to clean (due to the small opening on top) than plastic, and it’s also heavier and slippery when wet – which can cause serious damage if a container, full or empty, is accidentally dropped. For this reason we recommend a high quality plastic fermenter or a stainless steel fermenter.

Whatever the container is made out of, it should be food-safe quality and it’s helpful if it has a handle. Bonus points if the container is transparent or see-through since that allows you to see how much beer you actually have while also doing a visual check on what’s happening with the fermentation process without having to take the lid off. Though, keep in mind, clear material must be either covered or placed in a dark room, as light will damage the precious cargo inside.

The fermenter will also need to have an airtight lid or closure as well as what’s called an “airlock,” which is a device that allows the carbon dioxide produced by the fermenting process to escape the container without exposing the beer to the outside environment. (Airlocks actually do allow air / oxygen into the vessel if the temperature drops and the beer contracts, so be careful about temp changes.)

Separate from the fermenter, sanitized hoses and a high quality siphon will be required to transfer your brew between the brew kettle and the fermenter. Finally, an additional, or a “secondary fermenter,” may come into play if a transfer is needed to allow the beer to condition for a longer period of time.

Additional “basic fermentation” equipment that one should buy includes:

• A stick-on thermometer for the fermenter. 
• A hydrometer, which is simple rod-shaped device that measures the density of a liquid. This becomes useful after the beer has been fermenting about one to two weeks depending on the style of beer you are making and the fermentation process has begun to slow. 
• A “hydrometer test jar” which is a just a test tube for the hydrometer (you generally buy these separately). 
• And a “wine thief” or a turkey baster for taking a hydrometer sample.

How to Ferment Beer: The Complete Process

1. Chill the wort to 70 degrees Fahrenheit.
2. Using a hydrometer, take a “starting gravity” reading.
3. Siphon or pump the wort to a clean and sanitized fermenter (using a clean and sanitized siphon and hose).
4. Seal the fermenter and shake vigorously for 60 seconds. This should increase dissolved oxygen to about 8 parts per billion.
5. Open the fermenter again and add 1 - 2 packages of yeast (the exact amount depends on the yeast and the starting gravity).
6. Seal the fermenter once again.
7. Add a sanitized airlock, full of sanitizer solution.
8. Place the fermenter in a dark place with a stable temperature at or around 70 degrees Fahrenheit for ale yeast and 50 degrees Fahrenheit for lager yeast.
9. Wait 1 - 2 weeks for ales and 2 - 3 weeks for lagers. 
10. Once finished, take a final gravity reading
11. Siphon (don’t pour) the finished beer into bottles or a keg. More on this in another article.

Assuming a starting gravity as measured and recorded, it can be compared to final gravity to determine the alcohol by volume (ABV) of the beer. It’s easiest to use an ABV calculator for this.

Optimum Fermentation Conditions

To achieve the best possible results from your fermentation, pay attention to a couple of key variables that will impact the process. We’ve discussed about some of these in a separate article on yeast, but they’re worth repeating here:

Healthy Yeast

One essential element in properly fermenting beer is using healthy yeast that has not aged past its expiration date. Healthy yeast will be more likely to create the flavor profile desired in a finished beer. Yeast typically comes in one of two forms: dried or in liquid form. Liquid yeasts only have a shelf life of a few months but have a greater range of availability. Dried yeast is more shelf stable, but the varieties are limited. In either case, make sure to check the expiration date before using!

Fermentation Pressure

Basic ale fermentation, which is what we're covering in this article, is conducted at ambient pressure. However, some beer styles and yeast types, such as hoppy lagers, benefit greatly from pressure fermentation.

Proper Pitch Rate

Pitching the proper number of yeast cells is also an integral part of a successful fermentation. Read the directions on the back of the yeast package. It will provide a limit for how much sugar one package of yeast can ferment. This depends on two factors - the total amount of beer being fermented and the starting gravity of the beer. For example, most packages of White Labs liquid yeast are able to ferment 5 gallons of beer at a starting gravity of 1.048 or less. If a larger batch is being fermented or the starting gravity exceeds that number, two or more packages will need to be used.

A secondary option to help yeast get to work faster is making a batch of “starter,” which is basically a mini batch of beer that is made a day or two ahead of the actual brew day. This allows yeast to multiply before being pitched into the actual fermenter. We’ve created a separate tutorial on how to make a yeast starter.

Sugar and Nutrients

Yeast consume sugar and create carbon dioxide alcohol and flavor compounds – but not all sugars are created equally. Almost all sugar is readily fermentable. However, something like Splenda and other low or zero calorie sugars can’t be used to make beer. Lactose also cannot be used to make beer, as it is non-fermentable. For this reason, lactose, or milk sugar, is actually used to sweeten beers such as “Milk Stouts” and “Milkshake IPAs.”

Yeast also needs essential nutrients like zinc and calcium to thrive, both of which come from malt. However, if a simple sugar like dextrose is used to make hard seltzer, yeast nutrient should be added. Again, follow the directions on the back of the package.

Oxygen

Another key variable in a successful fermentation is the level of oxygen that is dissolved in the wort. To an extent, the more oxygen present in the beer, the more yeasts will grow – which is why you aerate the chilled wort prior to adding yeast to it. However, there is a limit. For this reason, wort needs either vigorously shaken in the fermenter for a minute or two before yeast is added or an oxygenation stone should be used.

For some beer varieties like lagers and high-gravity ales, even more oxygen may be required to achieve the desired end result. This may require a taken of pure O2. But beware, according to internet lore, there is also a risk of over-oxygenating beer, so we don’t recommend using pure oxygen for normal ABV beers.

Fermentation Temperature

Controlling the temperature is a critical component of achieving a successful fermentation and high-quality finished beer. Read the manufacturer’s guidelines to know what temperature is best for a particular style of beer. Ale yeasts typically thrive at temperatures in the range of 64 to 70 degrees F (16 to 24 C) while lager yeasts like it cooler – between 48 and 58 degrees F.

Temperature control is one of the most common oversights made by new brewers. Temperatures that range too high or too low will result in off flavors in the beer, as it impacts the yeast’s appetite for feeding. For example, if fermentation temperature is too high, yeasts can die, leaving behind unpleasant chemicals such as fusel or “hot alcohol.” If it’s too low, yeast will begin to go dormant and flocculate, or sink to the bottom of your beer without finishing the job. This can result in a sweet, low ABV beer that also has a green apple taste as a result of acetaldehyde the yeast didn’t have a chance to “clean up” before going dormant.

To avoid these problems, place your fermenter in a place with a normalized temperature, and follow the manufacturer’s guidelines for the temperature range of your yeast. Also, consider brewing seasonally – if your apartment is hot in the summer, brew a saison; if it's cold in the winter, consider using a hybrid yeast like a Kolsch or California Common strain. Yeasts such as Kveik have a huge temperature range and actually perform better at higher temps.

Fermentation Time

Fermentation time is a variable that even experienced brewers overlook. The excitement associated with wrapping up a project and tasting the final product can cause even veteran brewers to end fermentation before it's actually finished. Many ales will finish in about 7-10 days, but to be absolutely certain, a final gravity should be taken to determine if the process is truly done. The appropriate final gravity number varies by yeast and by recipe, but most ales with an ABV in the 5% range will ferment down to 1.010. Additionally, even if the expected terminal gravity has been reached, off flavors and aromas may exist that the yeast could clean up if given additional time. Always smell and taste a sample of the beer before kegging or bottling.

As mentioned earlier in the article lager beer takes longer than ale to ferment. However, the process can be sped up with the quick lager process that we used for our India Pale Lager beer.

Constant Monitoring

When it comes to mastering a fermentation process, it’s also critical to pay attention to what’s happening at all stages of the brewing process, keeping on a keen eye on indicators like those listed above: temperature, gravity, PH, oxygen, and carbon dioxide. That’s why its also extremely helpful to keep a log of your activities and results as a way to spot problems early on before they get away from you – and your beer.