When Is Primary Fermentation Complete For Beer? (2025)

Brewing beer at home is an exciting process, but it can be difficult to know when to move your beer from the primary fermenter to the next step. It is important to be patient and wait for primary fermentation to be complete, otherwise, you could end up with problems like over-carbonation. There are visual cues and specific measurements that will indicate when primary fermentation is done. Once active fermentation is over, the beer will brighten in colour and a large yeast cake will form at the bottom of the fermenter. However, the only way to be sure that primary fermentation is complete is to take hydrometer readings on consecutive days and check that they are stable and in line with your recipe's estimated final gravity reading.

Characteristics and Values of Primary Fermentation in Beer

Using a hydrometer to check density

Using a hydrometer to check the density of your beer is a great way to determine when primary fermentation is complete. A hydrometer is a simple tool that measures the density of a liquid by comparing it to the density of water. In the context of brewing beer, a hydrometer specifically measures the amount of dissolved sugar in the mixture. This is important because it indicates how well the fermentation process is progressing, as yeast converts sugar into alcohol.

To use a hydrometer, it is recommended to use a trial jar, also known as a "test jar," which is specifically designed for this purpose. The trial jar is typically made of clear plastic and is about 200mm long with a diameter of approximately 35mm. Here are the steps to follow:

• Fill the trial jar with the beer you want to test, filling it to about 35mm from the top.
• Gently place the hydrometer into the jar, being careful not to let it stick to the sides. You may need to give it a gentle spin to prevent this.
• Take the reading from the lower of the two levels you see when looking at the side of the test jar. This reading will indicate the specific gravity (SG) of the beer.
• It is important to note that the temperature of the liquid can affect the accuracy of the reading. For the most accurate results, the beer's temperature should be at 20°C. If the temperature differs significantly, you can use an online calculator or app to adjust the reading accordingly.
• Take readings on consecutive days (2-3 days apart).
• If there is no change in the readings over several days, indicating stable gravity, then you can be confident that the primary fermentation is complete.

It is worth mentioning that the starting gravity of a typical beer is usually around 1.045, and it will finish around 1.012, resulting in a drop of about 32 points. This drop can be divided by 7.362 to calculate the ABV (Alcohol by Volume). For example, if your starting gravity is 1.080 and it ferments down to 0.990, you would calculate the ABV as follows: 90 (point drop) divided by 7.362 equals 12.23% ABV.

Visual cues: colour, foam, bubbles

There are several visual cues that can indicate when primary fermentation is complete. These include changes in colour, foam, and bubbles.

One of the most important visual cues is the presence or absence of bubbles. While the absence of bubbles in the airlock does not necessarily mean that fermentation is complete, a reduction in bubbling activity can be an indication that fermentation is slowing down. It is important to note that carbon dioxide can continue to escape from the beer liquid even after fermentation is complete, resulting in some bubbling. Therefore, the absence of bubbles should not be the sole indicator of fermentation completion.

Another visual cue to look out for is the dissipation of foam from the headspace of the fermenter. During active fermentation, a foamy layer called krausen forms on top of the wort, which is composed of yeast cells, proteins from the wort, hop oils, and other residues. As fermentation progresses, the foam subsides and eventually flocculates. When fermentation is complete, the krausen sinks to the bottom of the fermenting vessel, indicating that fermentation is likely complete. However, it is worth noting that the appearance of krausen can vary, and it may not always be a reliable indicator of fermentation completion.

The colour of the beer can also provide visual cues about the fermentation process. As fermentation progresses, the beer may start to appear clearer as the yeast falls out of suspension. Additionally, the colour of the beer may brighten as it becomes clearer, indicating that fermentation is moving towards completion.

While visual cues can provide some indication of fermentation completion, it is important to rely on additional methods, such as hydrometer readings, to accurately determine when primary fermentation is complete. Visual cues may vary between batches, and relying solely on these cues can lead to inaccurate assessments of fermentation completion. Therefore, combining visual observations with other measurement techniques will provide a more comprehensive understanding of the fermentation process and its completion.

Autolysis: self-destruction of yeast

Autolysis, or the self-destruction of yeast, is a process that occurs when yeast inside the beer starts to expire. The yeast cells burst apart when they die, and the contents released can contribute to several off-flavours, none of which are pleasant. This process usually takes several months, but it can be accelerated by certain variables. For instance, the type of yeast used will impact the speed of fermentation, with turbo and Norwegian Kveik yeasts finishing faster than lager yeast. Similarly, the temperature of the liquid being fermented will also affect the speed, with warmer liquids fermenting faster than cooler ones.

To avoid issues with autolysis, it is recommended to use a secondary fermenter if you plan to condition or store your beer for longer than a month. It is also important to maintain the ideal fermentation temperature, even after active fermentation is complete.

While autolysis is not a common issue, it is still important to take steps to prevent it from occurring. By following best practices and using the right equipment, such as a brewing hydrometer, brewers can ensure that their beer ferments correctly and avoid the unwanted flavours that can result from autolysis.

A hydrometer is a simple tool that measures the density of a liquid compared to water. For homebrewers, this measurement is critical as it indicates when the yeast has converted all the sugar in the wort to alcohol and carbon dioxide. Stable hydrometer readings for several days in a row indicate that primary fermentation is complete.

In addition to using a hydrometer, brewers can also look for visual cues to determine when primary fermentation is done. During active fermentation, which lasts about 48-72 hours, there will be visible activity such as bubbling and foam formation. After this period, the beer will start to clear and brighten in colour, and a large yeast cake will form at the bottom of the fermenter. However, these visual clues are not the determining factor, and hydrometer readings are necessary to confirm that fermentation is complete.

Fermentation phases: lag, exponential, stationary

To determine when primary fermentation is complete, home brewers can look out for visual cues and take specific measurements. However, the only way to truly know when primary fermentation is complete is to take hydrometer readings. A hydrometer is a simple tool used to compare the density of any liquid to the density of water. For home brewers, this measurement is critical because it will allow them to see when the yeast has converted all of the sugar in the wort to alcohol and carbon dioxide. You will know that your fermentation is done with a hydrometer when the readings are the same for multiple days in a row and they are in line with your recipe’s estimated final gravity reading.

Fermentation has three phases: the lag phase, the exponential growth phase, and the stationary phase. The lag phase lasts for three to 15 hours. During this phase, yeast undergoes a process of acclimation to the environment. The yeast begins to uptake minerals and amino acids from the wort. The amino acids are then used to build proteins.

The exponential or logarithmic phase of yeast growth comes next and lasts for one to four days. During this phase, the yeast rapidly consumes sugar, and the cell count increases rapidly. CO2 is produced, which starts to dissipate through the airlock and create a surface layer of foam on the beer. Airlocks bubble during this time frame. The aroma that escapes from the airlock of most neutral ale yeast fermentations has an olive smell.

The final phase is the stationary phase of yeast growth, which lasts for three to 10 days. During this phase, yeast growth slows down, and most of the flavor and aroma compounds have been produced, including fusel alcohols, esters, and sulfur compounds. The beer is referred to as green because it does not yet have the acceptable balance of flavors. Beer is matured in the stationary phase of growth, also known as the conditioning phase. Yeast reabsorbs diacetyl that was produced during fermentation, and hydrogen sulfide escapes from the top of the fermenter as a gas.

ABV calculations

To determine when primary fermentation is complete, brewers use visual cues and specific measurements. Active fermentation is the most visible sign that beer is fermenting correctly. This period of active fermentation lasts about 48-72 hours. However, the only way to truly know when primary fermentation is complete is to take hydrometer readings. A hydrometer is a simple tool used to compare the density of any liquid to the density of water. For homebrewers, this measurement is critical to determining when your fermentation is done because it will allow you to see when the yeast has converted all of the sugar in the wort to alcohol and carbon dioxide.

To calculate the ABV (alcohol by volume) of your beer, you can use a simplified formula or an advanced formula. The simplified formula is:

> (OG - FG) * 131

However, this formula is not as accurate, especially at high ABVs. The advanced formula is:

> ABV = (76.08 * (OG - FG) / (1.775 - OG)) * (FG / 0.794)

This formula is more accurate, especially at high ABVs, and there is no reason not to use it if you are using a web calculator. There are also online ABV calculators that can determine the ABV of your beer based on the gravity change before and after fermentation. These calculators require an original gravity reading (OG) and a final gravity reading (FG) prior to adding priming sugar at bottling.

Frequently asked questions