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By Wyeast Lab
Fermentation is the heart of the beer brewing process, and thanks to it, the wort created from malted grains is transformed into alcohol and carbon dioxide through the action of yeast.
Beer fermentation is generally divided into three stages: primary fermentation, secondary fermentation, and conditioning (lagering).
It is during these fermentation stages that yeast produces all the alcohol, CO2, and the various aroma and flavor compounds found in the many types of beer that exist.
This is why temperature ranges, oxygen levels, the amount of yeast used, and the selection of the appropriate yeast strain will all significantly affect the final result of a finished beer.
1. Primary Fermentation
The primary fermentation stage begins when yeast is introduced into the cooled and aerated beer wort.
The yeast then rapidly uses the available oxygen to produce sterols, a vital compound for the expansion of the yeast culture.
Once the oxygen is depleted, the yeast enters an anaerobic phase where most of the sugars in the wort are converted into ethanol and CO2.
It is also during primary fermentation that yeast development occurs, with growth rates and quantities directly related to the production of aroma and flavor compounds.
Characteristics of Primary Fermentation
- Decrease in dissolved oxygen.
- Acidification/pH reduction.
- Growth or expansion of the yeast culture.
- Production of ethanol and CO2.
- Production of flavor compounds: esters, diacetyl, sulfur compounds, etc.
- Consumption of most of the wort sugars.
Primary Fermentation Temperatures
The temperature of primary fermentation must be regulated according to the desired flavor and aroma profile, and within the recommended temperature range for each yeast strain.
- Ale beers: 17–24°C (62–75°F).
- Lager beers: 8–14°C (46–58°F).
- Wheat and Belgian-style beers: 17–29°C (62–85°F).
2. Secondary Fermentation
Secondary fermentation is the stage that occurs after most of the wort sugars have been consumed by yeast and the fermentation rate decreases significantly.
The yeast converts some secondary metabolites, and flocculation and sedimentation then begin to occur due to the increase in alcohol content combined with the decrease in sugars and nutrients.
During secondary fermentation, diacetyl reduction also takes place. Depending on the profile, but generally for a standard beer, when fermentation reaches 40–50% attenuation, the temperature is raised by 3–5 degrees to allow the yeast to reabsorb diacetyl and break it down into acetoin and then into 2,3-butanediol.
Characteristics of Secondary Fermentation
- Decrease in the rate of ethanol and CO2 production.
- Diacetyl conversion.
- Reduction of some flavor compounds through yeast metabolism or CO2 absorption.
- Final gravity is reached.
- Yeast flocculation and sedimentation begins.
Secondary Fermentation Temperatures
- Ale beers: As in primary fermentation, higher temperatures increase diacetyl reduction rates.
- Lager beers: Some brewers intentionally raise the temperature to accelerate diacetyl reduction, although only for 24 to 48 hours.
- Wheat and Belgian-style beers: As in primary fermentation, higher temperatures increase diacetyl reduction rates.
Temperature, Density, and pH
To maintain consistency and determine fermentation performance, it is important to record the temperature, density, and pH of active fermentations daily.
Monitoring temperature can help prevent potential problems and allows corrective measures to be taken at the right time.
Monitoring and data collection in each fermentation provides information on yeast performance and health, which will help determine whether your culture meets the required standards for reuse in future propagations.
3. Conditioning
The conditioning stage occurs when final gravity has been reached and the fermenter is set to refrigeration temperatures (0–3°C / 31–38°F).
During this time, yeast continues to flocculate and sediment, conditioning the beer through the reduction of unwanted flavor compounds.
At this stage, Ale beers generally do not benefit as much from prolonged conditioning times as Lager beers do.
What happens instead is that all those flavors that are desirable in Ale beers will degrade over time, which is why it is recommended that conditioning be as brief as possible before bottling or canning.
It is also important to consider that oxygen exposure at this stage of the brewing process is extremely damaging to the final quality of the beer.
Characteristics of Conditioning
- Most of the yeast is removed from the beer.
- Formation and precipitation of proteins that cause turbidity.
- Reduction and softening of strong flavors.
- Reduction of sulfur compounds, diacetyl, and acetaldehyde.
- Flavor stabilization.
Conclusions
It is important to keep in mind that the fermentation process may vary depending on the type of beer and the preferences of the brewer.
These general guidelines provide an overview of the fermentation process in beer brewing, but each brewer can adjust parameters and stages according to their specific needs and goals.
Once cooling for conditioning or lagering begins, the yeast in suspension will continue to settle, and depending on the size of the fermenter, some yeast will be removed or discarded every 3 to 4 days until it quickly becomes beer.
If the yeast meets the quality standards required by the brewery for collection, this must be carried out at the end of secondary fermentation.
Frequently Asked Questions (FAQ)
What are the three main stages of beer fermentation?
Beer fermentation is generally divided into three stages: primary fermentation, secondary fermentation, and conditioning. During primary fermentation yeast converts most sugars into ethanol and CO2; during secondary fermentation diacetyl is reduced and the yeast begins to flocculate; during conditioning the beer is chilled near 0–3°C to clarify and stabilize flavor.
What temperatures are recommended for primary fermentation?
Ale yeasts ferment best between 17–24°C (62–75°F); Lager yeasts between 8–14°C (46–58°F); and wheat and Belgian-style yeasts between 17–29°C (62–85°F).
What is diacetyl and why does it matter in secondary fermentation?
Diacetyl is a fermentation byproduct that imparts a buttery or butterscotch flavor to beer. When attenuation reaches 40–50%, brewers raise temperature by 3–5°C so yeast reabsorbs and breaks down diacetyl into flavorless compounds.
Why is conditioning more important for Lager than Ale?
Lager beers benefit from extended cold conditioning because it clarifies the beer and smooths its characteristic flavor compounds. Ale beers are best conditioned briefly — desirable Ale flavors degrade over time, so packaging should follow as quickly as possible.
When should yeast be collected for repitching?
Yeast should be harvested at the end of secondary fermentation, before conditioning begins. Collecting during conditioning may yield stressed or contaminated yeast, affecting performance in subsequent batches.
