Fermentation is the core of the craft brewing process. The wort made from raw materials is transformed into beer by yeast during fermentation. This process is responsible for the production of alcohol, aroma, and flavor compounds in beer. It is often considered the magical stage of brewing. Fermentation can be categorized into three phases: lag phase, active phase, and stationary phase.
During fermentation, the wort sugars are consumed, and new yeast cells are produced along with ethanol and carbon dioxide. At the same time, flavor compounds are generated. As brewers, we have control over various elements of fermentation to ensure the timely completion of the beer.
The key elements include:
- Oxygen level
- Pitching rate
- Yeast selection
All these elements have an impact on the aroma and flavor compounds during fermentation.
Beer fermentation stage – lag phase
Once the yeast is added, it enters the lag phase. Although fermentation is not visibly active, processes are still occurring. Yeasts absorb oxygen from the wort, which is essential for their reproduction and healthy growth. Oxygenation of the wort is crucial in commercial brewing because yeasts require it to grow and produce vital cell wall components. Additionally, yeasts absorb vitamins and minerals necessary for their growth.
The fermentation temperature is crucial for proper timing. The chosen beer style and yeast determine the ideal fermentation temperature:
- Ale: 62-75 °F (17-24 °C)
- Lager: 46-58 °F (8-14 °C) *Note: Lager fermentation can begin at an elevated temperature (~60 °F/15.5 °C) until signs of fermentation (gravity drop, CO₂ production, head formation) become apparent. Once fermentation starts, cool it to the desired temperature.
- Wheat and Belgian styles: 62-85 °F (17-29 °C)
Beer fermentation stage – active phase
Depending on the beer style, visual signs of activity can be observed within 24-48 hours after pitching. Yeast transitions from the lag phase to the anaerobic phase. The lag period can be as short as 30 minutes.
During the active phase, yeast cell counts increase, and they start consuming the sugars present in the wort. Carbon dioxide is produced, resulting in the formation of a foam layer. As the yeast multiplies, ethanol and flavor compounds are generated. At this point, the fermentation becomes noticeable by its aroma. If neutral yeast, such as Fermentis US-05, is used, it may emit an odor reminiscent of olive oil.
The activity during this stage depends on the temperature, with higher temperatures leading to more vigorous yeast activity. Yeast consumes sugar in a specific pattern, starting with monosaccharides like glucose, followed by fructose and sucrose, before entering metabolism. In a typical beer wort, glucose accounts for about 14% of all sugars.
The primary sugar in brewing, maltose, comprises 59% of the wort sugar and contributes significantly to the beer’s flavor. Maltose serves as a critical ingredient that imparts desirable properties to the beer. When fermentation is at its peak, the foam on top of the wort usually changes color from yellow to brown, indicating the presence of oxidized hop resin.
Lagers – For certain lager strains, brewers increase the fermentation temperature after reaching 50 to 60% fermentation. This allows the beer to “free rise,” and the temperature can go as high as 20°C (68°F). The elevated temperature helps in reducing diacetyl.
- Cool down to 10°C (50°F) and maintain for 48 hours
- Cool to 5°C (41°F) and maintain for 24 hours
- Cool to 0 to 3°C (32°F) for maturation.
Beer fermentation stage – stationary phase
In the latter stage of fermentation, after most of the wort sugars are consumed, the fermentation rate decreases significantly. During this period, the remaining sugars are metabolized, and yeast converts some secondary metabolites. As alcohol content increases and resources deplete, yeast begins to flocculate and settle.
Yeast growth slows down, and the beer enters a stationary phase. Most of the flavor and aroma compounds have already formed, including fusel alcohols, esters, and sulfur compounds. Conditioning of the beer occurs during this resting period. Yeast reabsorbs diacetyl produced during fermentation, while hydrogen sulfide escapes from the fermenter as a gas. The specific gravity of the wort is measured to determine the extent of attenuation.
- Reduction of ethanol and carbon dioxide production
- Diacetyl conversion
- Reduction of certain flavor compounds through yeast metabolism or carbon dioxide scrubbing
- Reaching terminal gravity
- Yeast flocculation and sedimentation initiation
- Ales: same as primary fermentation (higher temperature speeds up diacetyl reduction)
- Lager: 40-60 °F (4-15 °C). Some brewers temporarily elevate the temperature to expedite diacetyl reduction, which typically lasts for 24 to 48 hours.
- Wheat and Belgian beers: same as primary fermentation (higher temperature speeds up diacetyl reduction)
If the brewery plans to repitch the yeast, they must ensure the quality of the harvested yeast meets viability and purity standards after secondary fermentation. Although these practices extend beyond most homebrew needs, there are simple techniques to determine whether reusing yeast is a suitable option for homebrewers.
The Turnkey Way to Start Home Brewing
The aforementioned stages of fermentation and the specific temperatures required are recommended by ACE’s brewing engineers. Understanding the fermentation process leads to better brewing practices. If you are ready to start your brewery, contact ACE for a turnkey solution, professional customized services, and equipment solutions. ACE looks forward to collaborating with you and providing assistance in establishing or expanding your brewery.