Fermentation is the core process in craft brewing. During fermentation, yeast converts the wort, made from raw materials, into beer. It is during fermentation that yeast produces alcohol, aroma, and flavor compounds, which is often referred to as “where the magic happens”. Fermentation can be divided into three phases: lag phase, active phase, and stationary phase.
Beer Fermentation Stage – Lag Stage
Once the yeast is added, it enters the lag stage. Although there is no clear visible fermentation, some activities are still taking place. Yeast absorbs oxygen from the wort, producing sterols that are essential for reproduction and healthy growth.
Yeasts are living organisms and require oxygen. That’s why aeration of the wort is important in commercial brewing. Yeasts need oxygen to grow and produce important cell wall components. In addition to oxygen, yeast also absorbs vitamins and minerals necessary for growth.
The fermentation temperature depends on the beer style and the type of yeast used.
Ale: 62-75 °F (17-24 °C)
Lager: 46-58 °F (8-14 °C) *Note: Lager fermentation can start at a higher temperature (~60 °F/15.5 °C) until signs of fermentation (gravity drop, CO₂ production, formation of a head) become apparent. Once fermentation begins, cool to the desired fermentation temperature.
Wheat and Belgian styles: 62-85 °F (17-29 °C)
Beer Fermentation Stage – Active Stage
Depending on the beer style, visible activity will start within 24-48 hours of pitching the yeast. The yeast transitions from the lag phase to the anaerobic phase. The lag period can be as short as 30 minutes.
The yeast cell count increases during the active phase. Yeast starts consuming the sugars produced during brewing. Carbon dioxide is produced, and foam forms on the surface.
As the yeast cell count increases, ethanol and flavor compounds are produced. At this point, the aroma of fermentation can be detected. If a neutral yeast like Fermentis US-05 is used, it may have an olive oil-like smell.
The activity during this stage is temperature-dependent, and higher temperatures result in more vigorous fermentation. Yeast consumes sugar in a specific pattern, starting with glucose, followed by fructose and sucrose. Glucose accounts for approximately 14% of all sugars in typical beer wort.
The main sugar in the wort is maltose, which contributes to the beer’s flavor. Maltose makes up 59% of the total sugar content in typical beer wort. Yeast utilizes maltose to impart desired characteristics to the beer. During active fermentation, the foam on the surface of the wort usually changes color from yellow to brown, and brown spots of oxidized hop resin may also appear.
Lagers: For certain lager strains, brewers increase the fermentation temperature after 50 to 60% of fermentation is complete. This process, known as “free rise,” raises the temperature of the beer to as high as 20°C (68°F). The elevated temperature helps in the self-purification of the beer and reduces diacetyl levels.
Cool down to 10°C (50°F) and hold at this temperature for 48 hours.
Cool to 5°C (41°F) and hold at this temperature for 24 hours.
Cool to 0-3°C (32°F) for maturation.
Beer Fermentation Stage – Static Stage
During the later stage of fermentation, after most of the sugars in the wort have been consumed, the fermentation rate decreases significantly. At this stage, most of the remaining sugars are consumed, and some secondary by-products are converted by yeast. As the alcohol content increases and sugar and nutrients become depleted, the yeast begins to settle and clump together.
Yeast growth slows down as the beer enters a stationary phase. Most of the flavor and aroma compounds have already been produced, including fusel alcohols, esters, and sulfur compounds. During this resting phase, the beer undergoes conditioning. The yeast reabsorbs diacetyl produced during fermentation, and hydrogen sulfide is released as a gas. The attenuation level is determined by measuring the specific gravity of the wort.
The production rate of ethanol and carbon dioxide decreases.
Reduction of some flavor compounds through yeast metabolism or carbon dioxide release
Attainment of terminal gravity
Yeast flocculation and sedimentation begin
Ales: Same as the primary fermentation temperature (higher temperatures increase diacetyl reduction rate)
Lagers: 40-60 °F (4-15 °C). Some brewers raise the temperature of the beer to speed up the reduction of diacetyl. This elevated temperature typically lasts for 24 to 48 hours.
Wheat and Belgian beers: Same as the primary fermentation temperature (higher temperatures increase diacetyl reduction rate).
If the brewery plans to reuse the yeast for re-fermentation, they must ensure that the harvested yeast meets certain viability and purity standards after the secondary fermentation. Although these methods are not commonly used in homebrewing, there are simple techniques to determine whether re-pitching yeast in homebrewing is a viable option.