Fermentation is the heart of the brewing process. During fermentation, wort created from raw materials is converted to beer by yeast. Fermentation is usually divided into three stages: primary, secondary, and conditioning (or lagering). Fermentation is when yeast produce all of the alcohol and aroma and flavor compounds found in beer. Manipulation of temperature, oxygen levels, and pitch rate as well as yeast strain selection will all dramatically affect the production of aroma and flavor compounds produced during fermentation.
The primary stage of fermentation begins when the yeast is introduced into cooled, aerated wort. The yeast quickly utilize the available oxygen to produce sterols, a vital compound for culture expansion. When the oxygen is gone, the yeast switch to the anaerobic phase where the majority of wort sugars are reduced to ethanol and CO₂. Yeast growth occurs during primary fermentation. The extent and rate of yeast growth is directly related to the production of aroma and flavor compounds.
Primary Fermentation Summary:
- Depletion of dissolved oxygen
- Acidification/reduction in pH
- Yeast growth or culture expansion
- Ethanol and CO₂ production
- Production of flavor compounds such as esters, diacetyl, sulfur containing compounds, etc.
- Consumption of most wort sugars
The temperature of the primary fermentation should be regulated according to the desired flavor and aroma profile. The following is a guideline:
Primary Fermentation Temperatures:
- Ales: 62-75 °F (17-24 °C)
- Lagers: 46-58 °F (8-14 °C) *Note: Lager fermentations can be started warmer (~60 °F/15.5 °C) until signs of fermentation (gravity drop, CO₂ production, head formation) are evident. Cool to desired fermentation temperature once signs of fermentation are observed.
- Wheat and Belgian styles: 62-85 °F (17-29 °C)
The secondary stage of fermentation refers to the stage of fermentation after the majority of the wort sugars have been consumed and there is a sharp decrease in the rate of fermentation. During this period, most of the final sugars are depleted and some secondary metabolites are converted by the yeast. Yeast flocculation and settling begins to occur due to the increase in alcohol content and the depletion of sugar and nutrients. Diacetyl reduction takes place during secondary fermentation and during the diacetyl rest that some brewers incorporate into the secondary stage of fermentation.
Secondary Fermentation Summary:
- Decreased rate of ethanol and CO₂ production
- Diacetyl Conversion
- Reduction of some flavor compounds by yeast metabolism or CO₂ scrubbing
- Terminal gravity is reached
- Yeast flocculation and settling begins
Secondary Fermentation Temperatures:
- Ales: Same as primary fermentation (higher temperatures will increase diacetyl reduction rates)
- Lagers: 40-60 °F (4-15 °C). Some brewers allow the beer to increase in temperature to speed the diacetyl reduction. This increased temperature is usually only sustained for 24 to 48 hours.
- Wheat and Belgian Beers: Same as primary fermentation (higher temperatures will increase diacetyl reduction rates).
The conditioning stage takes place when the terminal gravity has been reached and the tank is cooled to refrigeration temperatures (31-38 °F, 0-3 °C). During this time the yeast continues to flocculate and settle. The yeast also conditions the beer by reducing various undesirable flavor compounds. Ales do not benefit from long conditioning times like lagers do. The desirable flavors in ales will decrease with age and therefore it is recommended that conditioning be as short as possible before packaging. Exposure to oxygen at this stage is extremely detrimental to beer quality.
- Most of the yeast is removed from beer
- Formation and precipitation of haze forming proteins
- Reduction and mellowing of harsh flavors
- Reduction of sulfur compounds, diacetyl, and acetaldehyde
- Flavor stabilization