Fermentation is a vital step in brewing, transforming sweet wort into beer. While the initial stages of malting, mashing and making the wort start the process, yeast fermentation is the optimal phase of creation.
Yeast is a single-celled fungus who multiplies in vast numbers to dine on sugars. In nature yeast are decomposers of sugary organic matter like fruit, which contains mainly glucose and fructose.
In beer brewing yeast usually consume sugars from malt, specifically maltose, and excrete ethanol (alcohol) and carbon dioxide. CO2 adds the bubbles to beer, fizz to soda pop and sparkle to champagne.
The Windup
Cooling and Aerating the Wort: After boiling and chilling the wort, it needs to be cooled to the ideal temperature range for the chosen yeast strain. Ale yeasts do best between 15-24°C (60-75°F). Lager yeasts prefer a cooler 9-14°C (48-58°F).
Aeration provides the yeast with oxygen it needs to reproduce and build healthy cell walls before the anaerobic fermentation process truly begins. The three main methods to oxygenate wort are agitation, splashing, and injecting either air or pure oxygen.
Yeasts can live with or without oxygen. When oxygen is available, they perform aerobic respiration, transforming carbohydrates (sugar source) into carbon dioxide and water. Without oxygen, yeasts begin fermentation, converting carbohydrates into carbon dioxide and alcohol.
The Pitch
Pitching the Yeast
Introduce the yeast to the wort. Dried yeast (rehydrated according to package instructions) or liquid yeast starters are both viable.
The amount of yeast used, known as the pitching rate, can make a difference. Under-pitching can lead to off-flavors and sluggish fermentation. Over-pitching can shorten fermentation time and potentially suppress the development of desirable esters.
First Base
Primary Fermentation: This is the most active phase of fermentation. The yeast rapidly consumes sugars, producing alcohol, carbon dioxide, and a variety of other compounds that contribute to the beer's flavor profile.
Vigorous bubbling happens in the airlock as CO2 escapes. This phase typically lasts for 1-3 weeks, depending on the beer style, yeast strain, and temperature.
A well-balanced mix of nutrients optimizes yeast health. Yeast needs nitrogen, phosphates, and other minerals. Yeast nutrient additives are available.
Primary fermentation phase lasts from a few days to two weeks, depending on yeast strain and wort characteristics.
Second Base
Secondary Fermentation
Once the primary fermentation slows down, transferring the beer to a secondary vessel promotes clarification and flavor development. This step can help remove sediment (trub) and allow settling.
It can prevent beer from developing off-flavors from prolonged contact with the yeast cake. It can also be used for adding dry hops or other flavorings.
Typically, this stage lasts one to two weeks. Some brewers choose to extend it longer, up to four weeks, to allow flavors to develop further.
Third Base
Determining when fermentation is complete is one of the most crucial aspects of brewing. Measure specific gravity using a hydrometer. When gravity readings remain stable for two to three consecutive days, fermentation is usually complete.
Back before such measuring devices, beer is judged finished when the krausen falls. Krausen is the rich froth which may form on fermenting beer. It's sometimes beheld with joy, and sometimes skimmed off.
Another sign of completion is the transformation of aroma and flavor. The beer should have shed its initial "green" flavors from yeast activity, resulting in a smoother, more refined taste.
Fermentation time can vary significantly. Ales might be ready for packaging in just one week, while lagers typically require several weeks, sometimes up to three months, to reach a smooth, refined character.
Home Run!
Taste Test: After several days of stable gravity readings sample the beer. It should taste clean and not sweet.
Conditioning
Additional time for fermentation can improve the final product. Allowing the beer to condition, in the secondary fermenter or bottles/kegs, lets flavors mellow and blend. This phase can provide natural carbonation.
For Successful Fermentation
Temperature Control: Temperature fluctuations can cause off-flavors and stalled fermentation. Brewers may use fermentation chambers or temperature controllers. Temperature around 21°C (70°F) can benefit flavor.
Yeast Health and Nourishment: Consider adding yeast nutrient to the wort, especially for high gravity beers or those lacking sufficient nutrients.
Sanitation: Contaminants can ruin beer. Sanitize all equipment in contact with the boiled wort.
Time: Rushing the fermentation process can lead to incomplete fermentation and undesirable flavors.
Oxygen Exposure: After yeast is pitched, too much oxygen can cause bacterial growth, hindering fermentation or turning booze to vinegar.
Sugar Levels: The amount of fermentable sugars directly influences alcohol content. High sugar levels can overwhelm the yeast, leading to stuck fermentation. For example, high original gravity worts above 1.070 can be problematic.
pH Levels: An ideal pH for fermentation ranges from 4.0 to 5.0. Extremes in pH can corrupt yeast activity.
High alcohol concentration: Yeast goes dormant and sinks to the bottom at a certain alcohol level, between 8-20%. Different strains vary in tolerance.
Fermentation produces alcohol and compounds contributing to the aroma and flavor, including esters, phenols, fusel alcohols, and organic acids. Sour beers rely on wild fermentation.
These are intentionally fermented with wild yeast and bacteria for tart complex flavors. Wild yeasts can be unpredictable. As in wine, a batch may be started with wild, and a more stable S. cerevisiae added later.
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