Fermentation is the process by which yeast converts glucose in wort into ethanol and carbon dioxide gas, giving beer both alcohol content and carbonation. To start the fermentation process, the cooled wort is transferred to a fermenter to which yeast has been added.
Everyone knows that alcohol is produced through fermentation, but do you know why fermentation is necessary? Fermentation begins when yeast is added to the cooled wort. The whole fermentation process can be divided into three stages: yeast recovery stage, aerobic respiration stage, and anaerobic respiration stage. After the yeast is inoculated, its physiological activity is restored under the oxygenation condition of the wort. The amino acids in the wort serve as the main nitrogen source, while the fermentable sugars serve as the main energy source for respiration.
What is the purpose of beer fermentation?
Fermentation, facilitated by yeast, converts fermentable sugars into equal amounts of ethanol and carbon dioxide gas. There are two basic methods of fermentation in brewing: top fermentation and bottom fermentation.
During the fermentation stage of the brewing process, top-fermented beer contains yeast that ferments at the top of the fermenter. Top-fermented beers are fermented at higher temperatures.
During the fermentation stage of the brewing process, bottom-fermented beer contains yeast that ferments at the bottom of the fermenter. Bottom-fermented beers are fermented at cooler temperatures.
Since yeasts are living microorganisms, they don’t just ferment. They can produce many different molecules, each contributing to the flavor of the beer. This is where experience, knowledge, and understanding of the process and yeast come into play.
Each type of yeast produces different molecules, resulting in different beer flavors. Furthermore, temperature, glucose concentration, and other parameters affect how the yeast behaves and which molecules are formed! Some examples of the types of molecules formed are:
- Esters: contribute to the fruity aroma of beer
- Ketones (e.g., diacetyl): give beer a honey and butterscotch taste
- Sulfur volatiles
- Organic acid
- Fatty acid
Yeast recovery stage
The main component of yeast cell membranes is sterol, and the sterol content drops significantly after the last round of yeast reproduction. So, when the yeast is re-inoculated, it needs to synthesize sterols to create new cell membranes. The restoration of permeability and the resumption of reproduction are carried out through the biosynthesis of sterols with the participation of unsaturated fatty acids and oxygen. In the second stage, yeast cells do not reproduce, entering what is called the yeast stationary phase. Once the cell membranes are formed, permeability is restored, and nutrients enter, allowing the yeast to absorb the energy provided by sugar and accumulate it for future use.
Aerobic respiration stage
This stage refers to when yeast cells use fermentable sugar as the main energy source and reproduce under the presence of oxygen.
Anaerobic respiration stage
During this fermentation process, most of the fermentable sugars are broken down into carbon dioxide. These sugars are absorbed by yeast for fermentation. The order of glycolysis is glucose, fructose, sucrose, maltose, and maltotriose.
The yeast breeding period occurs 8-16 hours after adding yeast to the wort. When small bubbles appear and form white foam, the wort is immediately transferred to the fermentation tank after 20 hours of breeding. After 2-3 days of fermentation, the beer turns brown as hop resin and protein-tannin complexes begin to separate. Artificial cooling is required, but it should not be too intense to avoid premature yeast precipitation and affect fermentation. After 7-8 days of fermentation, the foam subsides and forms a cap, which is skimmed off along with precipitated polyphenol complexes. Hop resin, yeast cells, and other impurities should be cooled now to allow the yeast to settle.
The fermented liquor after the main fermentation of the wort is called young beer. At this stage, the carbon dioxide content is insufficient, and volatile substances such as diacetyl, acetaldehyde, and hydrogen sulfide have not been reduced to an appropriate level. The flavor of the beer is immature and not suitable for drinking.
Top-fermented beer matures when the equipment is turned around, and it develops a unique flavor but has a shorter shelf life. Generally, a post-fermentation period is not used, but fining agents are added. After the first stage of clarification, artificial carbon dioxide is used to achieve saturation.
How long does it take to ferment during the brewing process?
The duration of the fermentation process varies from batch to batch. The time it takes for a beer to ferment depends on factors such as temperature and yeast strain (dry yeast usually ferments faster). On average, fermentation takes at least two weeks to complete.
Does the type of yeast affect fermentation?
Different brewer’s yeast strains exhibit different behaviors during beer fermentation. The yeast strain used by the brewer affects the temperature required for wort fermentation. The type of yeast also impacts the flavor of the final beer. Some yeasts produce yeasty and bready flavors, while others produce a cleaner, crisper taste. It is important to select the right yeast strain for the beer style being brewed.