In many ways, fermentation is the most critical step in the brewing process. An often repeated saying amongst hobbyists is, "Brewers make wort. Yeast makes beer.".

Fermentation officially begins when yeast is added to the wort. When a beer is "fermenting", it means that the yeast is in the process of converting the sugars in the wort into alcohol and carbon dioxide (CO₂). 

The most important part of fermentation is sanitisation

Once all of your wort is in the fermenter, at the right temperature and well aerated or oxygenated, you can pitch the yeast (ie add yeast to your wort). However, it is important before you pitch your yeast to know what the density or gravity of the wort is. Then, you can seal the fermenter to ensure a closed environment for the fermentation process to begin. There should always be a way for excess CO₂ to escape the fermenter so that there is not too much pressure. This is often done with an airlock, blow-off tube or, in the case of pressure fermenters, a sounding valve attached to the fermenter. 

If your fermenter doesn't have temperature control. Be sure to place your fermenting wort in a place where the ambient temperature does not vary widely throughout the day & night. A basement or closet that maintains a consistent temperature between 20°C - 22°C (68°F - 72°F) is great for fermenting an ale. A chest freezer or refrigerator that maintains a consistent temperature between 7°C - 13°C (45°F - 55°F) is great for fermenting a lager.

The phases of fermentation are:

1. Lag Phase:
   • Description: This is the initial phase where yeast cells are introduced to the fermentation environment. During this phase, yeast cells adjust to the new conditions, taking in nutrients, and preparing for active growth.
   • Activities: Yeast cells are synthesizing enzymes needed for the fermentation process. There is little to no visible production of alcohol or carbon dioxide during this phase.
2. Logarithmic or Exponential Growth Phase:
   • Description: In this phase, yeast cells experience rapid multiplication. The population of yeast cells increases exponentially, leading to a significant increase in the metabolic activity of the yeast.
   • Activities: Yeast cells actively consume sugars and produce alcohol and carbon dioxide as by-products. This phase is crucial for establishing a strong yeast population capable of efficiently fermenting the available sugars.
3. Stationary Phase:
   • Description: As the concentration of alcohol increases and the availability of nutrients decreases, yeast growth slows down. The rate of cell reproduction equals the rate of cell death, resulting in a stationary population.
   • Activities: Yeast cells are still metabolically active but are no longer multiplying at a rapid pace. Fermentation continues, but the overall yeast population remains relatively constant.
4. Death or Decline Phase:
   • Description: In this phase, the yeast population begins to decline due to factors such as the accumulation of alcohol, depletion of nutrients, and other stressors.
   • Activities: Yeast cells may undergo autolysis, a process where cells break down, releasing enzymes and other compounds into the fermentation mixture. The decline in yeast population eventually leads to the end of the fermentation process.

Depending on the type of beer being brewed, fermentation can take 7 - 14 days for an ale, or several weeks for a lager. Look for signs that fermentation has completed when no bubbles are coming through your airlock or blow-off tube. A final gravity reading will help you determine whether fermentation is complete.