Brewing yeast strains are unicellular fungi that convert simple sugars into approximately equal parts of alcohol and carbon dioxide during the fermentation process. There are two main types of beer yeast varieties: saccharomyces cerevisiae, which is a top fermenting ale yeast, and saccharomyces pastorianusis, a bottom fermenting lager yeast.
A wide selection of home brewing ale and lager yeast can be found here:
Primary fermentation in beer brewing is the initial fermentation process where yeast will convert most or all of the wort sugars to alcohol and CO2 (carbon dioxide). After the yeast has been pitched into the wort, there is typically between 2 and 24 hour yeast lag time where the yeast acclimates to the fermentation environment and begins to replicate consuming sugars and the available oxygen in the wort; there is little alcohol conversion and CO2 generated during the lag phase.
Once the lag phase completes, a foamy head called a krausen begins to form in the fermentation vessel. The krausen is composed mostly of proteins, yeast, and the carbon dioxide that the yeast is rapidly producing. During primary fermentation the yeast is producing approximately equal parts of both alcohol and CO2. Depending on the style of beer, original gravity, quantity of yeast pitched, and fermentation temperature, the primary fermentation for an ale will last between 5-14 days, then it will then be transferred to a secondary fermentation vessel to allow the beer to condition and finish out its fermentation. In some cases only a primary fermentation is completed, and the beer may spending additional time in the primary fermenter or condition in the bottle, keg, or holding vessel.
Primary fermentation occurring two days after the yeast was pitched into an American Wheat style Hefeweizen. The krausen has formed and a great deal of alcohol and CO2 is being produced.
Beer fermentation is the metabolic conversion of malt and adjunct sugars to alcohol, acid, and CO2 using yeast or bacteria. As yeast and bacteria convert the wort sugars, approximately equal amounts of carbon dioxide and alcohol are produced. The alcohol being produced by the yeast is less dense than the sugars and water the yeast is metabolizing, so the gravity of the fermenting beer drops while fermentation continues to occur.
The two predominant types of fermentation are top fermenting which is used for ales, and bottom fermenting which is typical for lagers. The time frame for a fermentation to complete is dependent on a great many factors, just a few of them include the types of sugars that the wort is composed of, the amount of sugar in the wort (original gravity), the type of yeast and or bacteria used, the amount of yeast and or bacteria used, the health of the yeast and or bacteria, and temperature the fermentation occurs at. Most fermentations will take approximately two to six weeks to complete, but some may take upwards of a year before the beer is considered ready.
Below is a photo of beer during primary fermentation.
Autolysis is the destruction of a cell by the actions of its own enzymes. In brewing, autolysis typically occurs when yeast cells either decay over time or destroy each other. When the outer wall of the yeast cell is degraded and can no longer contain itself, it releases off flavors and odors into the beer. These odors are typically described as rubbery in aroma.
Autolysis is most common in aged beers, but can also occur in a fresh beer due to a variety of factors including unhealthy\aged yeast, stress caused by too rapid of a fermentation, excessive temperature changes and high alcohol levels. One of the best ways to reduce the impact of autolysis is to conduct a secondary fermentation as a means of removing the beer from the yeast cake. Other ways of reducing the impact of autolysis is proper aeration of the wort prior to fermentation, avoiding oxidization, keeping a consistent and appropriate fermentation temperature, and properly regulating the temperature of your finished beer.
Alcohol by weight (ABW) is the measurement of the alcohol content of a solution in terms of the percentage weight of the alcohol in the solution.
Since most of us are accustomed to thinking of beer in terms of alcohol by volume, you may find it beneficial to convert the two. To convert ABW to ABV, multiply the ABW by 1.25. For instance, an ABW of 5 equals an ABV of 6.25%. To convert ABV to ABW, you would multiply the ABV by .8.