Fermentation in homebrewing is the process of converting the wort into alcohol by yeast. The typical beer fermentation process generally includes three stages. First is the lag period directly after pitching, second is primary fermentation and third is secondary fermentation. When fermentation goes wrong from too lengthy a period or too warm of temperature autolysis can result causing defects and ruining weeks or more of careful fermentation.

Lag Period
The lag period is a seemingly inactive yet very important stage in wort fermentation. This period is when the yeast prepares itself for the processes involved in converting the various sugars into beer. The lag will normally last between 6-8 hours for ales and 12-18 hours for lagers. It's end will be indicated by activity in the airlock in the form of carbon dioxide escaping as bubbles. Once the yeast has become active the lag stage has moved into primary fermentation.

Primary Fermentation
Primary fermentation is the most important, most active stage where the wort is actually turning into beer. Yeast cells are multiplying and devouring the sugars in the wort. This process not only creates the beer but gives off carbon dioxide in large amounts. During the first few days of primary fermentation the air lock will be very active. As the sugars begin to be depleted the yeast growth slows down and the activity slows with it. This activity is a good indicator of the primary fermentation stage starting and also when it stops. Near the end of primary fermentation the bubbles will slow to few or none. Usually primary fermentation is considered complete when the activity trickles to just a few bubbles per hour. A better test is to check the specific gravity using a hydrometer. A hydrometer coupled with sight and smell will tell you exactly when it is time to rack for secondary fermentation.

Secondary Fermentation
Secondary fermentation is when the beer is racked to remove the old dead yeast and allowed to age. During this aging process the remaining active yeast begin to eat the harder to ferment sugars. This stage is also when the yeast begin to gather together into flocs and either rise to the top or fall to the bottom. This process is known as Flocculation and top-fermenting yeast rise to the top while bottom fermenting yeast fall to the bottom. Ales typically have a short secondary fermentation because ale yeast cannot consume all of the hard to ferment sugars remaining. Lagers, especially cold fermenting varieties have a long secondary fermentation. This long cold secondary fermentation is very important in lagers. It is the period where the large amounts of by products are reabsorbed by the yeast as they consume the remaining hard to ferment sugars creating the familiar taste of lager beer.

Autolysis
Autolysis is a defect of beer which results in a rotten smell and a taste not unlike wet cardboard. Once food has become scarce the yeast react by abandoning fermentation and begin building up it's glycogen stores. Once that process is complete the yeast then go dormant and drop to the bottom of the fermenting vessel. Once the yeast have finished consuming thier internal glycogen stores they begin to emit enzymes intended to break down neighboring yeast in order to create more food for themselves. As this process continues sulfury compounds are released resulting in the bad taste and smell. Autolysis is largely dependant on the type of yeast and the temperatures of fermentation. Racking the beer as soon as secondary fermentation is complete is the best practice in avoiding autolyzed beer.