What are the two stages of fermentation? Are both stages necessary?
Fermentation is generally divided into two stages: Primary fermentation and Secondary fermentation.
Primary fermentation refers to the initial, active phase when the yeast is forming the kraeusen head. During this stage the yeast is actively fermenting the malt sugars to alcohol and carbon dioxide.
Primary fermentation is generally over when the kraeusen head disappears. At this time about one -half to two-thirds of the fermentables have been converted. Primary fermentation will generally take anywhere from three to ten days, depending on the yeast strain, the fermentation temperature, and the dextrin content of the wort.
Secondary fermentation allows the slow reduction of the remaining fermentable sugars. This may take from seven to twenty-one days, depending on the yeast strain and the amount of fermentable sugars in the wort. Generally, ales will require a shorter period of secondary fermentation than lagers. During this period the yeast will begin to flocculate as the last of the sugar are used up. It is important that no oxygen be allowed into the beer at this time. Most brewers recommend racking the beer to a secondary fermentation vessel at the end of the primary fermentation.
Fermentation should proceed at the proper temperture for the yeast strain and style of beer being brewed. Sudden variations in temperature in excess of 6 degrees F. per day must be avoided during fermentation. Strong light should also be excluded from fermenting vessels (especially glass carboys)
Gas locks should be of the type that will not permit lock water to be sucked down into the carboy. Locks should not permit a great pressure to build up in the carboy.
If using a blowout technique (recommended), make sure that the blowout hose is sanitized before installing it on the carboy. Suspend it above the catch bucket, so that there is no possibility of blowout being sucked back into the carboy should the carboy be cooled. The hose should also be big enough that pelletized hops will not cause it to foul, resulting in a blown cork and kraeusen foam on the floor. Never fasten or seal the cork to the carboy!