Fermentation begins with the pitching of the yeast. "Pitching" is simply a brewer's term meaning to add the yeast to the fermenter. Before pitching, you need to make sure that you've properly cooled and aerated the wort. In some cases, you may need to prepare the yeast well beforehand; for more information, see below or under Preparing the Ingredients. The steps you may want to take to prepare your yeast depend on whether you are using dry or liquid yeast.
Before You Pitch
The wort should be cooled to below 80 degrees Farenheit,(even lower temps may be desireable for pitching yeast into lagers) and added to the fermenter. Before pitching the yeast, a sample of the wort should be taken and its specific gravity measured with a hydrometer.
Pitching Dry Yeast
One packet of dry yeast is typically enough to inoculate 5 gallons of wort; a yeast starter is not necessary. The dry yeast packet can be pitched directly onto the cooled wort; however, performance can be improved by following the directions for preparing the yeast.
Pitching Liquid Yeast
There are many varieties of specialized liquid yeast strains available to homebrewers, making liquid forms very appealing. However, the amount of yeast that is provided in the popular brands' (Wyeast and White Labs) packages is typically inadequate to inoculate 5 gallons of wort.
In order to overcome this inadequacy, many brewers make yeast starters for their liquid yeast. The purpose of a yeast starter is to increase the number of viable cells prior to adding the yeast to the wort, thus cutting down on lag time and making for a healthier fermentation. When yeast cells are added to fermentable wort, they begin to reproduce in amazing numbers; pitching the yeast onto sugar prior to adding them to the main wort allows the brewer to increase the cell count. It also gets the yeast cells working, reducing lag time (the time between pitching the yeast, and the start of fermentation).
When a beer is done fermenting, it is often transferred to a secondary vessel for conditioning and clarifying leaving a "yeast cake" at the bottom of the primary fermenter. This "yeast cake" is made up of a huge amount of viable yeast cells, in addition to protein solids and break material. While some may discard this cake, or "trub", others may harvest it in jars for future use (see washing yeast and freezing yeast ); another option is to pour the cooled wort directly onto it. Given the high yeast cell count, and the viability of those cells, this results in very short lag time and very vigorous fermentation. However, given that some of the byproduct of the previous beer remains in this trub, it is important the the style of the new beer does not clash with the prior style.
Since this method results in such a vigorous fermentation, some brewers opt to pour some of the trub out prior to pouring wort onto it. This allows the initial yeast reproduction period to take place again, which some brewers feel adds desirable qualities to their ales. Others worry about the natural heat that is produced by vigorous fermentation (thus resulting in higher fermentation temps), and also opt to pour a portion of the trub out.