In this chapter, we will discuss fermentation - how the yeast turns wort into beer. As important as the yeast process is to achieving a good batch, it is also the one that is most often taken for granted by beginning brewers. A lot of thought will be given to the recipe: which malts, which hops, but often the yeast choice will be whatever was taped to the top of the kit. Even if some consideration is given to the brand of yeast and the type, very often the conditions to which the yeast is pitched are not planned or controlled. The brewer cools the wort, aerates it a bit, and then pitches his yeast and waits for it to do its thing.
It has been common for brewing texts to over-emphasize the "lagtime" - the period of time after pitching the yeast before the foamy head appeared in the fermentor. This lagtime was the benchmark that everyone would use to gage the health of their yeast and the vigor of the fermentation. While it is a notable indicator, the lagtime accounts for a combination of pre-fermentation processes that have a great deal to do with the quality of the total fermentation, but that individually are not well represented by time.
A very short lagtime, for example, does not guarantee an exemplary fermentation and an outstanding beer. A short lagtime only means that initial conditions were favorable for growth and metabolism. It says nothing about the total amount of nutrients in the wort or how the rest of the fermentation will progress.
The latter stages of fermentation may also appear to finish more quickly when in fact the process was not super-efficient, but rather, incomplete. The point is that speed does not necessarily correlate with quality. Of course, under optimal conditions a fermentation would be more efficient and thus take less time. But it is better to pay attention to the fermentation conditions and getting the process right, rather than to a rigid time schedule.