This step is optional and could have several benefits and drawbacks depending on the brew, what you want to get out of it and how careful you are.
The gist of secondary fermentation is to transfer (‘racking’) the ‘finished’ beer out of the primary container and into a second device (typically glass), for a period of aging, ranging from a few days to months.
Let’s start with the downsides. Racking your beer gives you more chance to introduce an infection in your beer. It’s another process where you have to be extremely sanitary. It’s also another time when you can introduce oxygen into the beer, which can lead to stale flavors. It adds to the cost, since you need to buy an extra container. It also adds to the total time required before you can drink, which is never a good thing!
So why would anyone do a secondary fermentation? There are a couple of good reasons to consider doing a secondary fermentation on your beer. The first is probably obvious. It allows the beer to clear more, giving you a better-looking brew, with less sediment in the bottom of the bottle.
But why not just let your beer sit longer, in the primary fermentation bucket? Because plastic buckets are never fully air tight, and once the primary fermentation has slowed and is not producing large amounts of protective carbon dioxide, oxygen will affect the beer, producing those stale, oxidized flavors. If we’re going to let the beer sit after its main fermentation is done, it pretty much needs to be in glass, and away from the spent yeast that accumulates at the bottom of your fermenter.
Also, because yeast are clever little creatures, when they run out of sugar to eat in your wort, they will find other things to eat. One source of nourishment is dead yeast cells. Unfortunately, when the yeast go down this metabolic pathway, they don’t produce the carbon dioxide and ethanol that we all know and love. Instead, through a process known as “autolysis” they produce some off flavors you really don’t want in your beer.
But why age your beer so long? In essence, longer aging using secondary fermentation will generally smooth out the beer, giving you a more pleasant tasting brew. In the case of lager beers, this type of yeast requires a long, cold secondary fermentation. As yeast consume the sugars, they leave odds and ends of more complex sugars around, and will eventually turn to them for nourishment. It is not unusual for this process to take a month or more in lagers.
Ale yeasts, on the other hand, cannot process these more complex sugars and therefore require less time in a secondary fermentation. Once your ale has cleared to your satisfaction in the secondary, it has probably also completed any biological benefits from the secondary fermentation. Secondary fermentations for ales are usually on the order of a week or so, though it won’t hurt the beer to stay in the fermenter longer (but remember that hop flavors and aroma may fade over time). “Big” beers, such as barley wines and imperial stouts, may take a long time to finish fermenting, because there is more sugar to consume, and the yeast is struggling in the presence of the higher alcohol content.
Should you do a secondary fermentation? Some brewers only do it for lagers, some only with their “big” beers, and some (like me) don’t do it at all (yet!).