What is Secondary Fermentation?
Techincally, secondary fermentation is any period of additional fermentation or conditioning that takes place following the primary fermentation period. Generally speaking, secondary fermentation involves transferring a beer out of its primary fermentation vessel into a second sanitized vessel.
The point of secondary fermentation is to get the beer off the original cake of yeast and the sediments that have fallen to the bottom of the fermenter. When yeast goes dormant at the end of fermentation and begins to die off, the yeast cells will disintegrate and give off chemicals that negatively impact the flavor of your finished beer.
Is Secondary Fermentation Still a Valid Process?
The secondary fermentation process came into existence in the early days of home brewing. In those days, the available brewing yeast often wasn't the most viable, and less was known by home brewers about how to keep yeast healthy. As a result, the death and disintegration of the yeast cells (known as autolysis) was a very real possibility that could ruin a batch of beer. Also, brewing ingredients, equipment, and methods were less advanced than they are today, so there was often plenty of sediment remaining in the beer after fermentation finished. Transferring a beer to a secondary fermenter was a way to avoid off-flavors from autolysis and allow more of the sediment to fall out of the beer, making it clearer.
If you're using relatively fresh yeast, treating it well, and using good brewing practices, most experts say that secondary fermentation is mostly pointless. Yeast is unlikely to undergo autolysis during the first month or so of fermentation, and most modern tools and techniques result in clear beer.
Are There Risks Associated with Secondary Fermentation?
Any time you disturb a beer before it's bottled or kegged, there is a risk associated with this. If the beer comes into contact with too much oxygen, it will develop cardboard-like flavors. If the transfer tubing or secondary fermenter aren't properly sanitized, bacteria and wild yeast may enter the beer and cause spoilage, souring, or unwanted off-flavors. There's also a possibility of spillage... and no one wants that.
Are There Times When Secondary Fermentation is Worthwhile?
A good rule of thumb is that if your beer isn't going to be fermented longer than 4-6 weeks, there is no good reason to incorporate a secondary fermentation step.
However, there are two conditions under which you may want to consider a secondary fermentation:
- Extended fermentation or aging: The longer you plan to keep the beer in the fermenter, the more risk there is of autolysis occurring, though these days it's still pretty low. Anecdotal evidence (of mine and others) suggests that even up to five months, depending on yeast strain and recipe, isn't too long to keep a beer in the primary. A good rule of thumb, though, is to transfer to a secondary fermenter after about a month.
- Large additions in secondary: If you plan to add a lot of fruit puree or some other large addition during secondary, it may be better to put those ingredients in the secondary fermenter and carefully transfer the beer onto them, rather than dumping those ingredients into the primary fermenter. This is because you could churn the beer too much adding these items and cause it to oxidize. If you're careful, though, this may not be an issue.
I've personally had beers stay in their primary fermenter as long as three months without racking them off to a secondary. Those beers were as tasty and clear as any I've transferred to a secondary. Then again, I try to keep my yeast healthy in primary by providing it with yeast nutrient and oxygen from the start, so your experience may differ.
There's no substitute for experience. Perhaps the next time you brew a beer that you would normally transfer to a secondary fermenter, you might try only transferring half of it and leaving the other half on the original yeast cake, to see if you can tell a difference in the two. You may decide, as I have, that most of the time it's a waste of my time and effort.