Single stage vs two stage fermentation
Single-stage fermentation is the process of using only one fermenter. Two-stage fermentation uses two fermentation vessels: the initial fermentation occurs in the primary; once fermentation begins to slow, the beer is siphoned into a secondary fermenter. There is some debate as to weather single-stage or two-stage fermentation is better.
The purpose of two-stage fermentation is to separate the beer from prolonged contact with inactive yeast. This is only a concern if your beer sits on the yeast longer than two to three weeks at which time it will begin to undergo autolysis imparting a rubbery aroma into the beer. There is no real advantage to keeping you beer sitting around for longer than two to three weeks unless you are lagering.
There are advantages and disadvantages to both methods. The advantages for using only a primary include:
- Eliminating the risk of contamination during racking;
- Eliminating the risk of oxidation during racking;
- Eliminating the time and effort of the additional transfer;
- Eliminating the pick-up of oxygen and production of additional diacetyl which is not reduced by the yeast; and
- Eliminating the need to sanitize another vessel.
The disadvantages of using only a primary include:
- If you are fermenting in a plastic bucket and let the finished beer sit too long, oxygen can enter the bucket through an ill-fitting lid or by permeating through the plastic. This can be avoided by using a glass carboy;
- If you are dryhopping and plan to repitch your yeast, you will be forced to pitch the spent dryhops along with the yeast; and
- If you are not fining, you may end up with more yeast at the bottom of each bottle.
There is little advantage to using a plastic secondary or a secondary at all if using a glass primary. So advantages of using a glass secondary following a plastic primary include:
- Reduces the risk of oxidation during aging;
- Allows the reuse of yeast without the interference of dryhops;
- Reduces the amount of yeast in each bottle after conditioning;
- Reduces the risk of autolysis; and
- Don’t know when you’re going to have time to bottle.
The disadvantages of using a secondary include:
- Increased risk of oxidation during transfer;
- Increased risk of contamination during transfer;
- Increased time and effort; and
- Increased use of sanitizer.
Personally, I use a glass primary (except for fruit beers) and only use a secondary for lagers and high gravity beers except when time does not permit bottling on schedule