6 gal. carboy with about 4 gal. of beer. Major goof, but the beer turned out fine.
I stumbled upon a forum post in which the poster asked if he should use tap water or bottled water to top off the beer in his fermenter during secondary fermentation. Apparently someone informed him that it was necessary to “top off,” or raise the level of beer in the carboy, to reduce air space and limit the amount of oxidation that will occur.
One response to the post stated that some people use glass marbles to displace the beer, thereby raising the level, instead of adding water. As a home brewer since the late 1990s I’ve never heard of such a thing, nor the necessity for it.
Is It Necessary?
During fermentation yeast is consuming sugars, producing alcohol and Carbon Dioxide (CO2), and a lot of it early on, so the amount of regular old air present in the fermenter is minimal. And since the CO2 is emanating right from the fermenting beer itself, there will be a layer of that gas along the surface of the liquid, so no real oxidation can take place during the fermentation process.
When you rack your beer to a secondary fermenter the process will continue at a slightly lower rate, but CO2 will still remain as a buffering layer above the liquid, ultimately preventing any substantial oxidation.
If the airlock is bubbling, even infrequently, there is CO2 being released. There is no need to top off beer in the fermenter. If you find that your ending up with less than five gallons when making a five gallon batch of beer you need to plan ahead, not add plain water later.
During a 60 minute boil I typically lose about 1 quart due to evaporation. Expect to lose more to absorption if you’re doing a partial mash or all grain, and even more in trub in the fermenter. Typically I start with 6 gallons for my all grain batches, and 5.5 for extract brewing.
It’s Not Worth the Risk!
The recommendation of topping off the fermenter may have come from a mix up or carry-over from the process of wine making. During a brief web search I found one blog published by a brewer who actually compared the two almost equally and suggested topping off beer in the secondary for beer because he thought it made sense, since he does it for his wine.
Fermenting wine and beer are very different, though there are similarities. After several weeks of primary fermentation a wine is racked to a glass carboy, degassed, and fermentation is intentionally halted by adding sulfites. The wine is then topped off to minimize oxidation before being left to settle for 30 days or more. Sulfites also acts as a preservative and can help reduce oxidation* during long term aging in the carboy or in bottles.
Typically a commercial grade wine of the same variety is used to top off the carboy, not water. This is the recommended practice, although water may be used in some instances.=
The major difference is that the wine fermentation has been halted by the sulfites, and it sits in the carboy for about a month. Topping off is a good idea when it comes to wine production, and it will not adversely affect the wine because preventatives are present.
When racking a beer to the secondary, fermentation is not halted, no sulfites are added. The beer is typically bottled within 10 days of secondary, so reducing the air space in the carboy for beer production is not at all necessary, and is not a good idea. If you’re worried about oxidation during long term conditioning in the bottle you can use special oxygen absorbing bottle caps.
Personally I recommend never topping off beer in any fermenter, primary or secondary. The risk of contamination by pouring in water or adding glass marbles is greater than any potential oxidation that may occur in the first place. It’s just not worth it. Leave that beer alone!
* Reference: Wikipedia: Sulfite