Headspace...How Bad Is It Really?

A very common fear indoctrinated to new brewers is a fear of headspace. The way people talk about headspace is like a slight bump of a fermenter with a whiff of headspace will instantaneously turn any beer into vinegar. Acetobacter, the critter responsible for combining alcohol and oxygen to produce acetic acid (vinegar), is not that powerful. Yes, beers that have been aging for a long time and were inoculated with bottle dregs with acetobacter in it or became infected somewhere along the way, if agitated enough, can get very vinegary in a short period of time. That isn't because there was some oxygen contact. There had to be existing acetobacter slowly growing and using the oxygen to make acetic acid. Acetobacter can only make acetic acid when there is both oxygen and alcohol. Once either runs out acid production stops. In beer that hasn't been aged for very long (and we can even say months), unless inoculated or infected, shouldn't have enough acetobacter present to create vinegar overnight. Headspace only means there's a place for oxygen to hang out over the beer and slowly diffuse into the beer. Whether you have 10 cubic feet or 10 cubic inches of headspace only matters when you look at the surface area of the beer. Oxygen diffusion is much slower where the beer contacts glass or plasic, whatever your fermenter is made from.

Most commonly this fear is impressed on brewers when talking about brewing a beer in a fermenter much larger than the volume of beer going in. This seems mostly myth/misinformation than possessing any real basis. My experience certainly suggests the headspace issue is overblown. Look, most of us began fermenting five gallons in eight gallon buckets. That's three gallons worth of headspace plus the oxygen permeability of the HDPE. How many people find their buckets full of vinegar?

I've fermented lots of undersized beers in a 7.9 gallon bucket. I've fermented three gallon batches and left them for up to three months without a hint of vinegar. I've brewed one gallon batches in that bucket. I have one going right now. It's actually a second beer brewed on the cake of a prior one gallon batch. I intend on brewing two more one gallon batches on it before I clean it out. It's been in use, full of beer, since the beginning of March. It will probably get emptied and refilled in a few days and then emptied and refilled in early June, so it will go a full four months. I don't expect to taste any vinegar at all. My lambic solera sat for a year -- at one point I had to take the cap off and move it -- then was partially drained and refilled. No vinegar.

I even have some diluted wine I'm trying to turn into vinegar that's been sitting exposed for about a month. It's not even vinegary. Now some fruit flies did take a swim in it (and died) this week and it is starting to smell a little vinegary. So clearly oxygen exposure itself is not enough.


Make no mistake, I'm not suggesting beer never turns to vinegar, but if your beer is turning to vinegar, you probably have an infection issue/sanitation issue that could be addressed.