Fermenting in a Keg

The short answer

Yes, and the keg is already rated for it

A 5 gallon ball-lock corny keg is a pressure vessel with a typical burst rating far above anything fermentation will generate and a working pressure well clear of 15 psi. Fitting a spunding valve to the gas post gives you a genuine pressure fermenter for the cost of a valve, assuming you already own a spare keg.

That makes keg fermentation the cheapest honest route into pressure fermentation for a brewer who already kegs -- and it is also the approach with the most significant practical compromise.

The headspace problem

A 5 gallon keg ferments 4 gallons at best

This is the part that catches people. Krausen needs 20 to 25 percent of the vessel volume above the beer. In a 5 gallon keg that means a maximum of about 4 gallons of wort, and 3.5 is safer for a vigorous fermentation or a high-gravity beer.

So “ferment in a keg” actually means “brew a smaller batch.” That is fine if you plan for it -- and it is a genuine surprise if you pitch 5 gallons and come back to krausen blocking the gas post.

Two workarounds brewers use:

• Brew 4 gallon batches deliberately. Clean, simple, and you get a slightly-under-full keg of finished beer.
• Use a bigger keg. A 10 gallon corny ferments a comfortable 7 to 8 gallons. Harder to find and harder to fit in a chamber.

The dip tube problem

Fit a floating dip tube

A stock corny keg’s liquid dip tube reaches almost to the bottom of the keg -- which is where all the yeast and trub will be sitting. Draw through it and your first pours, or your whole transfer, come out cloudy.

The fix is a floating dip tube: a short tube with a float on the end, so the pickup sits just below the beer surface and draws the clearest beer in the vessel regardless of level. It is an inexpensive part and it is close to mandatory for keg fermentation.

How to do it

The practical sequence

• Clean and sanitize thoroughly, including the posts, the dip tubes and the lid gasket. A keg has more fittings than a bucket.
• Fill to no more than 4 gallons in a 5 gallon keg.
• Pitch, seal, and leave the spunding valve open for the first 12 to 24 hours so the yeast gets an unrestricted start.
• Set the spunding valve to 10-15 psi on the gas post once fermentation is active.
• Hold your temperature. A keg fits any chamber a keg fits, which is the form factor’s real advantage.
• Cold crash sealed, then transfer liquid post to liquid post into a purged serving keg -- see how to pressure ferment.

Should you?

When the dedicated vessel is the better buy

Ferment in a keg if you already own spare kegs, want to try pressure fermentation cheaply, and are happy brewing 4 gallon batches. It is a legitimate technique, not a bodge.

Buy a dedicated pressure fermenter instead if you want full 5 gallon batches, if you would benefit from seeing the fermentation, or if you want a cone for yeast harvesting. A FermZilla All Rounder with its kit is frequently cheaper than buying a keg plus a floating dip tube plus a spunding valve, and it does not cost you a third of your batch volume -- see best pressure fermenters and FermZilla vs KegMenter.

Either way you will want the keg hardware sorted: corny keg sizes and parts covers posts, dip tubes, lids and dimensions.