FermZilla vs Corny Keg for Pressure Fermentation

The deciding factor

Headspace, and why 19 liters is not enough

Fermenting in a corny keg looks like the obvious move. You may already own two or three, they are stainless, they hold more pressure than any plastic fermenter sold, and the ball lock posts mean a closed transfer is trivial. The reason it still is not the default answer is a single number: a 5 gallon corny keg holds 19 liters, and a 5 gallon batch of beer is 19 liters.

Krausen needs somewhere to go. In an open fermentation the foam rises, peaks and falls back. In a keg filled to capacity it rises into the headspace that is not there, finds the gas dip tube, and pushes yeast and protein into your spunding valve or PRV. At best you spend a week clearing a blocked valve. At worst the valve stops venting and the vessel holds pressure it was not meant to hold.

The fix is to under-fill: 2.5 to 3 gallons into a 5 gallon keg leaves real headspace and works reliably. That is a legitimate way to brew, and for split batches or experimental beers it is a good one. It is just not a 5 gallon batch. The practical detail of doing it properly is in fermenting in a keg.

The purpose-built answer

30 liters exists for exactly this reason

The FermZilla All Rounder is 30 liters, and that is not a coincidence. Thirty liters holds a 19 liter batch with around 11 liters of headspace above it, which is enough for a vigorous krausen on a hefeweizen without anything reaching the lid. The same logic sizes the stainless KegMenter at 30 L rather than at keg volume.

The All Rounder is also transparent, which is a bigger deal than it sounds for pressure fermentation. Under pressure you cannot open the lid to look, and a sealed opaque vessel gives you nothing but a pressure gauge. Being able to see krausen form, peak and drop tells you where fermentation actually is without a sample draw.

Pressure headroom

Where the keg genuinely wins

A corny keg is rated for roughly 130 psi burst and around 55 psi working. The FermZilla pressure kit tops out near 35 psi. In normal pressure fermentation you work somewhere between 10 and 20 psi, so both are comfortable -- but the margin is different, and margin is what you want when a spunding valve sticks.

Stainless also does not care about scratches, UV, or a strong caustic cleaner. PET does. A FermZilla is a consumable on a long timescale: it will serve you for years and then it will be crazed and cloudy and you will replace it. A corny keg bought reconditioned in 2026 will still be a corny keg in 2046.

That durability argument is the one that makes the KegMenter interesting: stainless, keg-style, but sized at 30 L so the headspace problem never arises. You lose visibility and gain decades.

What neither does

No trub cone, either way

• Both have a flat or near-flat bottom. A corny keg is flat. The All Rounder is cylindrical with no cone. Neither lets you dump trub or harvest a clean slurry from underneath.
• A floating dip tube is the workaround. It draws from the top of the liquid rather than the bottom, so you can transfer clear beer off the yeast cake without a cone. It is the single best accessory for either vessel.
• If yeast harvesting is the goal, neither is the answer. You want a true cone with a dump valve. The conical roundup covers that properly, and the FermZilla Tri-Conical is the cheap way in.

Cost, honestly

The keg is only free if you already own it

The argument for keg fermentation is almost always that the keg costs nothing because it is already in the garage. That is true, and it is a good argument. But count the additions: a spunding valve, a floating dip tube, and ideally a second lid you are willing to drill. Those are real purchases, and once they are made the gap to a FermZilla body plus pressure kit is smaller than the headline comparison suggests.

There is also an opportunity cost that nobody mentions. A keg fermenting beer is a keg not serving beer. If you have two kegs and one kegerator, dedicating one to a three-week fermentation halves your draft capacity. Brewers who ferment in kegs long-term tend to end up buying more kegs, which is the expensive version of buying a fermenter.

How we compared these

The basis for this

Compared on published manufacturer figures for volume, material, pressure rating and fittings, plus the documented homebrew consensus on krausen headspace requirements for a 5 gallon batch. We have not instrumented krausen height across both vessels and do not claim to. See how we pick.