Spunding and Pressurized Fermentation

What Is Spunding

Spunding is the practice of capturing naturally produced CO2 during the final stages of fermentation to carbonate the beer. A spunding valve — an adjustable pressure relief valve — is attached to a pressure-rated fermenter. It maintains a set pressure, allowing CO2 to build up in the headspace and dissolve into the beer.

How It Works

During active fermentation, yeast produces CO2 faster than it can dissolve. The spunding valve vents excess pressure, maintaining the fermenter at the target carbonation pressure. As fermentation slows and CO2 production decreases, the remaining sugar is consumed under pressure, carbonating the beer naturally.

Calculating Pressure

The target pressure depends on desired carbonation volumes and beer temperature. Use a carbonation chart or calculator:

• At 68 F, approximately 14 PSI produces 2.5 volumes CO2
• At 50 F, approximately 8 PSI produces 2.5 volumes CO2
• At 35 F, approximately 5 PSI produces 2.5 volumes CO2

When to Spund

Attach the spunding valve when fermentation is 2-5 gravity points from the expected FG. This ensures enough residual sugar remains to produce the desired carbonation. Attaching too early risks over-carbonation; too late risks under-carbonation.

Equipment

Pressure-rated fermenter — Corny kegs, Fermzilla, Spike Flex+, or commercial unitanks. Standard glass carboys and plastic buckets cannot handle pressure.

Spunding valve — an adjustable pressure relief valve with a pressure gauge. Commercial versions are available, or you can build one from a ball lock post, adjustable PRV, and gauge.

Pressurized Fermentation

Beyond just carbonation, fermenting under pressure (10-25 PSI from the start) suppresses ester and fusel alcohol production. This allows ale yeast to produce clean, lager-like beer at warmer temperatures — a technique called "pressure fermentation" or "fast lager."

At 15 PSI, an ale yeast fermenting at 68 F produces flavor comparable to the same strain at 58 F without pressure. This eliminates the need for a fermentation chamber for lager-style brewing.

Advantages

• No priming sugar needed (natural CO2)
• No separate carbonation step (beer is ready to serve after conditioning)
• Reduced oxygen exposure (closed system throughout)
• Cleaner fermentation at warmer temperatures (with pressure)

Limitations

• Requires pressure-rated equipment
• Gravity must be monitored carefully to time the valve attachment
• Over-carbonation is possible if FG is lower than expected
• Limited to CO2 volumes achievable from remaining fermentables

The Workflow

1. Brew and pitch yeast normally
2. Ferment open (airlock) until 2-5 points from target FG
3. Seal the fermenter and attach the spunding valve at target pressure
4. Allow fermentation to finish under pressure
5. Cold crash (still sealed) for clarity
6. Transfer to a serving keg via closed transfer
7. Serve immediately — the beer is already carbonated