Pressure Management
Part of Fermentation and Brewing
Fermentation produces carbon dioxide gas. In an open vessel, CO2 escapes harmlessly. In a sealed vessel, it builds pressure. Managed correctly, this pressure produces naturally carbonated drinks. Managed incorrectly, it shatters bottles, explodes crocks, and creates genuine safety hazards. Every fermenter must understand how to control the CO2 produced by their ferments.
How CO2 Is Produced in Fermentation
Yeast ferment sugar according to this simplified equation:
C6H12O6 → 2 C2H5OH + 2 CO2
One molecule of glucose yields two molecules of ethanol and two molecules of carbon dioxide gas. In practical terms: fermenting 1 kg of sugar produces approximately 0.49 kg of ethanol and 0.49 kg of CO2 — roughly 250 liters of CO2 gas at atmospheric pressure and room temperature.
A 20-liter batch of beer fermenting from a starting gravity of 1.050 to a final gravity of 1.010 ferments approximately 1.2 kg of sugar, producing approximately 300 liters of CO2. This gas must go somewhere.
Phases of CO2 Production
Understanding when CO2 is produced at what rate governs all pressure management decisions:
| Phase | CO2 Production Rate | Duration | Management |
|---|---|---|---|
| Lag phase | Near zero | 6–24 hours | No action needed |
| Active fermentation | High; continuous bubbling | 3–7 days | Open vent or active airlock |
| Secondary fermentation | Low; occasional bubbling | 7–14 days | Fitted airlock |
| Conditioning | Very low or zero | 2–8 weeks | Sealed vessel acceptable |
| Bottle conditioning | Low (carbonation building) | 1–4 weeks | Sealed bottles with headspace |
Airlocks: The Standard Solution
An airlock (also called a fermentation lock or water seal) allows CO2 to escape while preventing air and outside microorganisms from entering the vessel. It is the standard solution for vessels that cannot be left fully open.
How an Airlock Works
A simple airlock is a U-shaped tube partially filled with water. CO2 pressure from inside the vessel pushes through the water and escapes in bubbles, but outside air cannot push back against the internal pressure.
DIY Airlock Options
When commercial airlocks are unavailable:
Option 1: Tubing-in-water airlock.
- Insert a rubber tube or flexible reed through a hole in the lid.
- Seal the hole with clay, beeswax, or dough.
- Submerge the other end of the tube in a glass of water.
- CO2 bubbles through the water and escapes; water prevents airflow back in.
Option 2: Balloon or condom over vessel neck.
- Stretch a balloon or latex condom over the neck of a glass demijohn.
- Pierce with a pin to create one or two small holes.
- The balloon inflates during active fermentation (confirming active CO2 production) and deflates when fermentation slows.
- Small holes prevent explosive bursting; if pressure builds excessively, the balloon vents safely.
Option 3: Cloth cover (primary fermentation only).
- Secure a woven cloth over the fermentation vessel with a cord or rubber band.
- CO2 escapes through the weave; insects and dust are excluded.
- Only suitable for the 3–7 day active fermentation phase when CO2 production is high enough to prevent air entry. Not suitable for the conditioning phase.
Never seal a vessel airtight during active fermentation. Even with an airlock, verify it is functioning daily. A blocked or frozen airlock can build pressure rapidly enough to rupture glass vessels or launch stoppers across the room.
Vessel Selection for Pressure
Different vessel types have different pressure tolerances and appropriate uses:
| Vessel | Pressure Tolerance | Suitable For |
|---|---|---|
| Open crock or bucket | None | Active fermentation only; not for conditioning |
| Glass demijohn with airlock | Very low (airlock limits pressure) | All fermentation phases with functioning airlock |
| Ceramic crock with lid | Very low unless sealed with water trap | Traditional; best with water-seal groove |
| Thin glass bottle (recycled) | Low — 1–2 bar | Finished, still beer only; dangerous for carbonated |
| Champagne bottle (thick glass) | High — 5–6 bar | Safe for naturally carbonated cider and beer |
| Wooden barrel with bung | Moderate — depends on condition | Traditional conditioning vessel |
| PET plastic bottle (food grade) | Moderate — squeeze test | Useful field indicator of carbonation level |
Never bottle-condition in thin glass bottles (e.g., regular wine bottles). Wine bottles are designed for still wine and have negligible pressure tolerance. If fermentation re-activates in a sealed thin-glass bottle, the glass will shatter without warning, launching shards at high velocity. Only use thick-walled champagne bottles or ceramic swing-top bottles for any carbonated product.
Natural Carbonation: Bottle Conditioning
Naturally carbonated beer and cider are produced by sealing a small quantity of residual or added sugar into a sealed bottle with viable yeast. The yeast ferments this sugar, producing CO2 that dissolves into the liquid under the pressure of the sealed vessel.
Calculating Priming Sugar
Standard carbonation for beer and cider: 2.0–2.5 volumes of CO2 per liter of liquid.
At room temperature (20°C), fermented beer already contains approximately 0.85 volumes of dissolved CO2. To achieve a target of 2.5 volumes, you need to generate an additional 1.65 volumes through bottle conditioning.
Sugar required:
- 1 volume of CO2 per liter requires approximately 2 g of sugar
- For 1.65 additional volumes: 1.65 × 2 = 3.3 g sugar per liter
- For a 20-liter batch: 3.3 × 20 = 66 g of priming sugar total
| Target Carbonation | Sugar Required (per liter) |
|---|---|
| Low (still with slight fizz, 1.5 vol) | 1.3 g |
| Standard (2.0 vol) | 2.5 g |
| Lively (2.5 vol) | 3.5 g |
| High (cider/wheat beer, 3.0 vol) | 4.5 g |
Over-priming is the most common cause of exploding bottles. Err on the side of less carbonation rather than more. If uncertain whether fermentation is complete, wait: a beer racked to secondary and showing no airlock activity for 5+ days at stable room temperature is almost certainly done. Bottling an actively fermenting beer with added priming sugar is a reliable formula for explosive results.
The Squeeze Test
For plastic PET bottles, the squeeze test provides a non-destructive pressure indicator:
- Fill and seal a plastic PET bottle alongside your glass bottles.
- When the plastic bottle feels hard and barely compressible under firm thumb pressure, carbonation is at approximately 2.0–2.5 volumes.
- Refrigerate all bottles at this point to halt further fermentation.
This technique works because the plastic bottle’s rigidity is a direct function of internal pressure. Glass bottles provide no such feedback — you must rely on calculations and timing.
Burping Containers
Some fermentation vessels — particularly crocks, buckets, and loosely sealed jars — are designed to be periodically vented manually rather than through a continuous airlock. This is acceptable during active fermentation but requires attention:
How to burp a sealed jar: Gently loosen the lid enough for gas to escape (you will hear or see it), then reseal. Do this once or twice daily during active fermentation (days 2–7 typically). As fermentation slows, reduce frequency.
Wide-mouth jar lids can be modified into one-way valves by cutting a small X in the rubber seal and placing a small weight on top. CO2 pressure lifts the lid edge enough to escape; atmospheric pressure pushes it back down. This creates a functional self-burping lid without commercial equipment.
CO2 in Enclosed Spaces: Ventilation Safety
During vigorous fermentation, CO2 can build to dangerous levels in small, unventilated spaces. CO2 is heavier than air and accumulates near the floor.
| CO2 Concentration | Effect |
|---|---|
| 0.04% (normal air) | No effect |
| 0.5% | Mild headache after prolonged exposure |
| 1% | Drowsiness; increased breathing rate |
| 3% | Headache, dizziness, respiratory difficulty |
| 5%+ | Dangerous; rapid unconsciousness possible |
Active fermentation in a closed cellar with poor ventilation can reach 1–3% CO2 near the floor within hours. Always ventilate fermentation rooms. Never enter a sealed fermentation cellar without first opening doors and windows and waiting several minutes for air circulation.
Signs of a Pressure Problem
| Warning Sign | Likely Issue | Action |
|---|---|---|
| Stopper launching from vessel | Fermentation more active than expected; airlock blocked | Replace with open-top cover or vent immediately |
| Vessel walls bulging (plastic) | Active fermentation in sealed vessel | Vent immediately; do not open rapidly |
| Hissing or crackling from glass vessel | Pressure building; potential failure | Move away from people; vent carefully from a distance |
| Bottles seeping around caps | Over-carbonation | Refrigerate immediately to halt fermentation; open carefully over a sink |
| No bubbling after 48 hours | Fermentation stalled or airlock blocked | Check airlock is filled; check temperature; warm slightly |
Pressure Management Summary
Carbon dioxide produced during fermentation must be actively managed to prevent vessel failure and to achieve target carbonation in finished products. During active fermentation, an airlock or cloth cover allows CO2 to escape freely. During conditioning, a functioning sealed airlock maintains an oxygen-free environment while allowing slow CO2 escape. Bottle conditioning requires precisely calculated priming sugar (approximately 3.3 g/liter for standard carbonation) and thick-walled bottles capable of sustaining 5–6 bar of pressure. Never seal vessels airtight during active fermentation; never use thin-glass wine bottles for carbonated products; always ventilate fermentation rooms to prevent CO2 accumulation. The squeeze test on a sacrificial plastic bottle provides real-time carbonation level feedback without destructive testing.