Airlocks
An airlock lets gas out of a ferment without letting air in, and saves you opening the jar. How it works, which kinds there are and what to do if you don't have one.
4 min read First steps Reviewed October 26, 2026 19 facts, each with its source
What an airlock does
As vegetables ferment, bacteria turn their sugars into lactic acid and give off carbon dioxide: those are the bubbles you see in the brine1. That gas has to get out. An airlock is a one-way exit: the lid keeps air out and the valve lets the gas escape2.
It does two jobs. It stops pressure building up in the jar, because a closed container that can't vent its gas can burst3. And it keeps the carbon dioxide inside, which leaves the jar low in oxygen, the way fermenting bacteria like it4. That's why a jar with an airlock lid is one of the ways to keep mold away5.
When it's worth it
The University of Minnesota considers it necessary with mason jars6; other guides accept a regular lid that you loosen, as you'll see below. Quart jars with airlock lids are great for small batches7. If you ferment often, it's the easiest option: there's nothing to remember to open. They're sold online and wherever canning supplies are sold8.
Fervida's recipes always offer it as the first option, but none of them require it: they also give you a regular lid and, for sauerkraut, a cloth.
Kinds of airlock
| Kind | How it works | Keep in mind |
|---|---|---|
| Water airlock | A small chamber of water the gas bubbles through | You can see it bubble. Keep the water at its line |
| Silicone valve | A membrane that opens under pressure and closes by itself | No water. Keep brine off it |
| Lid with a built-in valve | The jar lid comes with the valve fitted | The easiest. Check the gasket |
Whichever you use, set it up the way the manufacturer says9.
A water airlock also lets you see how the ferment is going: when it stops giving off bubbles, fermentation is over10. Gas is a good sign, but only if the pH stays below 4.611, so it doesn't replace measuring it.
No airlock: loosen the lid
If you don't have an airlock, close the jar with a regular lid and loosen it now and then to let the gas out. That's called "burping" the jar, because sometimes you can hear it12. Open it only briefly, just long enough for the gas to escape, and close it again: each time it's open, some air gets in13.
UC Davis puts it simply: use a vessel with an airlock or a breathable top if you have one; if not, open the lid now and then14.
What it doesn't do
- It doesn't replace the weight. Mold grows where the ferment meets the air15, and an airlock won't stop a piece from floating up and poking out. Everything has to stay under the brine16.
- It doesn't stop overflows. Ferments bubble a lot and can spill out of the jar17. Fill it only as far as the recipe says: kimchi, for example, to two-thirds18.
Care
Wash the airlock and the lid like the rest of your equipment, in hot soapy water, and rinse them well19. If the ferment overflows and wets the airlock, take it off and wash it before putting it back.
Where every number comes from
Every number in the text leads to its figure: the exact sentence from the official guide it comes from, in its own language.
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1
Bacteria turn sugars into acid and give off carbon dioxide
“Bacteria, such as Lactobacillus spp., convert sugars into lactic acid and release carbon dioxide, which creates bubbles in the liquid or brine.”
University of Minnesota Extension · Preserving food at home: Fermentation (reviewed 2026) · read on October 25, 2026
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2
The lid keeps air out; the valve lets gas out
“A lid prevents oxygen from entering the jar. However, it is important to vent the jar with a one-way valve to release the gases that form during fermentation.”
K-State Research and Extension (Kansas State University) · Taking a New Look at Fermented Foods (MF3491, June 2020) · read on October 20, 2026
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3
The container must let gas out so it doesn't burst
“During the fermentation, containers should allow gas to vent to avoid containers exploding.”
BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.1: Fermented Vegetables (August 2024, updated December 2024) · read on October 25, 2026
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4
Carbon dioxide keeps the jar low in oxygen
“Additionally, the carbon dioxide produced during the fermentation process helps maintain the low-oxygen conditions necessary for the fermenting bacteria to flourish.”
University of Illinois Extension · Fermentation handout (2020) · read on October 20, 2026
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5
A jar with an airlock lid helps keep mold away
“Use a jar with an airlock lid”
University of Illinois Extension · Fermentation handout (2020) · read on October 20, 2026
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6
With mason jars, you need an airlock
“If you choose to use mason-style jars, you will need an airlock system to control the oxygen and carbon dioxide levels inside the container.”
University of Minnesota Extension · Preserving food at home: Fermentation (reviewed 2026) · read on October 25, 2026
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7
Quart jars with airlock lids are great for small batches
“Quart-size glass jars with airlock lids are great for small batches”
University of Illinois Extension · Fermentation handout (2020) · read on October 20, 2026
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8
Sold online and wherever canning supplies are sold
“Several different one-way valves are available on the internet or where canning supplies are sold.”
K-State Research and Extension (Kansas State University) · Taking a New Look at Fermented Foods (MF3491, June 2020) · read on October 20, 2026
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9
Follow the manufacturer's instructions
“If you are using an airlock system, follow the manufacturer instructions and place on the container.”
University of Minnesota Extension · Preserving food at home: Fermentation (reviewed 2026) · read on October 25, 2026
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10
It's fermented when the bubbling stops
“until normal fermentation is completed (when bubbling ceases)”
National Center for Home Food Preservation (USDA, University of Georgia) · Sauerkraut · read on October 20, 2026
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11
Gas is a good sign, but only if the pH stays below 4.6
“This can be a good sign, but only if the pH stays below pH 4.6”
UC Davis, Department of Food Science and Technology · Common issues with fermented fruits and vegetables (v1.0, 15-mar-2022) · read on October 20, 2026
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12
Without a valve, loosen the lid now and then to let gas out
“If a one-way valve is not used, the gases will need to be released manually by loosening the lid occasionally. This process is called “burping” because you may hear a little burp when the pressure is released.”
K-State Research and Extension (Kansas State University) · Taking a New Look at Fermented Foods (MF3491, June 2020) · read on October 20, 2026
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13
Open the lid only briefly
“To burp a container, briefly remove or crack the lid to allow built up carbon dioxide to escape. The lid should only be opened briefly to limit oxygen from entering the container.”
University of Minnesota Extension · Preserving food at home: Fermentation (reviewed 2026) · read on October 25, 2026
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14
Use a vessel with an airlock or breathable top if you have one
“Use a fermentation vessel with a breathable top or airlock if available.”
UC Davis, Department of Food Science and Technology · Common issues with fermented fruits and vegetables (v1.0, 15-mar-2022) · read on October 20, 2026
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15
Mold grows where the ferment meets the air
“Molds can grow on the surface of the ferment at the air interface.”
UC Davis, Department of Food Science and Technology · Common issues with fermented fruits and vegetables (v1.0, 15-mar-2022) · read on October 20, 2026
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16
Everything under the brine
“All produce should be submerged in the brine liquid.”
UC Davis, Department of Food Science and Technology · Common issues with fermented fruits and vegetables (v1.0, 15-mar-2022) · read on October 20, 2026
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17
Ferments bubble and can overflow
“Fermented vegetables are quite active and bubbly; they can ooze out of the jar if you are not careful.”
K-State Research and Extension (Kansas State University) · Taking a New Look at Fermented Foods (MF3491, June 2020) · read on October 20, 2026
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18
Kimchi, to two-thirds: it gives off gas and bubbles
“Fill container about 2/3 of the way full, as fermenting microorganisms will release carbon dioxide (CO2) and create bubbling and fizzing.”
Colorado State University Extension · Understanding and Making Kimchi (2025) · read on October 20, 2026
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19
Wash in hot soapy water and rinse with very hot water
“The fermentation container, plate, and jars must be washed in hot sudsy water, and rinsed well with very hot water before use.”
National Center for Home Food Preservation (USDA, University of Georgia) · Suitable Containers, Covers, and Weights for Fermenting Food · read on October 26, 2026