Oxygen and brine
Lactic acid bacteria work without air; mold and surface yeasts don't. Everything stays under the brine, with a weight, and a lid that lets gas out without letting air in.
7 min read First steps Reviewed October 27, 2026 37 facts, each with its source
Why air has no place here
The good bacteria in a vegetable ferment need an oxygen-free environment1. Air doesn't kill them: lactic acid bacteria grow with or without it2. The trouble is their rivals. Mold needs oxygen and grows right where the ferment meets the air3, and on a surface exposed to air, yeasts that spoil the ferment grow too4.
Salt alone won't stop them, because mold tolerates it better than most other microbes5. What stops them is taking away the air. That is why keeping vegetables submerged is a critical step, for safety and for quality6.
Brine is the barrier
Vegetables have to stay 1 to 2 inches (2.5 to 5 cm) under the brine7. The BCCDC guide asks for at least 3 cm of liquid on top8. That layer keeps the vegetables apart from the air.
Once fermentation gets going, the gas helps too. The carbon dioxide the bacteria release helps keep the jar low in oxygen9. But it doesn't protect what sticks out: vegetables that float up out of the brine can spoil10. In a BCCDC trial with pickles in a crock, molds, yeasts and a slimy layer grew wherever the pickles sat above the brine11.
How to keep everything under
The answer is a weight on top, such as a jar or a brine-filled bag12. The usual options:
| Weight | When it fits | Notes |
|---|---|---|
| Tempered glass13 or ceramic weight14 | Wide-mouth jars | Wash it and reuse it. If it's ceramic, make sure it's made for food, with no doubtful glaze |
| Cabbage leaf under the weight | Sauerkraut and shredded ferments | Stops small bits floating up15 |
| Plate with jars of water | Crocks and buckets | A plate slightly smaller than the opening that covers most of the vegetables16; on top, 2 or 3 sealed jars filled with water17 |
| Brine-filled bag | Any container | Seals the opening and works as a weight18 |
Fill the bag with brine, not water: if it leaks, what drips into the ferment is brine and it doesn't dilute it19. The NCHFP one uses 4½ tablespoons of salt in 3 quarts of water, the same ratio as the brine for topping up sauerkraut. Use food-grade plastic; never a garbage bag20.
Each weight needs its own care. With a brine bag, leave the crock alone until the bubbling stops21. With jars as weights, check it 2 to 3 times a week and skim off any film that forms22.
How full to fill the jar
Not too full, not too empty. Too much headspace leaves room for oxygen, and with it mold or a film on the surface23. Too little, and the ferment overflows, because it bubbles a lot24.
- Dry-salted: to the shoulder of the jar, leaving at least 2 inches (5 cm) free25.
- In brine: to the neck26.
Set the jar on a plate for the first few days in case it overflows.
How to close it
A lid keeps oxygen out, but the fermentation gas has to escape27. Fervida lets you choose among these closures; the towel only for dry salting:
| Closure | How it works | What it asks of you |
|---|---|---|
| Airlock lid | Lets gas out without letting air in27 | No burping needed |
| Regular lid | Keeps air out, but gas builds up | Loosen it now and then28, only briefly29 |
| Plate, weight and towel | A clean, heavy towel keeps insects and molds out30 | Check it 2 to 3 times a week |
Whichever you use, the lid goes on firmly, with no gaps, and the gas comes out from time to time, by opening it or through an airlock31. A jar closed tight and never loosened builds up gas: it can leak or even explode32.
Fermenting in glass jars is acceptable, though it loses more batches than a fermentation crock33. That is why, in a jar, the weight and the lid matter even more.
If something pokes out or the brine runs low
- If a piece pokes out, push it down with a clean spoon and put the weight back.
- If the brine runs low, add boiled and cooled brine34, ideally at the recipe's salt percentage.
- If a smooth white film forms, it is usually yeast: in sauerkraut it is removed from time to time35.
- If mold appears, fuzz or colored spots, the whole batch goes36.
The Mold or kahm yeast? tool helps you tell them apart.
In the fridge too
When the ferment moves to the fridge, the rule still holds. Keep it pressed down and under its liquid so it gets as little air as possible37. Every time you take some out, use a clean utensil and press the rest down again.
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
The good bacteria need an oxygen-free environment
“An anaerobic, low-oxygen environment is needed for the good bacteria to work.”
University of Minnesota Extension · Preserving food at home: Fermentation (reviewed 2026) · read on October 25, 2026
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2
Lactic acid bacteria grow with or without oxygen
“This means these bacteria produce acids, in particular lactic acid and can grow with and without oxygen and are salt tolerant.”
BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 1: Introduction to fermented food safety (2024) · read on October 25, 2026
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3
Mold needs oxygen and grows where the ferment meets the air
“Molds require oxygen to grow. 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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4
If air isn't kept out, yeasts and molds grow on the surface and spoil it
“When oxygen is not excluded, aerobic environments at the surface will allow growth of aerobic yeasts and moulds, causing spoilage.”
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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5
Mold tolerates salt better than most other microbes
“Molds also tolerate salt and sugar better than most other food invaders.”
USDA Food Safety and Inspection Service · Molds on Food: Are They Dangerous? · read on October 20, 2026
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6
Keeping vegetables submerged is critical for safety and quality
“This is a critical step to the safety and quality of the finished product.”
University of Illinois Extension · Fermentation handout (2020) · read on October 20, 2026
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7
Vegetables stay 1 to 2 inches under the brine
“Your fermenting product should be kept 1-2 inches below the brine.”
University of Illinois Extension · Fermentation handout (2020) · read on October 20, 2026
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8
At least 3 cm of brine above the vegetables
“There should be at least 3 cm or more of liquid brine between the fermented vegetables and the top of the brine.”
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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9
The carbon dioxide from fermentation helps keep the jar low in oxygen
“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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10
Vegetables that float and poke out can spoil
“Fermenting vegetables may float to the surface and poke up out of the brine where they may spoil.”
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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11
Where pickles sat above the brine, molds and yeasts grew
“At the sides of the crock, in the places where the pickles are above the surface of the brine, slimy spoilage agents, moulds and yeasts continue to grow.”
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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12
A weight on top, such as a jar or a brine-filled bag, keeps vegetables submerged
“Placing a weight (such as a jar or brine-filled bag) on top to keep them submerged is a good idea.”
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
Tempered glass weights
“Tempered glass weights”
University of Illinois Extension · Fermentation handout (2020) · read on October 20, 2026
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14
Ceramic weights
“Ceramic weights”
University of Illinois Extension · Fermentation handout (2020) · read on October 20, 2026
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15
A cabbage leaf stops floaters; a brine-filled bag works as the weight
“A cabbage leaf can help prevent floaters, a brine-filled bag can serve as the weight”
University of Illinois Extension · Fermentation handout (2020) · read on October 20, 2026
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16
The plate, slightly smaller than the opening and covering most of the vegetables
“The plate must be slightly smaller than the container opening, yet large enough to cover most of the shredded cabbage or cucumbers.”
National Center for Home Food Preservation (USDA, University of Georgia) · Suitable Containers, Covers, and Weights for Fermenting Food · read on October 26, 2026
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17
On the plate, 2 or 3 sealed jars filled with water
“To keep the plate under the brine, weight it down with 2 to 3 sealed quart jars filled with water.”
National Center for Home Food Preservation (USDA, University of Georgia) · Suitable Containers, Covers, and Weights for Fermenting Food · read on October 26, 2026
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18
A large bag of brine also works as a weight
“Fine quality fermented vegetables are also obtained when the plate is weighted down with a very large clean, plastic bag filled with 3 quarts of water containing 4-1/2 tablespoons of salt.”
National Center for Home Food Preservation (USDA, University of Georgia) · Suitable Containers, Covers, and Weights for Fermenting Food · read on October 26, 2026
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19
If the bag leaks, what drips out is brine
“If the bag leaks, the extra brine will be added to the produce.”
University of Minnesota Extension · Preserving food at home: Fermentation (reviewed 2026) · read on October 25, 2026
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20
Only food-grade plastic; never garbage bags
“Caution: Be certain that foods contact only food-grade plastics. Do not use garbage bags or trash liners.”
National Center for Home Food Preservation (USDA, University of Georgia) · Suitable Containers, Covers, and Weights for Fermenting Food · read on October 26, 2026
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21
With a brine bag, leave it alone until the bubbling stops
“If you weigh the cabbage down with a brine-filled bag, do not disturb the crock 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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22
With jars as weights, check 2 to 3 times a week and skim any film
“If you use jars as weight, you will have to check the kraut 2 to 3 times each week and remove scum if it forms.”
National Center for Home Food Preservation (USDA, University of Georgia) · Sauerkraut · read on October 20, 2026
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23
Too much headspace leaves room for oxygen, and mold or film forms
“Leaving too much headspace in a jar will allow extra space for oxygen to collect, causing mold or scum to form on the surface.”
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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24
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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25
Dry-salted, fill to the shoulder of the jar, with at least 2 inches free
“Fill only to the shoulder of the jar, leaving at least 2 inches of head space,”
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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26
In brine, fill to the neck
“Fill to the neck of the jar if using a brine.”
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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27
A lid keeps oxygen out; a valve lets gas escape
“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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28
Without a valve, release the gas by loosening the lid now and then
“If a one-way valve is not used, the gases will need to be released manually by loosening the lid occasionally.”
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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29
Open the lid only briefly, so little oxygen gets in
“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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30
A clean, heavy towel keeps insects and molds out
“Covering the container opening with a clean, heavy bath towel helps to prevent contamination from insects and molds while the vegetables are fermenting.”
National Center for Home Food Preservation (USDA, University of Georgia) · Suitable Containers, Covers, and Weights for Fermenting Food · read on October 26, 2026
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31
The lid on firmly, with no gaps, and the gas let out from time to time
“The lid should be applied firmly, with no air gaps. The CO2 should be released periodically by opening container or using an airlock.”
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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32
Built-up gas can make a container leak or explode
“Gas build-up may cause leakage from the container and has the potential to explode creating a physical hazard.”
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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33
Fermenting in jars is acceptable but loses more batches
“Fermenting sauerkraut in quart and half-gallon Mason jars is an acceptable practice, but may result in more spoilage losses.”
National Center for Home Food Preservation (USDA, University of Georgia) · Suitable Containers, Covers, and Weights for Fermenting Food · read on October 26, 2026
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34
If the juice doesn't cover the cabbage, boiled and cooled brine
“If juice does not cover cabbage, add boiled and cooled brine (1-1/2 tablespoons of salt per quart of water).”
National Center for Home Food Preservation (USDA, University of Georgia) · Sauerkraut · read on October 20, 2026
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35
In sauerkraut, yeast is removed from time to time
“For other ferments, such as sauerkraut, yeast should be periodically removed during fermentation.”
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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36
Mold anywhere means discarding the batch
“If you confirm mold growth on any part of a ferment, it should be immediately discarded.”
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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37
In the fridge, kimchi pressed down so it gets as little air as possible
“Keep it pressed down to minimize air exposure.”
Colorado State University Extension · Understanding and Making Kimchi (2025) · read on October 20, 2026