It smells rotten
The short answer Throw it out
Throw it out. A healthy ferment smells sour; if it smells rotten, the fermentation has failed. Throw out the whole batch without tasting it, and start again.
Reviewed October 26, 2026 · 18 facts, each with its source
AI
When in doubt, throw it out. Fervida can’t see your jar. No page can tell you something is safe: it can show you how to look.
What you see
A healthy ferment smells sour, like a pickle, even when the smell is strong. A rotten smell is something else: a smell of decay that makes you pull back and doesn't resemble the ferment at all. It often comes with other signs: soft or slimy vegetables, a thick brine, or mold on the surface.
If you're fermenting cabbage and it's the first week, check "It smells of sulfur" first: cabbage gives off a strong smell in those days that isn't the same thing.
Why it happens
The fermentation has failed. In a healthy ferment, lactic acid bacteria sour the vegetables before other microbes can grow. When something gets in the way, the microbes that rot food win. The usual causes:
- Too little salt, or a brine that is too weak.
- Too much heat: above 77 °F it can spoil.
- Vegetables poking above the brine.
What you can't smell matters most. When a ferment spoils, the pH can rise above 4.6, and then botulism spores can germinate. Their toxin can't be seen, smelled or tasted.
What to do
Throw it out
- Throw out the whole batch, including what looks unaffected. Don't taste it to see if it's fine.
- Wash the jar, the weight and the lid thoroughly before using them again.
- Start again with the recipe.
If you've already eaten some, see "What if I already ate some?".
How to prevent it
- The salt the recipe calls for, weighed. Never less.
- Every vegetable below the brine, held down with a weight.
- The ideal temperature, 68–72 °F. In summer, the coolest corner of the house.
- Vegetables as fresh as possible.
Where every figure comes from
Every statement on this page comes from a food safety guide. We keep a copy of each source and quote its exact words, in its original language.
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1
A vegetable ferment smells pleasantly sour, never putrid
“A fermented fruit or vegetable will smell pleasantly sour but not putrid.”
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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2
A rotten smell means a failed ferment
“A putrid smell indicates a failed 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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3
If it clearly smells spoiled, throw it out right away
“If a ferment smells noticeably spoiled, especially in combination with mold growth, it should immediately be 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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4
Everything below 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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5
In kimchi, an off smell unlike its own is a sign of spoilage
“Signs of spoilage include the presence of mold, an off-putting odor distinct from its characteristic fermented aroma, or an unusual texture.”
University of Georgia Cooperative Extension · Homemade Kimchi: A Science-Based Guide to Safe Fermentation (Circular 1362, 2025) · read on October 20, 2026
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6
If pickles turn soft, slimy or smell bad, throw them out
“Caution: If the pickles become soft, slimy, or develop a disagreeable odor, discard them.”
National Center for Home Food Preservation (USDA, University of Georgia) · Dill Pickles · read on October 26, 2026
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7
A brine that is too weak spoils pickles
“Salt brine too weak during fermentation.”
National Center for Home Food Preservation (USDA, University of Georgia) · Causes and Possible Solutions for Problems with Fermented Pickles · read on October 26, 2026
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8
Above 77 °F a ferment can spoil
“fermentation at temperatures higher than 25°C may result in 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
Show the remaining 10 claims
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9
When a ferment spoils and the pH rises above 4.6, botulism spores can germinate
“C. botulinum spores may germinate in fermented foods when pH rises above 4.6 during 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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10
Vegetables poking above the brine 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
Botulinum toxin can't be seen, smelled or tasted
“You cannot see, smell, or taste the toxin that causes botulism.”
CDC · Botulism prevention: home-canned foods · read on October 26, 2026
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12
Never taste food to find out if it's fine
“Never taste food to see if it's safe.”
CDC · Botulism prevention: home-canned foods · read on October 26, 2026
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13
If it is slimy or smells rotten, throw it out
“If fermented product is slimy, smells spoiled/rotten, discard it.”
University of Illinois Extension · Fermentation handout (2020) · read on October 20, 2026
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14
Clean the container thoroughly and start again
“Clean the container thoroughly and try again!”
University of Illinois Extension · Fermentation handout (2020) · read on October 20, 2026
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15
Salt is best weighed
“It is best to weigh salt for accuracy.”
University of Illinois Extension · Fermentation handout (2020) · read on October 20, 2026
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16
The ideal temperature, 68–72 °F
“Control room temperature– ideal=68-72°F”
University of Illinois Extension · Fermentation handout (2020) · read on October 20, 2026
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17
Vegetables as fresh as possible
“Use the freshest produce possible”
University of Illinois Extension · Fermentation handout (2020) · read on October 20, 2026
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18
Don't cut back on the salt the recipe calls for
“Do not attempt to make sauerkraut or fermented pickles by cutting back on the salt required.”
National Center for Home Food Preservation (USDA, University of Georgia) · General Information on Fermenting · read on October 26, 2026