Reading your jar: color, smell and bubbles
Your jar tells you a lot about how a ferment is going, but not everything. Learn to read what you can see, and remember what you can't: that's what pH is for.
6 min read First steps Reviewed October 27, 2026 32 facts, each with its source
What the jar tells you and what it doesn't
Looking at, smelling and listening to a ferment tells you how it's going: whether it has started, whether it has stalled, whether something is spoiling it. But there's one thing the jar will never tell you. Botulinum toxin doesn't change the color, smell, taste or texture of food1.
So this guide is about quality and spoilage. For safety, pH is in charge: check it now and then to make sure it doesn't rise above 4.62, and Fervida recommends waiting for 4.2 before the fridge, as a house rule.
Bubbles and gas
Bubbles are a good sign. Lactic acid bacteria turn sugars into lactic acid and release gas, which forms bubbles in the brine3. In kimchi they usually show up on the first or second day4; in sauerkraut they keep going for weeks, and fermentation is complete when they stop5.
In the first days, the vegetables can rise and ooze out of the jar6. Leave headspace at the top and set the jar on a plate.
One caveat: gas is a good sign only if the pH stays below 4.67. Bubbles say something is fermenting, not what. And if they stop, it doesn't always mean it's done: if the pH is still above 4.6, let it keep fermenting and test it every 1 to 2 days8.
Smell
A vegetable ferment on the right track smells pleasantly sour, never putrid9. Sauerkraut also has its touch of sulfur: its flavor is described by salty, sour and sulfur notes10. That strong cabbage smell isn't a fault.
A putrid smell is: it means a failed ferment11. If it smells clearly spoiled, and even more so with mold, discard it right away12. The University of Georgia's rule for kimchi works for every ferment: mold, an odor unlike the ferment's own aroma or an unusual texture are signs of spoilage13.
If it smells yeasty, look at the surface: it may be kahm, and if it spreads it causes off flavors. How to handle it is below.
Color and cloudiness
Color changes as a ferment cures, and so do flavor and acidity14. Most of these changes are normal:
- Red cabbage is a pH indicator. It shifts from purple to magenta to pink as it sours. If part of it turns purple instead of red, the pH has risen there15, almost always in vegetables left above the brine.
- Garlic can turn blue or purple. It carries pigments that change color in an acidic brine16.
- Dark pickles. They can come from hard water or ground spices; but if brass, copper or zinc utensils or containers were used, don't eat them17.
- Cloudy brine. With pickles, the brine goes cloudy in the first week and clears by the fourth18. That's fermentation.
- White sediment at the bottom. The fermenting bacteria make it, and there's nothing to fix19.
What shows up on top: kahm or mold
Kahm. A white, gray or pink film on the surface is usually yeast, what home fermenters call kahm20. In sauerkraut it's removed now and then21. Don't let it grow: too much yeast causes off flavors and textures and can lower acidity22. So after skimming it, test the pH to make sure it hasn't risen above 4.62.
Mold. Mold needs air and grows on the surface, where the ferment meets the air23. It's usually green, blue, brown or black24, but it can also be white, like the mold that grows on sauerkraut left outside the brine25. What gives it away isn't the color but the texture: it looks furry or velvety26, not like a flat film. If there's mold anywhere, the batch is discarded27. It can't be scooped out: its roots stay in the food28.
If you're unsure which one it is, the Mold or kahm yeast? tool walks you through it. And the best prevention for both is the same: everything under the brine and the air kept out, with the closure the recipe calls for.
Texture
Some softening is normal. By day 5 to 7, for example, kimchi is tangier, more aromatic and softer29.
What isn't normal is vegetables that are soft and slippery: if there are also signs of spoilage, don't eat them30. If pickles go fully soft, turn slimy or smell bad, discard them31. The usual causes are a brine that's too weak and too much heat32.
At a glance
| What you see | Usually | What to do |
|---|---|---|
| Bubbles | Fermentation | Nothing; test the pH at the end |
| Cloudy brine | Fermentation | Nothing |
| White sediment | Fermenting bacteria | Nothing |
| Blue or purple garlic | Garlic pigments | Nothing |
| Flat white film | Kahm yeast | Skim it off and test the pH |
| Purple red cabbage on top | pH rising | Check for mold; push it under and test |
| Fuzz of any color, white included, or spots | Mold | Discard the batch |
| Rotten smell | Failed ferment | Discard the batch |
| Soft and slimy | Spoilage | Discard the batch |
Nothing in this table replaces pH. A jar that looks perfect gets tested all the same.
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
Botulinum toxin doesn't change color, smell, taste or texture
“La presencia de toxinas botulínicas en las conservas no produce cambios organolépticos, por lo que no podemos detectar si un alimento está contaminado por cambios en su color, olor, sabor o textura.”
AESAN · Conservas caseras · read on October 26, 2026
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2
Check the pH now and then to make sure it doesn't rise above 4.6
“The pH can be checked periodically to ensure it does not rise above 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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3
Lactic acid bacteria release gas that makes bubbles in the brine
“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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4
In kimchi, bubbles usually show on the first or second day
“During the early stages of kimchi fermentation, typically within the first 1 to 2 days at room temperature, visible bubbling may occur as the lactic acid bacteria metabolize sugars, which produces carbon dioxide.”
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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5
Fermentation is complete when 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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6
Fermenting vegetables bubble and can ooze out of the jar
“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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7
Gas is a good sign only if the pH stays below 4.6
“Some fermentation microbes make carbon dioxide gas during the fermentation process. 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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8
No bubbles but still above 4.6: keep going and test every 1 to 2 days
“If bubbling has stopped but the pH is still above 4.60, continue to ferment and check the pH every 1 to 2 days.”
University of Minnesota Extension · Preserving food at home: Fermentation (reviewed 2026) · read on October 25, 2026
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9
It smells pleasantly sour, not 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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10
Sauerkraut's flavor has salty, sour and sulfur notes
“mostly by salty, sour, and sulfur notes (Trail and others 1996)”
USDA Agricultural Research Service (Johanningsmeier et al., Journal of Food Science) · Chemical and sensory properties of sauerkraut produced with Leuconostoc mesenteroides starter cultures (2005) · read on October 26, 2026
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11
A putrid 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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12
If it smells clearly spoiled, especially with mold, discard it
“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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13
Signs of spoilage: mold, an odor unlike a ferment's, or an unusual texture
“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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14
As it cures, color and flavor change and acidity rises
“During curing, colors and flavors change and acidity increases.”
National Center for Home Food Preservation (USDA, University of Georgia) · General Information on Fermenting · read on October 26, 2026
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15
If red cabbage turns purple instead of red, the pH has risen
“causing it to change color to purple instead of red due to an increase in pH”
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
Garlic can turn blue or purple in an acidic brine
“Garlic contains anthocyanins, watersoluble pigments that can turn blue or purple in acidic environments like vinegar or pickling brine”
University of California Agriculture and Natural Resources (Harris, UC Davis) · Garlic: Safe Methods to Store, Preserve, and Enjoy (ANR Publication 8568, October 2016) · read on October 25, 2026
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17
Dark pickles: hard water or ground spices; if brass, copper or zinc was used, don't eat them
“Dark or discolored pickles. (If brass, copper or zinc utensils and brining equipment were used, do not use pickles.) 1. Minerals in hard water. 1. Use soft water. 2. Ground spices used.”
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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18
Brine goes cloudy in week 1 and clears by week 4
“The brine becomes cloudy as the fermentation proceeds (week 1), then clears (week 4).”
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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19
White sediment is made by the fermenting bacteria
“White sediment in jar. 1. Bacteria cause this during fermentation. 1. None.”
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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20
A white, gray or pink film is usually kahm yeast
“White, grey, or pink films may form on the surface of fermenting fruits or vegetables. This film is typically yeast, often referred to by home fermenters as Kahm yeast.”
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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21
In sauerkraut, kahm is removed now and then
“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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22
Too much yeast causes off flavors and can lower acidity
“Excessive yeast growth can lead to off flavors or textures and reduce acidity.”
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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23
Mold needs air and grows on the surface
“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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24
Mold is usually green, blue, brown or black
“Mold is typically green, blue, brown, or black in color.”
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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25
Mold can also be white, like the mold on sauerkraut left outside the brine
“Note: White mold growth on sauerkraut stored outside brine”
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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26
Mold looks furry or velvety
“coin-size velvety circles on fruits, and furry growth on the surface of jellies”
USDA Food Safety and Inspection Service · Molds on Food: Are They Dangerous? · read on October 20, 2026
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27
Mold anywhere means it's discarded
“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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28
Mold can't be scooped out: its roots stay in the food
“The mold cannot be scooped out, as the roots will still be present in your food and could make you sick.”
Ask Extension (Clackamas County Extension, Oregon State University) · Pickle/olive brine surface mold spots #889210 · read on October 20, 2026
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29
By day 5 to 7, kimchi is tangier, more aromatic and softer
“By days 5 to 7, the kimchi develops a tangier flavor, a more pronounced aroma, and a softer 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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30
Soft or slippery pickles: if spoilage is evident, don't eat them
“Soft or slippery pickles. (If spoilage is evident, do not eat.)”
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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31
If they go soft, slimy or smell bad, discard them
“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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32
Causes: brine too weak or too much heat
“1. Salt brine too weak during fermentation. 1. Maintain salt concentration specified in recipe. 2. Cucumbers stored at too high a temperature 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