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Green, blue or black spots

The short answer Throw it out

Throw it out: green, blue, brown or black spots on a ferment are mold. Throw out the whole batch, including what's underneath, and wash the jar well.

Reviewed October 26, 2026 · 18 facts, each with its source

An open jar of kimchi with round, fuzzy spots of mold on its surface: blue-green with white rims, one black and one olive green 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

Dots, patches or circles of color on the ferment: green, blue, brown or black. They are usually raised, with a fuzzy or velvety look. They can sit on the surface of the liquid, at the edge of the jar or on a piece of vegetable poking above the brine.

Why it happens

It's mold. Its spores give it the color. Because it needs air, it grows wherever the ferment meets the air: on the surface, at the edge or on anything left out of the brine, especially pieces that float up. It can show up at any point in the fermentation, and when it does, the fermentation has failed.

What you see is only part of it. Its roots are very fine threads that reach into the food and can go deep. And a few molds, under the right conditions, produce poisonous substances.

What to do

Throw it all out

  1. Don't sniff it up close: mold can cause breathing problems.
  2. Throw out the whole batch, including the vegetables and the liquid underneath. Scooping the spots off doesn't work: the roots are still inside.
  3. Wash the jar, the weight and the lid in hot soapy water, and rinse them with very hot water before using them again.

If you're not sure it's mold, treat it as if it were. When in doubt, throw it out.

How to prevent it

  • Every vegetable below the brine, held down with a weight, so no piece floats up.
  • The jar filled just right, without a big gap of air at the top.
  • The lid on properly, with no gaps, with an airlock or opened briefly now and then to let the gas out.
  • A temperature of 68–72 °F (20–22 °C).
  • Everything clean before you start: the jar, the weight and the lid.

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.

  1. 1

    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

  2. 2

    Mold needs air and grows where the ferment meets it

    “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

  3. 3

    Mold can appear at any time and means the ferment has failed

    “Mold growth can occur anytime during the fermentation process and is a sign of 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

  4. 4

    If there is mold anywhere, the whole batch goes

    “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

  5. 5

    Every vegetable 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

  6. 6

    A lid with no gaps, with the gas let out by an airlock or by opening it

    “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

  7. 7

    It grows as fuzz, dots or velvety circles

    “gray fur on forgotten bologna, fuzzy green dots on bread, white dust on Cheddar, coin-size velvety circles on fruits”

    USDA Food Safety and Inspection Service · Molds on Food: Are They Dangerous? · read on October 20, 2026

  8. 8

    Mold gets its color from its spores

    “The spores give mold the color you see.”

    USDA Food Safety and Inspection Service · Molds on Food: Are They Dangerous? · read on October 20, 2026

Show the remaining 10 claims
  1. 9

    Its roots are fine threads that can reach deep into the food

    “Molds have branches and roots that are like very thin threads. The roots may be difficult to see when the mold is growing on food and may be very deep in the food.”

    USDA Food Safety and Inspection Service · Molds on Food: Are They Dangerous? · read on October 20, 2026

  2. 10

    Some molds produce poisonous substances

    “And a few molds, in the right conditions, produce "mycotoxins," poisonous substances that can make people sick.”

    USDA Food Safety and Inspection Service · Molds on Food: Are They Dangerous? · read on October 20, 2026

  3. 11

    Don't sniff mold: it can cause breathing problems

    “Don't sniff the moldy item. This can cause respiratory trouble.”

    USDA Food Safety and Inspection Service · Molds on Food: Are They Dangerous? · read on October 20, 2026

  4. 12

    With green, blue or black mold, the ferment is discarded

    “if your fermented or pickled products develop green, blue, or black mold, they must be discarded.”

    Ask Extension (Clackamas County Extension, Oregon State University) · Pickle/olive brine surface mold spots #889210 · read on October 20, 2026

  5. 13

    Mold can't be scooped out: its roots stay in the food

    “The mold cannot be scooped out, as the roots will still be 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

  6. 14

    Wash the jar, weight and lid in hot soapy 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

  7. 15

    When in doubt, throw it out

    “When in Doubt—Throw it Out”

    University of Illinois Extension · Fermentation handout (2020) · read on October 20, 2026

  8. 16

    Vegetables that float up out of the brine 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

  9. 17

    A half-empty jar leaves room for air, and with it for mold

    “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

  10. 18

    The ideal temperature is 68–72 °F

    “A temperature between 68 and 72 degrees Fahrenheit is ideal.”

    University of Minnesota Extension · Preserving food at home: Fermentation (reviewed 2026) · read on October 25, 2026