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A flat white film

The short answer It can be fixed

It can be fixed: a flat white or gray film is usually kahm yeast. Skim it off, check that everything is still below the brine and keep an eye on it. If it's fuzzy or has colored spots, it's mold, and everything goes.

Reviewed October 26, 2026 · 19 facts, each with its source

An open jar of sauerkraut with a flat, matte white film of kahm yeast on the brine, softly wrinkled and broken into a few loose patches at one side 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 thin, flat layer covering the surface of the liquid, like a sheet of fabric or a white powder floating on top. It can be white, gray or slightly pink, and cover a patch or the whole mouth of the jar. It has no fuzz and no raised dots: look at it from the side and it's still flat. It often comes back after you skim it.

Why it happens

It's almost always yeast, the kind known as kahm. It grows where the brine meets the air, so the surface of the liquid is its place, all the more if the jar has a big gap at the top.

It isn't mold, but it shouldn't be left either. It feeds on the ferment's acid and, if it builds up, lowers the acidity, which is what protects the ferment. It also brings off flavors and can spoil the texture.

What to do

It can be fixed: skim it off

  1. First, look at it closely. If it's fuzzy or has green, blue, brown or black spots, it's mold, and the whole batch goes.
  2. If it's flat, lift it off with a clean spoon or a paper towel, without stirring the liquid.
  3. Check that every vegetable is still below the brine. If not, press them down with the weight or top up with brine from the same recipe.
  4. Repeat whenever it comes back, ideally daily.
  5. If you have strips or a pH meter, check the pH: it must not rise above 4.6.

If it's also slimy or smells rotten, throw it out. When in doubt, throw it out.

How to prevent it

You can't always prevent it entirely, so check the jar two or three times a week and skim off any film that forms. What holds it back most:

  • Every vegetable below the brine, with a weight on top.
  • The jar filled just right, without a big gap of air.
  • The lid on properly, with no gaps, with an airlock or opened briefly now and then to let the gas out.

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

    White, gray or pink films can form

    “White, grey, or pink films may form on the surface of fermenting fruits or vegetables.”

    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

    That film is usually yeast: kahm

    “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

  3. 3

    Too much yeast spoils the flavor and texture and lowers 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

  4. 4

    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

  5. 5

    The pH must not 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

  6. 6

    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

  7. 7

    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

  8. 8

    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

Show the remaining 11 claims
  1. 9

    A white layer growing on top of the brine may be kahm

    “If your fermented brine has a white bloomy residue growing on top, it may be Kahm yeast.”

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

  2. 10

    Skimming it daily is recommended

    “we recommend skimming the yeast off daily”

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

  3. 11

    It forms when the brine is exposed to air

    “A scum can form on the surface of the brine if it is exposed to air as yeast and mold grow.”

    Oregon State University Extension Service · Pickle Fact Sheet (SP 50-466, revised April 2024) · read on October 26, 2026

  4. 12

    Yeasts feed on the acid if they're left to build up

    “Wild yeasts and bacteria that feed on the acid thus reducing the concentration if allowed to accumulate.”

    National Center for Home Food Preservation (USDA, University of Georgia) · Causes and Possible Solutions for Problems with Fermented Pickles · read on October 26, 2026

  5. 13

    Skim it off as often as needed

    “Remove scum as often as needed.”

    National Center for Home Food Preservation (USDA, University of Georgia) · Causes and Possible Solutions for Problems with Fermented Pickles · read on October 26, 2026

  6. 14

    Lactic acid holds back harmful bacteria and preserves the ferment

    “these bacteria convert sugars into lactic acid, which inhibits harmful bacteria and acts as a preservative.”

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

  7. 15

    If it's 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

  8. 16

    When in doubt, throw it out

    “When in Doubt—Throw it Out”

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

  9. 17

    A weight on top keeps the vegetables under

    “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

  10. 18

    A half-empty jar leaves room for air, and for mold or film

    “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

  11. 19

    Check the jar two or three times a week and skim off any film

    “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