There's a film on my ferment. Do I throw it out?

A flat, white or off-white film, sometimes wrinkled and sometimes faintly grey or pink, is almost always kahm yeast. Skim it off, check that the pH has not risen, and carry on. Fuzzy growth, round raised spots, or anything green, blue, brown or black is mold, and the batch should be thrown out whole. That is the position of UC Davis and Illinois Extension food safety specialists [1] [2]. This page shows how to tell the two apart, how to skim kahm properly, why mold cannot be scooped off, and how to set up a ferment so neither one gets a start.

What you see, what it means, what to do

If you cannot tell which it is, both UC Davis and Illinois Extension give the same rule: when in doubt, throw it out [4] [2].

Skimming kahm yeast without losing the batch

Kahm yeast grows only where brine meets air, so it comes back if you leave the conditions the same. USDA's troubleshooting table for fermented pickles says surface scum should be removed as often as needed [3]. To do it cleanly:

1. Work with clean hands and a clean spoon. A wide, shallow spoon or a sanitized paper towel laid on the surface and lifted off works well.
2. Lift the film off in pieces. Try not to stir it into the brine. A little that breaks up and sinks is not a problem.
3. Wipe the inside of the jar above the liquid. Film often climbs the glass and the weight.
4. Rinse or swap the weight if film is growing on it.
5. Take a pH reading. UC Davis advises checking that pH has not risen above 4.6 [1]. A ferment that was well below 4.6 and is now climbing back toward it is losing its acid. Snyder and coauthors say ferments "that fail to reach a pH of < 4.6" or in which "a continued pH decline is not recorded" should be discarded [5]; a pH that is climbing back up fails that test.
6. Close the gap. Push the peppers back under the brine, top up with brine at the same salt strength if the level has dropped, and fit an airlock or a better weight.

If kahm keeps coming back every day, the jar is getting too much air. See the lid comparison on jars, weights and airlocks.

Why harmless kahm still matters

Kahm yeast does not make you sick, but it eats the acid that keeps the ferment safe. The yeasts that form surface films are oxidative: with oxygen available, they consume lactic acid. In a USDA study, two such yeasts used up more than half of the lactic acid in a fermented cucumber medium within three to four days, and the pH passed 4.6 after six days. After 20 days the pH had climbed above 5.6, and spoilage bacteria that could only grow above pH 4.5 had moved in [6]. Without oxygen the same yeasts were held back and the acid stayed put [6].

USDA's troubleshooting table for fermented pickles says the same thing in plain terms: scum on the brine during curing comes from wild yeasts and bacteria that consume acid, and should be removed as often as needed [3].

Film yeasts also bring off flavors. UC Davis notes that excess yeast can cause off flavors or textures [1]. A mash left under a thick film for weeks often tastes musty or flat even if the pH held.

Why mold means the whole jar goes

Mold grows into the food beneath what you can see, so skimming the visible part leaves the rest behind. UC Davis says a ferment with confirmed mold on any part "should be immediately discarded" [1]. Mold also raises pH as it grows; UC Davis gives the example of white mold on sauerkraut stored above the brine raising the pH enough to turn red cabbage purple [1].

Dispose of a moldy batch without breathing in spores: put the lid on, bag the jar or empty it into the trash, and wash the vessel, weight and lid in hot soapy water before sanitizing them.

Where the older advice to skim mold came from

You will find older and secondhand sources telling you to skim mold off and keep going. USDA's home canning guide, last revised in 2015, says to check fermenting containers several times a week and "promptly remove surface scum or mold" [7]. That wording is about routine care of large crocks of kraut and pickles, and it does not say the food under visible mold is fine to eat.

Current extension advice is stricter, and this page follows it: skim yeast film, discard anything with mold [1] [2]. The two positions agree on the practical point that matters most: do not let anything grow on the surface unchecked.

Keeping both off your ferment

Both kahm and mold need air. Most prevention comes down to taking it away.

• Keep every piece under liquid. UC Davis advises keeping all produce submerged and using weights to do it [1].
• Use an airlock or a well-fitted lid. UC Davis recommends a firmly sealed lid with no air gaps, releasing gas through an airlock or by opening it now and then [1].
• Leave less air in the jar. A jar that is mostly empty space has more air above the brine. Use a size that fits the batch, or press a weight or brine bag right onto the surface.
• Salt in the normal range. USDA-ARS researchers note that yeasts and molds are more salt-tolerant than most lactic acid bacteria, so extra salt does not reliably stop surface growth [5]. A maker on The Hot Pepper forum reported kahm at about 4 percent salt with a whey starter [8]. Air control matters more than salt here.
• Keep it in the 65 to 72°F range. Snyder and coauthors give that as the optimal range for vegetable ferments [5].
• Start clean. Wash and sanitize equipment, and do not use produce that is already moldy or soft [4].
• Check two or three times a week, as USDA's guide advises for jars [7].

When the problem is not on the surface

A film is easy to see. Soft peppers, a slimy brine, a ferment that stopped bubbling at day 10, or a sulfur or rotten smell are harder to read. Each of those has its own diagnosis on when a ferment stalls or smells wrong.

References

1. DiCaprio E, Marco M, Finnegan P, Hanlon M, Common issues with fermented fruits and vegetables, UC Davis and UC ANR, version 1.0, March 15, 2022. https://ucfoodsafety.ucdavis.edu/sites/g/files/dgvnsk7366/files/media/documents/Troubleshooting%20fermented%20fruits%20and%20vegetables%20FINAL.pdf
2. University of Illinois Extension, Fermenting FAQ, undated, accessed October 2026. https://extension.illinois.edu/food-preservation/fermenting-faq
3. National Center for Home Food Preservation, Causes and Possible Solutions for Problems with Fermented Pickles, adapted from So Easy to Preserve, 6th ed., 2014 (reviewed 2020). https://nchfp.uga.edu/how/ferment/general-information-on-fermenting/causes-and-possible-solutions-for-problems-with-fermented-pickles/
4. DiCaprio E, Marco ML, Kim H, Ting KC, Safely Ferment Fruits and Vegetables at Home, UC Davis, version 1.0, March 15, 2022. https://eatlac.ucdavis.edu/sites/g/files/dgvnsk13106/files/media/documents/Safely%20Fermented%20Fruit%20and%20Vegetable%20Factsheet.pdf
5. Snyder A, Breidt F, Andress EL, Ingham BH, Manufacture of Traditionally Fermented Vegetable Products: Best Practice for Small Businesses and Retail Food Establishments, Food Protection Trends 40(4):251-263, 2020. https://www.ars.usda.gov/ARSUserFiles/60702500/Fermented%20Veg%20Manuscripts/p437.pdf
6. Franco W, Pérez-Díaz IM, Role of selected oxidative yeasts and bacteria in cucumber secondary fermentation associated with spoilage, Food Microbiology 32:338-344, 2012. https://www.ars.usda.gov/ARSUserFiles/60701000/Pickle%20Pubs/p378.pdf
7. USDA and National Center for Home Food Preservation, Complete Guide to Home Canning, Guide 6: Preparing and Canning Fermented Foods and Pickled Vegetables, revised 2015. https://nchfp.uga.edu/papers/guide/GUIDE06_HomeCan_rev0715.pdf
8. The Hot Pepper forum, Fermenting peppers, 10% salt too much? (thread, page 2), September 2014. https://thehotpepper.com/threads/fermenting-peppers-10-salt-too-much.50031/page-2