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Is this mold?

Three questions. One answer: eat, fix or bin.

Illustration: a jar of sauerkraut surrounded by cabbage, garlic, dill and peppercorns
Check my jar Rules from USDA & FDA guidance · sources
Question 1 of 3

Which one looks like your jar?

Look at the top, from the side, in daylight. Brine dyed by beet, red cabbage or turmeric is fine.

Open it. Does it smell rotten?

Tangy, sharp, fresh, a little sulphur from cabbage: all normal.

Is everything under the brine?

Mold can only start where air touches food.

Eat

That's fermentation.

A flat white film is kahm yeast. Skim it off. Cloudy brine and bubbles are normal. Taste it; fridge when it's sour enough for you.

Acid is what keeps it safe: at pH 4.6 or below, botulism bacteria can't grow.1

Fix

Push it under. Check tomorrow.

Press everything below the brine with a clean spoon. Low on liquid? Top up with brine: about 25 g salt per litre (quart) of water.2 Weigh it down with a leaf or a small bag of brine.

Mold needs air. The brine keeps air out.

Work out the salt for your next jar

Bin

That's mold. Bin the whole jar.

In soft, wet food, mold spreads below what you can see. Don't scoop around it. Wash the jar hot. Next time, keep everything under the brine.

USDA advice for moldy soft foods: discard.3

Work out the salt for your next jar

Bin

Wrong microbes won. Bin it.

Sour, fresh or a little sulphury is fine. Rotten or putrid means spoilage got ahead, usually too little salt or too warm a room. Next jar: weigh the salt, find a cooler spot.

When in doubt, throw it out.3

Work out the salt for your next jar

Keys 1 / 2 / 3 to answer · Esc to start over

Free · salt calculator

How much salt?

Too little salt and a jar goes soft. Too much and it stalls. Pick your jar, type the weight, get grams.

20 g salt
2 % of 1000 g vegetables

No scale?A tablespoon of salt weighs anywhere from 12 to 20 g, depending on the crystal. Spoons are how jars go soft or pink.

Topping up later?Use brine, never plain water: 25 g salt per litre (quart) for shredded jars.2

Twenty recipes in the book, every one weighed like this.See the recipes →
Scrub through two weeks

What normal looks like.

Day
0

Packed

Pressed under its own brine. Lid loose.

An illustration rendered from two reference frames, not a recording of one jar. Around 20 °C (68 °F). Cooler kitchens run slower.2
Who wrote this

I'm Michael. The Quiet Jar is a one-person kitchen business in Poland, where kiszonki (kraut, pickles, the whole cellar shelf) are just what you do in autumn. I binned my first jars anyway, because nobody would just say: flat is fine, fuzzy isn't. This is the book I wanted at jar one.

  • sponsors
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  • health claims
  • stock photos
The book

Fermentation Without Fear

You're not scared of cabbage. You're scared of getting it wrong. This book, written in Poland, shows you what normal looks like, day by day.

  • Your first jar in 20 minutes, with one cabbage and salt
  • By the end of week one, you know flat from fuzzy
  • Every safety rule traced to USDA and FDA guidance
The bookPrint PDF (A5) + phone PDF
$7.99
For the kitchen wall
Book + fridge-door chartAdds the photo poster, A3 and 11 × 17 in
$9.99

Launch price until 31 October

Get the book
Pick your option at checkout · instant downloadRead 6 pages first

The 14-day jar promise. No jar you trust within 14 days? Email me for a full refund, and keep the files. How it works

What's inside

  1. 8 pp
    Part oneIs this safe?Flat or fuzzy, eat, fix or bin.
  2. 33 pp
    Part twoFour jars, day by dayWhat you'll see, what to do.
  3. 21 pp
    Part threeTwenty recipesKraut to hot sauce, weighed.
  4. 9 pp
    Part fourKeep goingFixes, labels, fridge sheet.

Is it for you?

Yes, if
  • you've never fermented, or binned a jar in a panic
  • you want to know why a jar is safe, not just a recipe
  • you cook in cups and °F, or grams and °C
Not if
  • you want canning or pressure-cooking
  • you want gut-health promises

Questions

I've never fermented. Is this for me?

Yes. Week one is cabbage and salt, the most forgiving jar there is.

What if my jar still goes wrong?

Every page says what to do next: eat, fix or bin. If the book doesn't help, email me within 14 days for a full refund. You keep the PDF.

Is this medical or health advice?

No. It's food-safety practice from USDA and FDA guidance, with sources. No health claims.

I'm in the US. Will the measurements work?

Yes. Temperatures are in °C and °F, ingredients in grams with ounces, pounds and quarts next to them. The only tool I ask you to buy is a kitchen scale.

What do I need to start?

A 1 litre (quart) glass jar, a kitchen scale, salt and a cabbage. The kit page lists the rest, and none of it is special equipment.

How do I get it?

Right after checkout you get a download link on screen and by email: one PDF to print (A5) and one laid out for your phone. About 14 MB each. Updates are free.

See the full contents
  1. Read this at 3 am 3
  2. The kit 5
  3. The 2 % rule 6
  4. Part one · Is this safe? 7
  5. How it works 8
  6. Flat or fuzzy? 9
  7. Eat, fix or bin 10
  8. Three questions 11
  9. Temperature 12
  10. Checking the acid 13
  11. Six myths 14
  12. Part two · Four weeks 15
  13. Week 1 · Cabbage 17
  14. Week 2 · Cucumbers 25
  15. Week 3 · Carrot & ginger 33
  16. Week 4 · Kimchi 41
  17. Part three · Twenty recipes 48
  18. Part four · Keep going 69
  19. Troubleshooting 70
  20. Questions people ask 71
  21. Storing and serving 72
  22. The fridge-door sheet 73
  23. Labels to print 74
  24. Your next four weeks 75
  25. Index of methods 76
  26. Sources 77
Help me choose

What should book two be?

One tap. The jar with the most votes gets written next.

Sources
  1. US FDA, 21 CFR 114.3: acidified foods, equilibrium pH 4.6 or below. eCFR
  2. National Center for Home Food Preservation (USDA-funded): sauerkraut, salt ratio, keeping food submerged, fermenting temperatures. nchfp.uga.edu
  3. USDA FSIS: Molds on food, are they dangerous? fsis.usda.gov

Last reviewed September 2026 · book edition 1.0