In 151 of 152 blue cheese samples bought in American stores, researchers found the mycotoxin roquefortine C. The median was 903 µg/kg and the maximum 6,630. People eat that cheese without a second thought, and at those levels it is considered safe.
A coin-sized patch of mold on a loaf, however, means the whole loaf goes in the trash. The entire subject comes down to the difference between those two cases: what the mold is, what it grew on, and how far it has already traveled.
What you see on the surface and what you do not
Mold is a microscopic multicellular fungus. The colored patch you can see is made of spores sitting on the tips of stalks. Underneath it are fine threads that grow into the food. The Food Safety and Inspection Service of the US Department of Agriculture (FSIS) puts it plainly: these “roots” are hard to see, and they can run deep.
That gives two risks. First, some molds produce mycotoxins under the right conditions, and in the dangerous species the toxin often concentrates in the threads and around them, sometimes spreading through the whole product. Second, bacteria you cannot see may be growing alongside the mold. How often that happens nobody has counted: FSIS gives it as a caution without pointing to a study.
Where the toxin sits: in the mold or already in the food
An Italian group tested exactly this in 2020 on the rind of grana cheese. Blocks were inoculated with five fungal species, held for 14 days at 50-86°F (10-30°C), and then measured for where the toxin ended up.
In three species out of five, including Penicillium roqueforti, around 90 percent of the toxin stayed in the mycelium itself. In the other two the picture was reversed: Aspergillus versicolor left 71 percent of its sterigmatocystin in the cheese rind, and Penicillium verrucosum 58 percent of its ochratoxin A. Part of the toxin is already in the cheese, and that is the reason for cutting with a margin.
All five fungi grew best at 59-77°F (15-25°C). FSIS adds that mold grows in the fridge as well, and tolerates salt and sugar better than most other microorganisms. That is why it turns up on jam and on salted sausage.
What to cut and what to toss
The FSIS logic rests on two properties of the food: moisture and density. In a dense dry product the threads do not travel far. In a moist, soft or porous one they can be anywhere.
| Food | What to do | Why |
|---|---|---|
| Hard cheese (cheddar, parmesan, gouda) | Cut off at least 1 inch (2.5 cm) around and below the patch | Mold usually does not penetrate deeply |
| Gorgonzola and stilton with stray mold on the surface | Cut as for hard cheese | Mold outside the recipe may be dangerous |
| Brie and camembert with stray mold | Toss it | Same reason, and the product is soft |
| Soft cheese (curd, cream cheese, chèvre), and any grated or sliced cheese | Toss it | Moist product; the knife spreads mold across every piece |
| Yogurt, sour cream | Toss it | Moisture, accompanying bacteria |
| Bread and baked goods | Toss it | Porous product |
| Jam, preserves, jelly | Toss it | The mold may produce mycotoxin |
| Firm vegetables (cabbage, peppers, carrots) | Cut off at least 1 inch (2.5 cm) around and below the patch | Dense product |
| Soft vegetables and fruit (cucumbers, peaches, tomatoes) | Toss it | Contamination below the surface |
| Cooked grains, pasta, leftover meat, casseroles | Toss it | Moisture, accompanying bacteria |
| Cooked sausages, bacon, hot dogs | Toss it | Same reason |
| Hard salami, dry-cured ham | Scrub the mold off the surface | For these products surface mold is normal |
| Peanut butter, nuts, pulses | Toss it | Products made without preservatives carry a high mold risk |
Hard cheese: how to cut it properly
The FSIS instruction has three parts, and the middle one is the part people skip. Cut off at least 1 inch (2.5 cm) around the patch and below it. Keep the knife away from the mold itself, so that you do not carry it over to the clean part. Then wrap the cheese in fresh film and throw the old wrapping away.
Where the 1 inch (2.5 cm) comes from is not explained in the research: no measurement justifying that margin appears in the available work. It is an agency rule of thumb. The grana data show why a margin is needed at all, but they do not produce its size.
Culinary mold is a separate story. Roquefort, gorgonzola, brie and camembert are made with mold on purpose, and FSIS states directly that those fungi are safe to eat. In the same American study, roquefortine C was not found once in 46 white-rind cheeses. The mold treated as dangerous is the mold that is not part of the recipe. Some dry salami also carries a thin white coating that can be eaten, though no other mold should be growing on it.
Bread: why the whole loaf
Bread is porous. Inside the crumb the mold threads have somewhere to go, while in dense cheese the journey is harder. That is the basis for the FSIS advice to toss the loaf entirely.
It is worth knowing the limit of this advice. Measurements of mycotoxin in the crumb at different distances from the patch do not exist in PubMed or in EuropePMC. The advice rests on the structure of the product. Cutting the patch out of bread is a bet that the threads went no further, and the eye cannot check that.
What reaches grain and bread from the field is covered in our piece on glyphosate in grains. Mycotoxin works differently: it appears on its own when grain or bread is stored badly.
Jam: why you do not lift the mold out with a spoon
Plenty of people lift the white circle off the top of the jam and finish the rest. FSIS advises against it: the mold may be producing a mycotoxin, and microbiologists do not recommend scooping it out and eating what is left.
The closest measurements come from apples. A Spanish group in 2006 inoculated apples with Penicillium expansum and cut samples around the rot in 0.5 cm layers. Between 2 and 6 percent of all the patulin had moved into tissue that looked healthy. No such experiment exists for jam, so the figure cannot be carried across one to one. What it does show is the mechanism: the toxin travels beyond the visible rot.
Patulin has a legal limit. The Ukrainian regulation on maximum levels of contaminants (Ministry of Health order No. 368 of May 13, 2013) allows 50 µg/kg in juice and cider, 25 µg/kg in products made from whole apples including compote and purée, and 10 µg/kg in apple products for children. The figures match the European regulation 2023/915.
Boiling the jam again will not help either. A 2007 review of mycotoxin stability during processing concludes that processes reduce concentrations substantially but do not remove the toxins completely. Noticeable reductions come from treatment above 302°F (150°C), and a pan of jam never gets there. Potato glycoalkaloids also hold up to 302°F (150°C), so boiling leaves them intact too, as our piece on green potatoes explains.
How dangerous is this really
To judge the risk it helps to know what real mycotoxin poisoning looks like. In 2004 an outbreak of aflatoxicosis in eastern Kenya produced 317 cases and 125 deaths. Home-grown corn in the affected households carried 354.53 µg/kg of aflatoxins (geometric mean) against 44.14 in control households. Case households were more likely to store damp corn inside the home rather than in a crib.
That is months of eating heavily contaminated grain as a staple. One patch on a loaf is nowhere near that scale. For patulin the EU set a tolerable daily intake of 0.4 µg per kg of body weight, and even for infants eating apple purée a calculation in Qatar came out below that limit.
So a moldy loaf is worth binning calmly. Tolerable limits for mycotoxins are set for daily intake, and the job is to avoid adding them to the diet where avoiding them is easy.
What to do when you find mold
- Do not sniff the food: according to FSIS this can cause breathing problems.
- If the food is covered in mold, wrap it in a bag or film and put it in a covered trash can that children and animals cannot reach.
- Clean the spot in the fridge or pantry where it was sitting.
- Check the food next to it: on fruit and vegetables mold spreads quickly.
- Do not leave perishable food out of the fridge for more than two hours, and eat leftovers within 3-4 days.
How microorganisms move from food to the board, the knife and the dish towel is covered in our piece on cross-contamination in the kitchen.
The short version
- The visible patch is spores. The threads run into the food, and the toxin can be inside them and around them.
- Hard cheese and firm vegetables can be saved: cut at least 1 inch (2.5 cm) around and below the patch, keeping the knife away from the mold.
- Bread, jam, soft cheese, yogurt, soft fruit and cooked leftovers go in the trash whole.
- Culinary mold in roquefort or brie is safe. The dangerous kind is the mold that is not part of the recipe.
- Heat lowers mycotoxins without removing them, so reboiled jam does not become safer.
- Real mycotoxin poisonings come from months of eating contaminated grain. A patch on a loaf is a reason to toss the loaf.
What the E numbers on a label mean, and why they say nothing about risk on their own, is covered separately. Where preservatives are absent by law is shown in our piece on bagged salads. How long microgreens keep at different temperatures sits in the storage terms reference. Whether thawed food can go back into the freezer, and what that costs in quality, is covered in our piece on refreezing. How a best-before date differs from a use-by date, and when a visual check actually settles anything, is covered in our guide to use-by, best-before and sell-by dates. The wider picture is in our food safety hub. Molds and edible mushrooms belong to the same kingdom, and in mushroom growing the same green mold takes out whole blocks: see our guide to growing mushrooms at home. Why folk methods for testing wild mushrooms fail is covered in our piece on wild mushrooms and the death cap. When a normal-looking jar means nothing at all is covered in our piece on botulism in home canning: spoiled low-acid food may show very little evidence.
The refrigerator works against mold and against bread texture at the same time. A 2021 Munich study weighed both sides of that trade: what the cold actually costs.
