There's Mould on My Home-Canned Food

Discard the whole jar. Do not scrape the mould off and eat what’s underneath.

Mould in a sealed jar is not a blemish — it is evidence the process or the seal failed. The Québec government states that boiling water processing is “enough to avoid food spoilage such as mould”, so mould in a finished jar means something did not work. Disposal steps below.

Why “scrape it off” is the wrong model

It is the most repeated piece of advice on this topic, and it comes from thinking about a jar the way you’d think about a block of cheese or a loaf of bread — a food that was always open to the air, where mould is an expected visitor on the surface.

A sealed jar is the opposite case. It was supposed to be a closed system. Nothing was supposed to be able to get in, and the processing was supposed to have killed what was already there. So mould is not a visitor. It is a report that the closed system was not closed.

That changes the question completely. You are no longer asking how much of this food is spoiled? You are asking what else got in, and when? — and there is no way to answer that from the outside.

There is a second, smaller problem with scraping, and it is the one most people have heard: what you can see is the fruiting body, and the network that produced it extends into soft food beyond the visible edge. But even if you could remove every trace, the first problem would remain.

What mould actually tells you, and what it doesn’t

Precision matters here, because overstating it is its own kind of dishonesty.

What it does tell you:

• The barrier failed, or the process did. Québec’s guidance is explicit that boiling-water processing prevents this class of spoilage.
• The jar has been open to the outside for some unknown period.
• Something has been metabolising your food. Spoilage organisms consume acids as they grow, which can raise the pH of a food that started out acidic enough to be safe.

What it does not tell you:

• It is not evidence of botulism. Mould and Clostridium botulinum are different organisms, and we are not going to tell you the jar contains toxin. What we will say is that Public Health Ontario’s review gives C. botulinum’s growth range as above pH 4.6, and a jar whose acidity has been altered by spoilage is a jar you can no longer make claims about.

The honest summary is that a spoiled jar is an unknown jar — not a proven-dangerous one. The response is the same either way, because there is no test that would move it from unknown to known.

What we are not going to tell you

We cannot tell you whether your jar is safe. This page reports what Canadian authorities say about mould in home-canned food as a class; it is not a diagnosis of the one in front of you.

And the usual reasoning does not get you there:

• Not “it’s only a little”. The amount visible has no relationship to how long the seal has been open.
• Not “it’s high-sugar jam”. Sugar and acid are why jam usually resists this. Mould growing anyway is the finding.
• Not smell. Botulinum toxin is odourless — HealthLink BC states contaminated food “may look and smell normal”.
• Not taste. Québec’s instruction is plain: “Do not taste any food that looks unusual, is foaming or smells bad.”

Québec finishes that same section with the sentence this whole site is built around: “When in doubt, throw it out!”

How to dispose of it

HealthLink BC’s steps:

1. Place the contents in a waterproof container.
2. Put that container in the garbage.
3. Do not feed it to pets or other animals.
4. Wash your hands well with warm soapy water.
5. Wash every utensil and surface the food, the container, or your hands may have touched.

Do not compost it — you would be distributing spores through the garden. Do not taste it while emptying the jar.

If the lid is also bulging, do not open the jar at all. Bin it sealed and read the bulging-lid page.

The jar itself is salvageable. Wash it thoroughly, then run a fingertip around the rim — a chip you cannot see is one of the commonest reasons a seal fails in the first place. The lid is not reusable and never was.

Where the mould got in

Worth working through, because it points at what to change — and because the rest of the batch depends on the answer.

• A nick or chip on the jar rim. The single commonest cause. Invisible, and enough to stop a seal forming properly.
• A reused SNAP lid. The sealing compound is shaped by its first use. Lids are single-use, and HealthLink BC says the same.
• Food or residue on the rim at the moment the lid went on.
• Wrong headspace. Too little and the contents siphon past the lid during processing, leaving food in the sealing surface.
• Processing time cut short, or not adjusted for altitude — see altitude adjustments.
• A band left on in storage, which can hold a failed lid in place so the failure is invisible until mould appears.
• Open-kettle “canning” — pouring hot food into jars and letting them seal on cooling, with no processing at all. This is not a canning method and it is the classic route to a mouldy jar.

The rest of the batch

Whatever failed in this jar was probably available to fail in its sisters.

• Inspect every remaining jar from the run before opening any of it — lid shape, seal, residue under the rim, contents through the glass.
• Identify the cause if you can. A single chipped rim is a one-jar explanation. A shortened processing time is a whole-batch explanation, and it changes what you should do with the rest.
• If you cannot identify it, you have no basis for treating the others differently from this one.

Next time

• Check every rim with a fingertip before filling. It takes seconds and it is the highest-yield habit on this list.
• Wipe every rim with a clean damp cloth before the lid goes on.
• Use fresh lids, every time.
• Follow the recipe’s headspace exactly, and adjust the processing time for your altitude.
• Remove bands before storing, so a failed seal shows itself immediately instead of months later.
• Never open-kettle. If a recipe says to pour hot jam into jars and invert them, it is not a tested Canadian method.

Related

• How to tell if home-canned food has gone bad — the full inspection routine
• The lid on my canning jar is bulging — when not to open the jar at all
• My canning jar is leaking in storage — the other visible sign of a failed seal
• Botulism — what every Canadian home canner needs to know — why pH 4.6 is the number underneath all of this

If anyone has eaten from this jar and develops double or blurred vision, drooping eyelids, slurred speech, difficulty swallowing, a dry mouth or muscle weakness — call 911 or go to an emergency department immediately. Botulism antitoxin works best given early. Do not wait to see whether it passes.