How Can You Tell If Home-Canned Food Has Gone Bad?

The hardest part of home canning isn’t the process — it’s opening a jar nine months later and trusting it.

This is the page to bookmark for that moment. We cover what to look for, what’s normal versus what’s a red flag, and why the most common piece of bad advice on this topic (“just taste a tiny bit to check”) can put you in hospital.

The cardinal rule

When in doubt, throw it out.

You can replace ten jars of jam for $40. You cannot replace a hospital stay or the people involved. The economics of canning safety are completely unbalanced — almost any cost of replacement is less than any cost of poisoning. We say “throw it out” a lot in this article and we mean every instance.

What to check BEFORE opening the jar

Most spoilage shows up on the outside. Inspect every jar before you even unscrew the band.

The lid

• Bulging or domed upward. Discard without opening. Gas pressure has built up inside, which is a strong indicator of microbial activity — potentially Clostridium botulinum. Don’t taste, don’t smell up close, don’t even open the jar.
• Concave (dipped down) and rigid. Normal. This is what you want — the vacuum is holding the lid down.
• Concave but flexes when pressed. The seal is gone (or never formed). Discard the contents.
• Lid is loose, lifted, or has popped off. Discard the contents.
• Rust or food residue around the lid edge. Could indicate the contents siphoned during processing or the jar has been re-pressurized by spoilage. Inspect more carefully. If there’s any swelling along with this, discard.

The contents (through the glass)

• Cloudy liquid where it should be clear. Pickle brine and pickled-fruit syrup should be clear. Cloudy = bacterial growth. Discard.
• Floating food that wasn’t floating before. Sometimes a sign of gas being produced under or in the food. Suspicious; tilt and inspect carefully.
• Food packed differently than the rest of the batch. If one jar of the same recipe looks visibly different from its sisters — settled differently, separated more, lost colour — treat it with extra suspicion.
• Visible mould. Any colour. Discard. Don’t try to scoop the mould off and use the rest; mould networks extend invisibly through soft food.
• Discoloration. Dark spots, brown areas, or unexpected colour changes from when you canned it.

The smell (from a distance)

When you open the jar, hold it at arm’s length and waft the air toward you — do not put your nose over the jar. Spoiled food can release sulphur compounds, gas, or even tiny aerosols of liquid. Smell carefully:

• Sour, rancid, fermenty, yeasty, or off in any way = discard.
• Smells like what you canned, just more intense = normal aging.

If the smell is at all ambiguous, treat it as off. The cost of being wrong in one direction is a flushed jar; the cost of being wrong in the other can be permanent.

What’s normal versus what’s a red flag

There are some appearance changes that look alarming but are normal:

Appearance	Normal or red flag?
Slight liquid loss / lower fill line	Normal — happens during processing
Layer of darker fruit at the top of jam	Normal — gravity over months
White crystals on top of jam or jelly	Normal — sugar crystallization, harmless
Cloudy brine in pickles	Red flag — bacterial activity
Greyish-purple fruit (e.g. peaches, pears)	Usually normal — oxidation from air contact
Pink tinge in canned pears	Normal — natural anthocyanin pigment
Faint hissing or fizzing when opening	Red flag — gas under pressure
A pop when you press the lid down	Red flag — seal was already broken
Visible bubbles rising through a jam or sauce	Red flag — gas being produced
Stuck-shut lid that won’t twist off	Normal — strong vacuum seal, use a butter knife to gently pry

Why tasting to check is dangerous

The most widespread piece of bad canning advice on the internet is “just taste a tiny bit to see if it’s off.” Don’t do this with home-canned food.

Botulism toxin — the cause of botulism, the worst-case canning outcome — is:

• Odourless — you can’t smell it.
• Tasteless — you can’t taste it.
• Colourless — you can’t see it.
• Active in microgram doses — a single mouthful from a contaminated low-acid jar can be a lethal exposure.

If you’ve already cooked the contents at boiling temperature for 10+ minutes (some sources say 15), the toxin itself is destroyed by heat — but doing this means you’ve already opened the jar in your kitchen, contaminating surfaces. And you’ve trusted yourself to cook it long enough.

The reliable rule is the upstream one: don’t open or taste a suspicious jar at all. Inspect externally; if it fails any check, discard without opening.

How to discard safely

Don’t just throw a suspicious jar in the indoor garbage. The bacteria or toxin can survive there and contaminate pets, kids, or the kitchen.

For a sealed-but-suspicious jar (bulging, etc.):

1. Place the unopened jar in a doubled plastic bag.
2. Seal the bag tightly.
3. Put it in the outdoor garbage immediately, ideally on collection day.
4. Wash your hands thoroughly.

For a jar you’ve already opened that turned out to be spoiled:

1. Wear disposable gloves if you have them.
2. Tip the contents into the toilet and flush — the dilution and sewage treatment handle the contamination.
3. Wash the jar in very hot soapy water; the jar itself is reusable, but the SNAP lid goes in the bag for outdoor garbage.
4. Wipe down any surface the spoiled food touched with diluted bleach (about 5 mL household bleach in 1 L water).
5. Wash your hands and clothes.

Health Canada recommends extra care with anything that contacted low-acid spoiled food.

When to call for help

Call your provincial public health line

If you suspect you’ve eaten something contaminated but have no symptoms yet, call your provincial public health office for advice. Most provinces have a 24-hour line.

Call 911 if you have symptoms after eating canned food

Botulism symptoms typically appear 12 to 72 hours after eating contaminated food, sometimes longer. The classic progression:

• Early: blurred or double vision, drooping eyelids, slurred speech, difficulty swallowing, dry mouth, dizziness.
• Mid: descending muscle weakness — face, then arms, then chest, then legs.
• Severe: respiratory failure.

If any of these symptoms appear after eating home-canned food, call 911 immediately and tell the operator you suspect botulism and need antitoxin treatment. Bring the jar (sealed in a bag) to the hospital if you can. Antitoxin must be administered quickly to be effective.

Storage rules that prevent most spoilage

Most home-canned food never goes bad in the pantry because the cannery was done right. To stay in that category:

• Process for the time the recipe says, at your altitude band. Open your Bernardin edition or our altitude-adjustments guide.
• Inspect every jar after 24 hours — if it didn’t seal, reprocess or refrigerate within that window.
• Store in a cool, dark, dry place. Pantry shelves below 21 °C are ideal. Avoid temperature swings and direct sunlight.
• Label with the canning date and rotate (oldest at the front of the shelf).
• Use within 18 months for best quality. Safe for longer if seals hold and contents look normal, but flavour declines.

What never to do

• Never reuse a SNAP lid. (Full rules here.)
• Never reduce processing time even by a few minutes.
• Never water-bath low-acid foods even with a tested-recipe vibe. (Why.)
• Never taste a suspicious jar — see above.
• Never store unsealed jars at room temperature.

Next steps

• Water-bath canning pillar — the safe method for high-acid foods.
• Pressure canning pillar — the only safe method for low-acid foods.
• Health Canada — Botulism — official botulism resource with up-to-date symptoms and treatment information.
• Jar didn’t seal — what now? — what to do at the 24-hour mark when a jar fails.