When to Toss a Jar of Preserves: Mold, Bulging Lids and Other Warning Signs

Toss the jar without hesitation if you spot any of these eight signs:

• Mold — any colour (white, green, blue, black, pink), no matter how small a patch, whether on the surface or just on the underside of the lid.

• A bulging lid — the centre of the lid sits up instead of down, and the button pops when pressed.

• Gas escaping outward when you open it — a hiss coming out of the jar, foam, or brine spraying out.

• Rising bubbles in a jar that hasn't been moved at all.

• Cloudiness, sliminess or flecks in brine that should be clear.

• Dried trickles of contents running down the outside of the jar from under the lid — a sign something leaked out under pressure.

• An unnatural colour or smell — yeasty, sharply sour, cheesy, musty, or rotten.

• A cracked jar, rusted lid, or a seal that isn't tight anywhere around the rim.

Just one of these is enough. There's no such thing as "just a little" or "only on top" — a jar is either fine or it belongs in the bin. And keep in mind there's a ninth warning sign you can't detect at all: botulinum toxin has no colour, smell or taste, and spoiled low-acid preserves can look perfectly normal.

If there's mold in your preserves, throw out the whole jar, because what's dangerous about the mold stays in the jar even after you scrape it off. What you see is only the fruiting body — the coloured fuzz on the surface. The fungal threads reach much deeper, and some molds release mycotoxins that spread freely through the soft, moist environment of jam.

How far can it spread? Lab studies show that patulin — a mycotoxin typical of apples and pears — migrates from the mold patch into surrounding healthy-looking tissue too. In a firm apple, its concentration drops roughly tenfold with every half centimetre; in softer, wetter matrices like tomato or jam, it spreads much further. So in soft jam, scraping off a centimetre or two doesn't solve anything — unlike in a firm apple, where patulin won't travel more than a few centimetres. The softer and wetter the contents, the further the toxin travels — in tomato, it diffuses roughly an order of magnitude faster than in a firm apple. Microbiologists and the USDA agree on this point: throw out moldy preserves entirely, because soft contents lack the firm structure that would otherwise stop fungal threads and toxins from spreading. That's exactly why, with hard cheese, you're allowed to cut away a moldy spot with at least 2.5 cm of margin around and beneath it — with jam, that option doesn't exist.

And one more reason people overlook: as mold grows, it consumes acids and raises the pH locally. What was originally safely acidic can turn into an environment where even bacteria that used to be held back by acidity can now grow. A moldy jar isn't just unpleasant to look at — it's a microbiology problem already in motion.

Sugar slows mold growth but doesn't stop it. Jam sits at a water activity of around 0.80, while some osmophilic and xerophilic fungi can grow down to as low as 0.61 — well below what ordinary preserves offer them. High sugar content, in other words, isn't a protective wall, just a speed bump.

Heat treatment alone isn't a guarantee either. The genera Byssochlamys, Talaromyces and Neosartorya (now classified under Aspergillus, section Fumigati) produce heat-resistant ascospores that survive the pasteurisation temperatures used for fruit products — they're notorious spoilage organisms specifically in acidic fruit preserves and juices.

If you still find mold in a sealed jar, it means one of two things: either the heat treatment wasn't sufficient, or the seal wasn't airtight and let in air carrying spores. Either way, it points to something in your process, not bad luck. A white coating on the surface of preserves is almost always mold or yeast — the same rule applies to chilli jam and strawberry jam alike.

A bulging lid means there's excess pressure inside the jar — and that pressure is produced by microorganisms breaking down the contents and releasing gas. Don't open a jar like that; throw it away.

A properly canned jar runs on a vacuum seal. As the contents cool, they contract, the air above the surface pulls back, and outside atmospheric pressure pushes the lid inward. That's why the centre of the lid sits dished down, and pressing it with a finger produces no click. On a twist-off lid, that's the little button people call the "pop": it should sit down and stay silent.

When the centre sits up instead, there are only two possibilities, and both end the same way:

• A vacuum seal never formed — the lid didn't seal properly right after canning. If you catch this within 24 hours, the jar can still be saved: either put it straight in the fridge and eat it within a few days, or transfer the contents to a clean jar with a new lid and re-process it.

• A vacuum seal formed and gas broke it — the jar sat in storage and the lid bulged over time. At this point there's nothing to save. Throw it out.

Read dried trickles running down from under the lid the same way. They look like a minor cosmetic issue, but they mean something leaked out under pressure — which means gas was building up inside.

Hissing where gas escapes outward from the jar means excess pressure, and that jar should go in the bin; a short pop followed by air being drawn inward, on the other hand, is exactly the sound a properly canned jar should make. What matters is the direction the air moves — out, or in.

Hissing outward always comes from excess pressure. That pressure doesn't appear in a sealed jar on its own — yeasts or bacteria produced it. You'll know it's this kind if the hiss lasts longer than an instant, and it comes with foam rising to the surface, brine spraying out, or a strong yeasty or alcoholic smell.

If you're not sure which direction it is, the lid will tell you: a jar that had a dished-in centre before opening had a vacuum seal. A jar with a bulging lid that hisses had excess pressure — and according to the previous section, you shouldn't have been opening that one anyway.

One legitimate exception: with fermented foods, bubbling and hissing are normal, because lactic acid bacteria are producing that gas on purpose. Fermented sauerkraut or pickles in brine are allowed to bubble. Sterilised preserves, which are supposed to be microbially dead, are not.

Motionless bubbles stuck to the side of the jar are harmless — that's just air trapped during filling. Bubbles rising upward in a jar that's sitting perfectly still mean fermentation is happening inside. Don't move or carry the jar while checking; assess it right where it stands.

Read cloudiness in context:

• Sterilised fruit compote or pickled cucumbers should have clear brine. Cloudy, milky or stringy brine means throw it out.

• Fermented foods are supposed to have cloudy brine — that's the bacterial culture, and it's fine.

• A fine sediment at the bottom from spices, hard water, or fruit starch is harmless, and you can tell it apart from cloudiness because it has settled and the brine above it stays clear.

Also watch for things people blame on age that are actually warnings: sliminess or a stringy texture, an unnaturally dark or grey colour in the contents near the lid, contents soaked up under the lid, and any smell that doesn't belong to that ingredient. Open the jar away from your face and never sniff directly over it — with low-acid foods, the aerosol from a burst jar can be unpleasant on its own.

Be especially cautious with low-acid preserves. Spoiled green beans, peas, corn, mushrooms in water, or meat may show absolutely no visible warning sign at all. With these, "it looks fine" doesn't count as evidence.

Botulism is a real risk with low-acid preserves — green beans, peas, corn, carrots, mushrooms in water, meat and pâtés. With fruit jam, compote from acidic fruit, or vegetables in vinegar brine, it's practically not a risk. Acidity is what makes the difference.

The bacterium Clostridium botulinum doesn't grow in environments with a pH of 4.6 or lower. Fruit, vinegar and lemon juice reliably keep things below that threshold, which is why classic preserves like apricot compote or mushrooms pickled in vinegar are safe from a botulism standpoint. With mushrooms, the contrast is textbook: mushrooms in acidic brine, yes; mushrooms canned in plain water alone, absolutely not.

The problem is that spores of Clostridium botulinum survive boiling. The contents of a jar in a pot or oven never rise significantly above 100°C — an oven has hot air around it, sure, but the preserves inside only heat up to boiling point, and spores survive that kind of heat treatment just fine. Only a pressure canner, which reaches 116–121°C, actually destroys them — and that's equipment most home kitchens simply don't have. That's why low-acid foods can't be safely canned at home in a pot or oven, no matter how convincing the method sounded.

The toxin itself, on the other hand, is heat-sensitive, and ten minutes of boiling destroys it. But that isn't a way to rescue a suspect jar — you never know how much toxin was there, whether the heat reached every part of it, or what else was growing alongside it.

If a suspect jar is still sealed and holding pressure, don't open it — wrap the whole jar, lid included, in a sturdy bag and put it out with general waste. Only boil it if it's already open, no longer sealed, or cracked: with a low-acid jar like that, simply binning it isn't enough — boil the jar and its contents for 30 minutes first to destroy any toxin present. The procedure from the US National Center for Home Food Preservation (NCHFP) goes like this:

1. Wear gloves and work well away from children and pets.

2. Lay the jar, lid still on, on its side in a large pot.

3. Cover it with water so the level sits at least 3 cm above the jar.

4. Bring to a boil and boil for 30 minutes.

5. Let it cool, wrap the jar and its contents, and dispose with general waste.

6. Spray any surfaces, the sink, the pot, and anything the contents touched with a fresh solution of 1 part chlorine bleach (5–6% sodium hypochlorite) to 5 parts water, let it sit for 30 minutes, repeat, rinse, and wash your hands thoroughly.

With moldy jam or compote, this whole protocol is unnecessary — just discard the contents, and boil or dishwasher-clean the jar. Whatever the case, three things are off-limits: don't put spoiled preserves on the compost, don't pour them down the sink, and don't feed them to animals. Dogs are notably sensitive to mold toxins, and poultry are often sensitive to botulinum toxin as well.

Preserves last longest in a cool, dark, dry place — ideally at 10–21°C. Specifically:

• Darkness. Light breaks down pigments and vitamins and speeds up rancidity. Jars left on a windowsill spoil faster than almost anywhere else.

• 10–21°C, never consistently above 24°C. Heat speeds up every chemical and microbial process; stored long-term above 24°C, contents lose colour, flavour and vitamins, and above 35°C they can spoil outright or cause the lid to pop. A cupboard above the stove or a shelf next to a radiator is about the worst spot you could pick.

• No freezing. Frozen contents expand and can crack the glass or damage the seal, so once thawed, the jar no longer seals properly. An unheated garage in winter is a real risk.

• Dryness. Damp corrodes lids with rust, and rust eats through the sealing layer from the inside.

• Remove the clamps from Weck-style jars once cooled, and the screw band from twist-off jars. The band hides a bad seal — without it, a jar that isn't sealed properly will give itself away.

• Don't stack jars on top of each other. Weight from above deforms lids and can quietly break the seal.

• Label the date and contents, and always use the oldest jar first.

• Check the lids 24 hours after canning, not six months down the line.

A properly canned jar, properly stored, holds its full quality for roughly a year. That's not an expiry date — after a year, the contents aren't usually dangerous, just gradually duller in colour, texture and flavour. That's why it's worth only canning as much as you'll realistically use within a year.

Two things hold true here: the older the jar, the more likely a minor seal failure has had time to show up, so check every warning sign from the first section that much more carefully. Plum jam after two years being darker is perfectly fine. Plum jam after two years with a bulging lid is rubbish, no matter how good it looks.

Once you open a jar, the vacuum seal is gone and the normal shelf life of a chilled food begins. Jam and marmalade keep in the fridge for about a month; reduced-sugar versions won't last as long, while vegetables in vinegar brine keep longer, protected by their acidity.

The most common way mold gets into preserves isn't bad canning technique — it's the spoon you use. Always use a clean, dry one, never one you've licked, never one that's been sitting on a chopping board. Saliva and crumbs bring both microorganisms and nutrients into the jar. And wipe the jar rim and lid threads completely dry before closing it — that's exactly where mold takes hold first.

A spoiled jar is almost always down to one of four things: insufficient temperature or too short a processing time, a dirty jar rim (a drop of jam on the rim stops the lid seating properly), a reused or damaged lid with a worn seal, and an unsuitable method for that particular ingredient. Fixing these takes far less effort than later trying to figure out whether a jar is still safe to eat.

If you want to know exactly where things most often go wrong, continue here: