There is mould on my jam — is the rest still all right?

A jar of jam keeps for a year because the sugar binds the water so tightly that almost nothing can grow, and because it went into a sterile jar hot and sealed itself as it cooled. Mould means one of those two defences failed. It is worth identifying which, because otherwise the whole batch will follow.

Safety

Step by step

1. Throw the jar out without tasting it.Not just the top. Mould in a soft wet food sends threads well below the surface and some moulds produce toxins that are not destroyed by heat and are not confined to the visible growth. A jar of jam is not worth the gamble.
2. Check the rest of the batch.Look at every jar for surface growth, a lid that has lifted, seepage down the outside of the glass or a lid that clicks when pressed. If one jar failed for a batch-wide reason, others will follow within weeks.
3. Ask whether the jars were properly sterilised.Washed is not sterilised. Jars need washing in hot soapy water and then heating dry in a 140°C oven for fifteen minutes, with lids done separately in boiling water because oven heat destroys the rubber seal in most of them.
4. Ask whether it went in hot.Hot jam into hot jars, sealed immediately, is what creates the vacuum and what kills anything on the rim. Jam left to cool in the pan and then ladled into cold jars has no defence at all, and this is the single commonest cause.
5. Ask about the headspace and the rim.Any drip of jam left on the rim breaks the seal and feeds the mould. Wipe every rim with clean kitchen paper before the lid goes on. Leave about 5 to 10mm of headspace — filling to the brim leaves no air to contract and so no vacuum forms.
6. Ask about the sugar.A reduced-sugar jam does not keep in a cupboard, whatever else you did right. If the recipe was cut back, the jars belong in the fridge or the freezer.
7. Store the good jars properly.Cool, dark and dry. A cupboard above the cooker or a sunny windowsill shortens the life of a preserve substantially, and warmth is what lets any survivor get going.

Tips

• Once opened, keep jam in the fridge and use a clean spoon each time. Butter and crumbs from a used knife are what starts most mould in an opened jar.
• Waxed discs are traditional but they are not a substitute for a proper lid and a vacuum seal — they mainly stop a skin forming.
• Label every jar with the contents and the date it was made. A jar of unknown age is a jar you will end up throwing away anyway.

Common mistakes

• Scraping the mould off and eating the rest — It is the advice most people were given and it is not safe for a soft, wet, low-acid-tolerant food like jam. What you can see is the fruiting body; the threads and any toxins are already through the jar.
• Reusing the metal lid from a shop-bought jar for a second or third batch — The sealing compound inside those lids deforms the first time it is used and often will not pull down a second time. The jar looks sealed, is not, and moulds within a couple of months.

If it doesn't work

Questions people ask