How to Sterilise Jars for Jam, Chutney and Pickles

A jar can look spotless and still spoil a preserve. Moulds and yeasts you cannot see will happily sit in a washed jar, and some of them cope with the sugar in jam and the vinegar in chutney. Sterilising the jars, filling them hot and sealing them straight away closes that gap, and it takes little more than an oven, a dishwasher or a big pan of water.

Why sterilising matters

A spoiled jar turns furry, fizzy or boozy-smelling. Throw out any jar that shows mould or yeast growth or smells fermented, yeasty or of alcohol. Mould on jam and jelly can carry toxins, so a mouldy jar goes in the bin, not back on the shelf.

Clean jars do not make up for a weak recipe. That matters for bramble jelly, and for green tomato chutney, the usual home for the tomatoes a short Scottish season leaves unripe on the vine at the first frost. The amount of sugar or vinegar is part of what keeps a preserve safe, so stick to a tested recipe and don't cut either. For pickles, use vinegar of at least 5% acidity (check the label) and don't change the recipe's proportions of vinegar, water and vegetables.

A sterilised jar does not make every food safe to keep at room temperature. Garlic, herbs or vegetables kept in oil carry a botulism risk, because oil is not acidic and shuts out the air. Never store them in oil in the cupboard. Sterilising is for high-sugar and high-vinegar preserves made to a tested recipe.

Wash first, whichever method you use

Before every use, wash jars in hot water with detergent and rinse them well, or put them through the dishwasher. Rinse properly: leftover detergent can give the preserve odd flavours and colours. Washing alone does not sterilise a jar, so follow it with one of the three methods below.

Check every jar as you wash it. Old jam jars and shop jars are fine to reuse, but throw out any that are cracked or chipped. There is more on choosing jars and lids for preserving in its own guide.

The three methods compared

All three work for glass jars. Check the jar maker's instructions; some (Kilner, for example) advise against the oven for their jars and prefer the water bath.

Whichever method you use, jars must be fully dry before they are filled. Once they are sterilised, stand them upside down until you are ready to fill them.

Oven method

1. Wash and rinse the jars.
2. Stand them on a baking tray.
3. Heat in the oven at 160°C (gas mark 3) for 10 minutes.
4. Stand them upside down until you fill them.

Keep lids and seals out of the oven and sterilise them separately (see below).

Dishwasher method

1. Load the washed jars into the dishwasher.
2. Run them through the hottest cycle, all the way to the end, without detergent.
3. Check the jars are fully dry inside, then stand them upside down until you fill them.

Water bath method

1. Stand the clean jars upright in a large, deep pan.
2. Cover them with cold water so it fills the jars and sits about 2.5 cm above the tops.
3. Bring the water to the boil and keep it boiling for 10 minutes.
4. Lift the jars out one at a time as you need them, drain them upside down and fill them while still hot, once they are dry inside.

Don't try to sterilise a jar by pouring boiling water into it. Boil it in the pan instead.

Lids and seals

Use new lids every time, and never an old, dented or bent one. The glass jar can go round again and again. The lid that makes the seal can't. On two-piece lids, the screw band can be reused if it is washed, dried and stored dry; the flat lid underneath is single-use.

Sterilise lids and rubber seals separately from the jars in a pan of hot water, not a hard boil. Kilner gives 82°C for 10 minutes for its lids and seals.

Filling, headspace and sealing

Aim to have the jars sterilised and dry by the time the preserve is ready, and fill them while they are still hot from sterilising, so hot jam or chutney goes in straight from the pan (for jam, once it has reached its setting point). Hot jam or chutney in a sterile jar, with the lid on at once, is what lets a vacuum seal form as the jar cools. Pickles packed cold don't work this way, so follow the tested recipe's own sealing and storage advice for those.

Fill each jar close to the rim. The gap left between the preserve and the lid is called the headspace. For jam and jelly, leave about 6 mm (1/4 inch) of space at the top. Too much air in a jar of chutney makes it darken in storage, so fill it well up.

Screw the new lid on straight away, while the preserve is still hot.

A poured layer of paraffin wax is no longer recommended as a seal for any jam or jelly, because mould can grow on the surface underneath. The seal you can rely on, and test, is a new lid put on while the preserve is hot.

Checking the seal

Leave the jars to cool at room temperature, undisturbed, for 12 to 24 hours. Don't retighten the lids while they are hot.

Once cool, press the middle of each metal lid. A sealed lid is curved down slightly in the centre and stays put. If it springs back up when you let go, or looks flat or bulging, the jar has not sealed. Put an unsealed jar in the fridge and use it soon, as you would an opened jar. Date every jar and store sealed jars in a cool, dark, dry place. Once a jar is opened, keep it in the fridge. Storage times are in how long preserves keep.

Common mistakes

• Reusing old lids. Fit a new lid every time.
• Filling wet jars. Jars must be fully dry, whichever method sterilised them.
• Skipping the rinse. Detergent left in the jar can taint the preserve.
• Using chipped or cracked jars. Check each one as you wash it and throw out damaged glass.
• Pouring boiling water into jars instead of boiling them in a pan.
• Letting the preserve cool before lidding. The lid needs to go on hot for the vacuum to form.
• Leaving a big gap at the top. Extra air darkens chutney in storage.

Common questions

Sources