Water bath canning, jar to seal

Water bath canning is the method behind every jar of jam, chutney, pickle and bottled fruit in the British tradition. It is straightforward, it needs no specialist pressure equipment, and done properly the results keep for a year or more on a shelf.

It is safe only for acid foods. If you have not read water bath or pressure, read it before this page. Everything here assumes that decision has already been made correctly.

Before you start

Work from a tested recipe. Not a family recipe of uncertain provenance, not something adapted, not something scaled up. The quantities of vinegar, sugar and acid in a tested recipe are doing safety work as well as flavour work, and the process time was measured for that food in that jar size.

This app does not print process times, for the reasons on the method-choice page. Have your NCHFP or Ball tables to hand, on paper.

The kit

The rack matters more than it looks. A jar resting on the base of the pan is heated from below by direct contact rather than by the water, which both risks thermal shock cracking it and means the contents do not reach temperature the way the tested time assumes. A folded tea towel, a round cake rack, or a ring of jar bands all work.

On the lids: the flat sealing disc has a compound gasket that deforms to fit the jar rim on first use. It cannot be relied upon a second time. Bands and jars go on for decades; discs are consumables and worth stockpiling, since they are the one part of this system you cannot improvise.

Preparing jars and lids

1. Check every jar rim with a fingertip, all the way round. Any chip, crack or nick means the jar will not seal. Set it aside for dry storage instead.
2. Wash jars, lids and bands in hot soapy water and rinse well.
3. Keep the jars hot until filling: in the canner as it warms, or in a low oven, or straight out of a dishwasher cycle. Hot jars going into hot water avoids thermal shock, and hot jars take hot contents without cracking.
4. Do not boil the sealing discs. Modern discs need only to be clean and, if the manufacturer says so, warmed in hot water. Boiling can damage the sealing compound on some brands. Follow the packet.

Sterilising: if the processing time is 10 minutes or more, the jars are sterilised by the process itself and do not need separate sterilising. For anything processed under 10 minutes, sterilise the jars first by boiling them for 10 minutes.

Filling: headspace and air bubbles

Headspace is the gap between the food and the rim, and it is a real specification, not an approximation.

Too little headspace and the contents expand during processing, force their way past the seal, and the jar fails to seal or seals over food residue. Too much and there is more air than the process can drive out, which leaves oxygen in the jar and causes discolouration and poor keeping.

1. Fill to the stated headspace using a funnel.
2. Remove air bubbles. Run a plastic tool down the inside of the jar in several places and turn the jar. Trapped air pockets change how heat moves through the jar and leave oxygen behind. Do not use metal against glass.
3. Re-check the headspace after debubbling and top up if it has dropped.
4. Wipe the rim. Thoroughly, with a clean damp cloth, all the way round. A trace of jam or brine on the rim is the single commonest cause of a failed seal.
5. Fit the disc, then the band, and tighten to fingertip tight only. Snug, using fingertips rather than a full grip.

Processing

1. Fill the canner with enough hot water to cover the jars by 25 to 50 mm once they are in, and heat it.
2. Lower the jars in on the rack, upright, not touching each other or the sides.
3. Top up with boiling water if needed to get 25 to 50 mm of cover. Cover matters: an exposed jar top is not being processed.
4. Bring to a full rolling boil, then start timing. Not before. Timing from when you put the jars in, or from when it looks like it is simmering, is a real and common error.
5. Keep it at a full boil for the whole time. If it drops off the boil, bring it back and start the timing again from zero.
6. Adjust for altitude if you are above 1,000 feet. The figures are below.
7. Turn the heat off, remove the lid, and wait 5 minutes before lifting the jars. This lets the pressure and temperature equalise and greatly reduces siphoning, where liquid is drawn out of the jars.
8. Lift the jars out straight up, keeping them upright, and set them on a towel or board with a gap between each.

Altitude, because water boils cooler the higher you are, so the same time at the boil delivers less heat. Add this to whatever the recipe says:

In Britain this is a footnote rather than a rule. Even Flash in Staffordshire, the highest village in the country, is about 1,518 ft, so the worst case for a British kitchen is the 5 minute band. Upland Wales, the Pennines, the Peak District and parts of the Highlands are the places to check. If you are unsure of your elevation, look it up once and write it in the front of whichever preserving book you use.

Cooling and the seal check

Leave them alone for 12 to 24 hours. Undisturbed, out of a draught, not moved, not prodded, not turned over.

You will hear the lids ping as they seal. That is satisfying and it is not the test.

After 12 to 24 hours, check every jar, three ways:

1. Look. The lid should be visibly concave, dished downward in the centre.
2. Press. Press the centre of the lid firmly. It should not move at all. If it flexes down and springs back, it has not sealed.
3. Lift. Remove the band entirely and lift the jar an inch by the edge of the lid alone. A sealed lid holds the full weight of the jar.

The lift test is the definitive one and the one people skip.

Then remove the bands for storage. This is deliberate and worth doing:

• A band left on can hold a failed lid in place, so a jar that has lost its seal looks fine and you find out by eating it.
• Moisture trapped under a band rusts it onto the jar.

Jars that did not seal: perfectly good food. Put them in the fridge and eat them within a few days, or freeze them, or reprocess within 24 hours with a new disc, a re-wiped rim, and the full process time again.

Storing

• Cool, dark and dry. Warmth degrades quality and colour; light bleaches; damp rusts lids.
• Label every jar with contents and date. Every one. You will not remember, and an unlabelled jar of something red is a jar you will eventually throw away.
• Do not stack jars directly on each other. The weight can break a seal.
• Best quality within about a year, though a properly sealed jar remains safe well beyond that. Rotate.
• Check the seals occasionally, especially on anything more than a year old.

What goes wrong

Anything that has lost its seal in storage is not safe, whatever it looks like. Read recognising canning failure before you make any judgement about a suspect jar, and in particular before you smell or taste anything.