Sterilising Jars for Jam and Preserves: What Works in an Irish Kitchen
Oct 09, 2026
Sterilising Jars for Jam and Preserves: What Works in an Irish Kitchen
Wash the jars in hot soapy water, rinse, then heat them: 15 to 20 minutes in the oven at 140°C (120°C fan), or a full hot dishwasher cycle, or ten minutes at a rolling boil in a pan. Fill them while they are still hot, seal immediately, and leave them alone to cool.
That is the whole job. The part that actually goes wrong is not the method — it is the timing, and almost nobody writes about it.
There is a second thing worth knowing before you start, because it decides how careful you need to be: the way most people in Ireland and Britain make jam depends on sterile jars in a way the American method does not. More on that below, and it is the reason a half-hearted rinse is riskier here than the recipe books suggest.
The three methods that work
All three get the glass hot enough for long enough. Pick on convenience and on how many jars you are doing, not on some notion that one is more thorough.
| Method | How | Best for |
|---|---|---|
| Oven | Washed jars upright on a baking tray, no lids, 140°C conventional or 120°C fan for 15–20 minutes. Leave the door shut and the heat off until you need them. | The default. It holds the jars hot and dry until the jam is ready, which is exactly what you want. |
| Dishwasher | A full hot cycle, 60°C or above, finishing as the jam reaches setting point. A rinse-only or eco cycle does not get hot enough. | Big batches. Twelve jars at once with no juggling of trays. |
| Boiling water | Jars fully submerged in a deep pan, brought to a rolling boil for 10 minutes. Lift out with tongs, drain, use straight away. | Clip-top jars with rubber seals, and anything you would rather not put in an oven. The only method that also does the seals. |
A small Irish detail on the ten minutes. American preserving guides tell you to add a minute of boiling time for every 1,000 feet above sea level, because water boils cooler as you climb. Meelin in Co. Cork, generally reckoned the highest village in the country, sits at roughly 290 m — right on that first 1,000 ft line. Which means that for every kitchen in Ireland, ten minutes is simply ten minutes. You can ignore the altitude tables entirely.
Lids and seals need their own treatment. Do not put rubber rings, plastic lids or the flat discs of two-part lids in the oven — dry heat perishes the sealing compound and you lose the seal you are relying on. Boil them for ten minutes in a separate pan, or pour boiling water over them in a bowl and leave them five minutes.
And the one to skip
The microwave. You will find it suggested online — a splash of water in each jar, two minutes on full — and it is a bad idea on two counts. Heating is uneven, so parts of the jar never reach temperature while others get hot enough to crack; and any jar with a trace of metal in the decoration or a metal lid left on is a genuine hazard. There is no version of this that is faster or safer than the oven.
Timing is the actual skill
Here is where most batches go wrong, and it has nothing to do with how clean the jars were.
Hot jam poured into a cold jar can crack it — thermal shock, and it happens at the base where the glass is thickest. Cool jam into a hot jar is safer but sets badly and traps condensation under the lid, which is where mould starts. What you want is hot jam into hot jars, both arriving at the same moment.
So the jars go in the oven roughly when the fruit goes in the pan, and they stay in there with the heat off until the jam hits setting point. You are not sterilising for forty minutes — you sterilised in the first twenty and you are simply holding them warm after that.
Setting point is 104–105°C on a sugar thermometer. Without one, use the cold saucer test: a teaspoon of jam onto a plate chilled in the freezer, left a minute, then push it with a fingertip. If the surface wrinkles, it is done. Take the pan off the heat while you test, or you will overshoot.
Then fill to about 5 mm from the rim, wipe the rim clean, and put the lid on straight away while everything is hot. The seal forms as the contents cool and contract — that is the ping you hear from the kitchen twenty minutes later. Leave the jars untouched until they are cold.
Do you even need to pre-sterilise? It depends which method you are using
This is the bit that gets skipped, and it is the difference between “be reasonably careful” and “this step is load-bearing”.
There are two traditions. In the American one, filled jars go into a boiling-water canner and are processed for ten minutes or more. That processing sterilises the jar along with its contents, so the National Center for Home Food Preservation is clear that pre-sterilising is redundant for anything processed that long.
The Irish and British tradition is different. We fill hot and seal, and that is it — no water bath afterwards. It works because jam is a hostile place to live: high sugar, high acid, very little free water. But it works only if the jar was already sterile, because nothing after the fill is going to make it so.
So the short version: if you are filling and sealing the way most Irish recipes describe, pre-sterilising is not a precaution you can shrug off — it is the only sterilising step in the whole process. And it only holds for proper high-sugar jams, marmalades and jellies. Low-sugar or “reduced sugar” preserves, chutneys, anything with vegetables, and anything you intend to keep outside the fridge need a different approach; the Food Safety Authority of Ireland is the place to check before you improvise.
If you are new to it and want a straightforward recipe to practise the timing on, ILoveCooking’s easy jam is a sensible starting point — small batch, nothing exotic, hard to ruin.
What Irish hard water does to a clean jar
Here is something specific to making preserves here rather than anywhere else.
Much of Ireland sits on limestone, and limestone means dissolved calcium and magnesium in the water. When water carrying those minerals evaporates off glass, the minerals stay behind as a fine white film. On a drinking glass you notice it and move on. On a jar of pale jelly it is more annoying: a cloudy haze on the inside of the glass that looks, to anyone you give the jar to, exactly like the jar was not properly washed.
It shows worst on the things where clarity is the whole point — crab apple jelly, quince, elderflower, redcurrant. A dark damson jam hides it completely.
| Your supply reads | Classified as | What to do about it |
|---|---|---|
| Under 100 mg/l | Soft | Nothing. Air-dry the jars if you like. |
| 100–200 mg/l | Moderately hard | Drain upside down, then finish in the oven. A faint film otherwise. |
| 200–300 mg/l | Hard | Oven-dry, never air-dry. Use filtered water for jellies. |
| Over 300 mg/l | Very hard | Oven-dry, and expect visible scale in the preserving pan too. |
Find your own number. Uisce Éireann lets you check the hardness of your supply zone by Eircode. It takes a minute, and it is worth knowing for the kettle and the taps as much as for the jam — our guide to brass taps goes into what the same water does to a tap finish.
Three practical rules follow from all that:
- Do not air-dry sterilised jars in a hard water area. Invert them to drain for a minute, then let the residual oven heat take the last of it.
- Never towel-dry. Even a clean tea towel puts lint and handling back into a jar you have just sterilised. It undoes the step.
- For jellies, consider the water itself. If clarity matters and your supply is hard, the minerals are not only on the glass — they are in the liquid you added.
The kit that genuinely helps
We sell taps, not preserving equipment, so take this as a list rather than a shop window — most of it turns up in Irish cookshops and hardware shops for very little.
- A wide, heavy-based pan. A proper maslin or preserving pan is wide and shallow so water evaporates fast, and heavy enough that sugar does not catch. A stockpot is the wrong shape and jam made in one takes far longer to set.
- A sugar thermometer. The saucer test works, but a thermometer turns setting point from a judgement into a number.
- A jar funnel. Wide-mouthed, sits in the neck of the jar. Keeps the rim clean, and a clean rim is most of a good seal.
- Jar tongs or a lifter. If you are using the boiling-water method you need some way to get a hot wet jar out of a pan that is not oven gloves.
- A jelly bag or muslin. Only for jellies, and never squeeze it — squeezing is what makes jelly cloudy.
- Labels, and the date. Write what it is and when you made it. You will not remember in February.
What you do not need: a dedicated steriliser appliance. The oven you already own does the same job.
Where a boiling water tap actually helps — and where it does not
Since this is our end of the kitchen, let us be straight about it.
A boiling water tap does not sterilise jars. It delivers water at around 98–100°C, which is hot, but sterilising needs the jar held at temperature for a sustained period — twenty minutes in the oven, or ten at a rolling boil. A tank of a couple of litres will not fill a preserving pan either. Anyone telling you a tap replaces that step is selling you something.
What it does do is remove the kettle from a preserving session, and a preserving session asks for boiling water constantly:
- Loosening skins. Pour boiling water over tomatoes, plums or peaches, wait half a minute, and the skins slide off. This is the single most repetitive hot-water job in preserving.
- Scalding tools between batches — the funnel, the ladle, the thermometer.
- Topping up a water bath that has dropped below the boil without stalling it.
- The washing up afterwards, which with set sugar is its own event.
So: a convenience during the two weekends a year you make jam, and genuinely useful the other fifty for tea, blanching and pasta. If you make jam twice a year and nothing else, the kettle is fine and we would rather tell you so — the honest case for one of these rests on everyday use, which is what our boiling water tap running costs guide works through with real electricity numbers.
One practical note before you order: all three of ours need 1 bar of pressure and a socket in the cupboard below. If your hot water comes from a cylinder in the hot press fed by an attic tank, check your pressure first — our water pressure guide shows you how with a bucket and a timer.
Instant Boiling Water Tap — 3 in 1 Chrome
- Boiling, hot and cold from one tap
- 2.4 L tank and filter included
- Needs 1 bar and a socket below
- The safe default finish
Instant Hot Water Tap — 3 in 1 Matt Black
- Same tank and filter, matt black finish
- Suits a dark or handleless kitchen
- Needs 1 bar and a socket below
- Shows water marks least of the three
Brushed Brass Boiling Tap — 3 in 1
- Warm satin brass rather than chrome
- 2.4 L tank and filter included
- Needs 1 bar and a socket below
- Brushed finish hides limescale spotting
All three come with a filter, which is the part that matters if your water is hard — see boiling water taps for the full detail, or kitchen taps and everything for the kitchen if a standard mixer is what you are after.
Frequently Asked Questions
Do I really need to sterilise jam jars?
Yes, if you are making jam the usual Irish way — filling hot jars and sealing them without a water bath afterwards. In that method the pre-sterilising is the only sterilising step there is. If instead you process the filled jars in boiling water for ten minutes or more, the processing does the job and pre-sterilising is redundant.
What temperature and how long should jars go in the oven?
140°C in a conventional oven or 120°C fan, for 15 to 20 minutes, washed and placed upright on a baking tray with no lids on. Turn the heat off afterwards and leave them inside so they stay hot until the jam is ready.
Can I sterilise jars in the dishwasher?
Yes, on a full hot cycle of 60°C or above. A rinse-only or eco cycle does not reach a high enough temperature. Time the cycle so it finishes as your jam reaches setting point, because the point is to have hot jars ready at the right moment.
Can I sterilise jars in the microwave?
Not reliably, and we would not. Microwave heating is uneven, so parts of the glass never reach temperature while others get hot enough to crack, and any metal in the lid or the decoration is a hazard. The oven is both safer and no slower.
Do the lids need sterilising too?
Yes, but not in the oven. Dry heat perishes rubber seals and the sealing compound in metal lids, which is the very thing holding your seal. Boil lids and rubber rings for ten minutes in a separate pan, or cover them with boiling water in a bowl for five minutes.
Should jars be hot or cold when I fill them?
Hot. Hot jam poured into a cold jar can crack it through thermal shock, usually at the thick base. Cool jam in a hot jar sets poorly and traps condensation under the lid, which is where mould starts. Aim to have both hot at the same moment.
Can I reuse shop-bought jars and lids?
The jars, yes, provided there are no chips and the rim is perfectly smooth — run a finger around it. Lids are the weak point: the flat disc of a two-part lid is single use because the sealing compound only compresses once, the screw band is reusable, and rubber rings on clip-top jars should be replaced. Twist-off lids from shop jars will do a few times if the liner is undamaged and not deformed, but they are not designed for it.
Why did my jam go mouldy on top?
Almost always the seal rather than the jam. The usual causes are a jar that was not sterile, jam left to cool before the lid went on, a rim that was not wiped clean, too much headspace, or not enough sugar to hold the preserve. Fill to about 5 mm from the rim and lid it immediately while hot.
Does hard water affect jam making?
It affects how the jars look more than how the jam keeps. Dissolved calcium and magnesium leave a fine white film on glass as water evaporates, which reads as a dirty jar through pale jellies. In a hard water area, drain jars upside down and finish them with the oven's residual heat rather than air-drying, and never towel-dry a sterilised jar.
Does a boiling water tap sterilise jars?
No. It delivers water at roughly 98 to 100°C, but sterilising requires the jar to be held at temperature for a sustained period — twenty minutes in an oven or ten at a rolling boil — and a tank of two or three litres will not fill a preserving pan. What a boiling tap genuinely saves is the repeated kettle boiling that a preserving session demands: loosening fruit skins, scalding tools and the washing up.
