Canning is not difficult. It is, however, one of the few things in the kitchen where the margin for improvisation is zero — and most canning guides are written as though everyone lives at sea level and nobody ever learned a method from their grandmother.
This one covers both. Two decisions determine whether a jar is safe: which method the food needs, and what your elevation is. Get those right and the rest is procedure.
In this guide
- Find your altitude first
- Which method your food needs
- Equipment
- Water bath, step by step
- Pressure canning, step by step
- Headspace and sealing
- Cooling, labelling, storage
- Safety and the outdated methods
- Where to start
Find your altitude first
This is the step almost every canning article skips, and it’s the one that quietly ruins batches.
Water boils cooler as you go up — roughly 2 °F for every 1,000 feet. At sea level it’s 212 °F. At 5,000 feet it’s nearer 202 °F. If you process at the recipe’s sea-level time anyway, the food spends the same minutes at a lower temperature than the recipe was validated for. The jar seals, it looks perfect, and it is under-processed.
So look up your elevation now, before you do anything else, and write it on a card taped inside a cupboard door. Then use these two tables for every batch, forever.
Boiling water bath — add to the recipe’s processing time
| Your altitude | Add this much time |
|---|---|
| 0–1,000 ft | None — use the recipe as written |
| 1,001–3,000 ft | +5 minutes |
| 3,001–6,000 ft | +10 minutes |
| 6,001–8,000 ft | +15 minutes |
| 8,001–10,000 ft | +20 minutes |
Pressure canning — the time stays the same, the pressure changes
| Your altitude | Dial gauge | Weighted gauge |
|---|---|---|
| 0–1,000 ft | 11 PSI | 10 lb |
| 1,001–2,000 ft | 11 PSI | 15 lb |
| 2,001–4,000 ft | 12 PSI | 15 lb |
| 4,001–6,000 ft | 13 PSI | 15 lb |
| 6,001–8,000 ft | 14 PSI | 15 lb |
| 8,001–10,000 ft | 15 PSI | 15 lb |
If you live above 1,000 feet, a weighted gauge is the easier tool. The rule is simply 15 lb and you’re done — no band to look up, no gauge to read, no gauge to get tested. Dial gauges give you finer control and are perfectly safe, but they add a step and a failure point.
Which method your food needs
There is one number behind this entire decision: pH 4.6.
Clostridium botulinum cannot grow in an environment more acidic than pH 4.6. Above that threshold it can, and its spores survive boiling water indefinitely — which is why “I boiled it for ages” is not a safety argument. Killing those spores takes 240 °F, and the only way a home kitchen reaches 240 °F is under pressure.
| Water bath canning | Pressure canning | |
|---|---|---|
| For | High-acid foods, pH 4.6 or below | Low-acid foods, pH above 4.6 |
| Reaches | 212 °F (less at altitude) | 240 °F and above |
| Foods | Fruits, jams, jellies, pickles, applesauce, chutneys, relishes, vinegar-based salsas, acidified tomatoes | Meat, poultry, fish, beans, corn, carrots, potatoes, green beans, soups, broths, mixed vegetable dishes |
| Why it works | Acidity blocks botulinum. Boiling handles mould, yeast and the bacteria that remain | Only 240 °F destroys botulinum spores. Nothing else does |
Common foods and their pH
| Food | pH range | Method |
|---|---|---|
| Apples, berries | 3.0–3.6 | Water bath |
| Peaches, plums | 3.2–4.0 | Water bath |
| Tomatoes (acidified — see below) | 4.3–4.9 | Water bath or pressure |
| Green beans | 5.6–6.5 | Pressure |
| Carrots | 5.8–6.3 | Pressure |
| Corn | 6.0–6.5 | Pressure |
| Potatoes | Low-acid | Pressure |
| Chicken, beef | 5.5–6.5 | Pressure |
When in doubt, pressure can it. There is no food that is unsafe to pressure can and plenty that are unsafe to water bath.
Tomatoes always need added acid. Every time, every variety.
Look at the table: tomatoes run 4.3 to 4.9, straddling the 4.6 line. Some are safely acidic, some are not, and you cannot tell by tasting — sweetness masks acidity, and modern low-acid varieties taste no different. Vine-ripened and overripe fruit tends lower in acid, not higher.
So acidify every jar, whether you’re water bathing or pressure canning:
- Pints: 1 tablespoon bottled lemon juice, or ¼ teaspoon citric acid
- Quarts: 2 tablespoons bottled lemon juice, or ½ teaspoon citric acid
Bottled lemon juice, not fresh. Fresh lemons vary in acidity from fruit to fruit; bottled juice is standardised, which is the entire point. Add it to the jar, then fill.
Equipment
For both methods
- Canning jars — glass, no chips or cracks, especially around the rim. Run a finger round the rim of every jar; a chip you can feel will not seal
- Two-piece lids — a flat lid with a sealing compound, plus a threaded band
- Jar lifter — not tongs. Tongs drop jars
- Canning funnel — essential for narrow-mouth jars, useful for all of them
- Bubble remover — plastic or wood. A chopstick works. Not metal, which can scratch the glass
- Ladle, clean towels, timer, labels
Water bath specifically
- A large stockpot with a rack, deep enough to hold at least 1 inch of water above the jar tops with room to boil. Jars must not sit directly on the pot bottom
Pressure canning specifically
- A pressure canner with a dial or weighted gauge. Not a pressure cooker and not an Instant Pot — those are not tested or rated for canning, regardless of what the box says about a canning setting
Useful extras
- Magnetic lid lifter, food mill or strainer for sauces, wide-mouth jars for bulky foods
About sterilising jars — the advice changed, and most sites haven’t caught up.
If your process time is 10 minutes or more, you do not need to pre-sterilise. Wash the jars, rinse them, keep them hot, and the processing sterilises them. That’s most of what you’ll ever can.
If the process time is under 10 minutes — some jellies run 5 — then you do need to pre-sterilise: boil the jars 10 minutes at under 1,000 feet, plus 1 minute for every additional 1,000 feet. At 5,000 feet that’s 15 minutes. Or sidestep it entirely by processing for 10 minutes plus your altitude adjustment instead.
Simmering jars at 180 °F, which you’ll see recommended constantly, keeps them hot. It does not sterilise them.
Water bath canning, step by step
- Fill the canner and start it heating. It takes a long time to bring that much water to a boil, and you want it ready when the jars are.
- Prepare the produce. Fresh and unblemished only — canning preserves what you put in it, it doesn’t improve it. Wash, peel and chop as the recipe directs.
- Wash the jars and keep them hot. Pre-sterilise only if your process time is under 10 minutes, as above.
- Make the recipe, and have the food hot when it goes in the jar.
- Fill, leaving the right headspace — see the table below. Add acid to tomatoes before filling.
- Release trapped air. Run the bubble remover around the inside of the jar.
- Wipe the rim with a clean damp cloth. A smear of jam on the rim is the most common cause of a failed seal.
- Lid and band, fingertip tight. Turn the band until it meets resistance, then just snug with your fingertips. No force.
- Lower the jars in with the lifter. Water must cover the tops by at least an inch. Bring back to a full rolling boil.
- Start timing when the water returns to a rolling boil — not when the jars go in. Process for the recipe time plus your altitude adjustment. Keep it boiling the whole time; if it stops, bring it back and start the timing over.
- Turn off the heat, wait 5 minutes, then lift the jars out straight up and set them on a towel or rack with space between them.
Pressure canning, step by step
Same beginning, and then one step that people skip and absolutely should not.
- Prepare food and jars as above, using a tested recipe written for pressure canning.
- Put 2 to 3 inches of water in the canner, or whatever your canner’s manual specifies. Unlike a water bath, the jars are not submerged.
- Load the jars on the rack and lock the lid.
- Vent steam for a full 10 minutes. Leave the vent open and let steam pour out steadily for ten minutes by the clock before you close it. Do not skip this. Trapped air makes the gauge read a pressure the contents never actually reach — the dial says 13 PSI, the jars are nowhere near it, and everything looks completely normal. This is the single most common way a pressure-canned batch goes wrong.
- Close the vent and let pressure build to your altitude’s figure from the table above.
- Start timing once you hit pressure. Hold it steady for the full recipe time — adjust the burner, don’t let it swing. A drop below pressure means bringing it back up and starting the time again from zero.
- Turn off the heat and leave it alone. Let the pressure fall to zero on its own. Never run it under cold water or force the vent — you’ll get siphoning, broken seals, and possibly a face full of steam.
- Wait 10 minutes after it reaches zero, then open the lid away from you, and lift the jars out.
Dial gauges need testing once a year. A dial that reads 2 PSI high is invisible — it looks correct and under-processes every batch all season. Most county extension offices test them free or nearly free. Weighted gauges don’t drift and don’t need testing, which is one more reason they’re the simpler choice.
Headspace and sealing
Headspace is the gap between the food and the rim, and it isn’t a rough guide. Too little and food gets forced under the lid as it expands, which siphons liquid out and breaks the seal. Too much and there isn’t enough air driven out to pull a strong vacuum.
| What you’re canning | Headspace |
|---|---|
| Jams and jellies | ¼ inch |
| Fruits | ½ inch |
| Tomatoes | ½ inch |
| Pickles | ½ inch |
| Low-acid vegetables and meats | 1 to 1½ inches |
Note how much more room low-acid foods need. If you’ve only ever made jam, a half-inch habit will carry over and be wrong for green beans.
Cooling, checking, labelling, storing
- Cool undisturbed for 12 to 24 hours. Don’t poke the lids, don’t move the jars, and don’t retighten the bands — the lid needs to flex and let air escape as it cools. Tightening now prevents sealing.
- Don’t stack jars while they cool, and keep them out of a draft. Cold air on hot glass cracks it.
- Check the seal at 24 hours. Press the centre of the lid. If it flexes and pops back, it hasn’t sealed. If it’s firm and concave, it has.
- An unsealed jar isn’t a disaster — refrigerate and eat it within a few days, or reprocess within 24 hours with a new flat lid.
- Take the bands off for storage. A band left on can hold a failed lid in place and hide a broken seal, and it rusts onto the jar.
- Label every jar — contents, date, and a batch number if you made more than one that day. Future you will not remember.
- Store cool, dark and dry. Light degrades colour and nutrients; heat shortens shelf life and can stress seals.
How long does it keep?
12 to 18 months for best quality. Properly sealed jars often stay safe well beyond that, but texture, colour and nutrition decline. Rotate — oldest first — and if you’re still working through last year’s green beans when this year’s come in, can less.
Safety, and the methods your grandmother used
Use tested recipes. This is the whole game.
A canning recipe isn’t a suggestion the way a soup recipe is. The processing time was determined by testing how heat penetrates that specific food, at that density, in that jar size. Change the ratios and the number no longer applies.
So: don’t reduce the vinegar in pickles, don’t cut the sugar in jam to make it healthier, don’t swap jar sizes, don’t thicken with flour or cornstarch, and don’t add extra onions or peppers to a tested salsa. Get recipes from the National Center for Home Food Preservation, a university extension service, or a current edition of a reputable canning book.
About the way it used to be done.
If you learned canning from someone who inverted the jars to make them seal, sealed jam with paraffin wax, did open-kettle canning with no processing at all, or water bathed green beans — she wasn’t being reckless. That was the standard advice at the time, and the evidence changed.
Here’s the uncomfortable part, and it’s why these practices persist: unsafe canning usually gets away with it. Botulism is rare. A family can water bath low-acid food for forty years and never have an incident, and that experience feels like proof. It isn’t — it’s a low-probability event that eventually isn’t. The reason the rules tightened is that when it does happen, it is catastrophic and the food gave no warning at all.
Not currently considered safe, whatever anyone’s experience:
- Open-kettle canning — filling hot jars and letting them seal without processing
- Inverting jars to force a seal
- Paraffin wax over jams and jellies
- Oven canning — dry heat doesn’t penetrate the way water does, and jars can shatter
- Dishwasher canning
- Water bathing low-acid food for any length of time
- Steam canners for low-acid food — they only reach boiling temperature, same as a water bath
- Reusing flat lids. Bands yes, flat lids never — the sealing compound only works once
If you suspect a jar
Signs: a bulging or unsealed lid, leaking, spurting liquid when opened, cloudiness, off colour, mould, or any unusual smell.
Do not taste it. Not even a little, not even to check. Botulinum toxin frequently gives no off taste or smell at all, and the amount that causes illness is extremely small. Treat any suspect low-acid food — including tomatoes — as though it contains toxin, even when it looks fine.
Disposing of it properly
- Still sealed? Put the whole unopened jar in a heavy rubbish bag, seal the bag, and put it in the bin or take it to a landfill.
- Unsealed, open or leaking? Detoxify it first. Wearing disposable gloves, lay the jars and lids on their sides in a large pot — 8 quarts or bigger — add water carefully to cover them by at least 1 inch, cover the pot, bring to a boil and boil for 30 minutes. Cool, then bin the lot.
- Clean up after. Make a fresh solution of 1 part unscented household chlorine bleach to 5 parts water. Wet every surface, utensil and anything else the food touched, leave 30 minutes, wipe up with paper towels (bag those and bin them), apply again, leave another 30 minutes, then rinse and wash thoroughly. Discard the gloves afterwards.
- Keep animals and children away from anything you’re disposing of.
Where to start
Pick one of these, make a single small batch, and don’t try to preserve an entire harvest on your first day.
- Strawberry jam (water bath) — short, forgiving, and you find out quickly whether you enjoy this
- Pickled cucumbers (water bath) — the vinegar does the acid work for you
- Green beans (pressure) — the classic first pressure-canning project
- Chicken broth (pressure) — if you keep backyard chickens, this is the one that pays for the canner
Then soups, stews, sauces and whole-harvest storage. Canning isn’t the only answer either — drying and a root cellar handle things canning handles badly, and between the three you can put away most of what a garden produces.
More food preservation
- How to dry and store herbs
- Building a DIY root cellar
- Herbal vinegars for cooking and cleaning
- Beeswax wraps for food storage
- Using kitchen scraps in the garden
Common questions
Do I really need to adjust for altitude?
Yes, above 1,000 feet. Water boils about 2 °F cooler per 1,000 feet, so processing at the sea-level time means the food never reaches the temperature the recipe was validated at. For high-acid foods that mostly costs you quality; for low-acid foods it’s the exact failure mode that causes botulism.
Can I water bath green beans if I process them for a really long time?
No. Botulinum spores survive boiling water indefinitely — more time at 212 °F doesn’t help, because the temperature is what matters, not the duration. Low-acid food needs 240 °F, and only a pressure canner gets there.
Is an Instant Pot a pressure canner?
No, even the models with a canning button. They aren’t tested or rated for canning, they don’t hold pressure the way a canner does, and the USDA doesn’t recognise them for it. Use a real pressure canner for low-acid food.
Do I have to sterilise jars first?
Only if your process time is under 10 minutes. At 10 minutes or more, washing and keeping them hot is enough — the processing does the sterilising. If you do need to, boil them 10 minutes plus 1 minute per 1,000 feet of elevation.
Why bottled lemon juice instead of fresh?
Because fresh lemons vary in acidity from fruit to fruit and season to season, and bottled juice is standardised to a known level. When the acid is doing the safety work, you need a number you can rely on.
My jar didn’t seal. Is the food ruined?
Not at all. Refrigerate it and eat it within a few days, or reprocess within 24 hours using a brand new flat lid. Check the rim for a chip or a smear while you’re at it — that’s usually the culprit.
Can I reuse the lids?
Bands yes, as long as they aren’t rusty or bent. Flat lids no — the sealing compound is designed to work once and won’t seal reliably a second time.
Should I leave the bands on in storage?
No. A band can hold a failed lid in place and hide a broken seal, which is the opposite of what you want. It also rusts onto the jar. Take them off, wash them, store them separately.
Can I halve the sugar in a jam recipe?
Not in a tested recipe without changing its safety profile and its set. If you want lower sugar, use a recipe that was tested at that sugar level, or one written for low-sugar pectin.
How can I tell if home-canned food has gone bad?
Bulging or unsealed lids, leaking, liquid spurting when you open it, cloudiness, mould, off colour or smell. But understand the limit of that list — botulinum toxin often produces no detectable sign at all, which is why the processing method matters more than your inspection. Never taste to check.
Sources
- National Center for Home Food Preservation, University of Georgia. Do I need to pre-sterilize my jars before canning?
- National Center for Home Food Preservation. Canning Tomatoes and Tomato Products — acidification amounts.
- Penn State Extension. Time, Temperature and Pressure in Canning Foods.
- Penn State Extension. Let’s Preserve: Basics of Home Canning — headspace by food type.
- Utah State University Extension. Utah Altitude Adjustments for Home Canning.
- Iowa State University Extension. Safely disposing of unsafe home canned goods.
General guidance for home canners. Always follow a current tested recipe from NCHFP or your local extension service for the specific food you’re preserving.
