Water-Bath Canning: A Step-by-Step Guide for Home Preservers
Water-bath canning is the method for preserving high-acid foods in sealed glass jars using a pot of boiling water. It works by heating the jar contents to a temperature that destroys moulds, yeasts, and most bacteria, while the vacuum seal that forms on cooling keeps pathogens out. It does not work — and should not be used — for low-acid foods such as green beans, corn, or meat. For those, a pressure canner that reaches 240°F (116°C) is required.
The following guide covers the full process from equipment selection through to storage. Processing times and headspace measurements come from USDA-tested recipes. If you are working from a family recipe that predates current safety standards, check it against the USDA National Center for Home Food Preservation before proceeding.
What can be processed in a boiling-water bath
The determining factor is acidity. Foods with a pH at or below 4.6 are considered high-acid and can be safely processed at 212°F (100°C) — the temperature of boiling water at sea level. Foods above that threshold require pressure canning.
- High-acid (suitable for water-bath): most fruits, jams and jellies, fruit butters, pickles made with vinegar (5% acidity or higher), tomatoes with added acid, salsas made to tested recipes
- Low-acid (pressure canning required): vegetables, beans, corn, meat, poultry, fish, soups, and tomatoes without added acid
Tomatoes sit near the 4.6 pH boundary, which is why all tested tomato canning recipes include either bottled lemon juice (two tablespoons per 1 L jar) or citric acid (half a teaspoon per 1 L jar). Fresh lemon juice is not a substitute because its acidity varies. This addition is not optional.
Equipment you need
You do not need a purpose-built water-bath canner. Any large pot deep enough to keep jars covered by at least 2.5 cm (1 inch) of water during processing will work. A rack to keep jars off the pot floor is necessary — the bottom of the pot gets hotter than the water and can crack jars. A canning rack, a round cake rack, or folded towels will do.
- Large pot with lid, minimum 30 cm deep
- Jar rack or equivalent
- Mason-style canning jars with new lids and bands (lids are single-use; bands can be reused)
- Jar lifter
- Canning funnel (strongly recommended)
- Bubble remover or thin plastic spatula
- Clean cloth for wiping jar rims
- Kitchen timer
Jars entering and exiting a boiling-water bath. The jar lifter keeps hands clear of 100°C water. (NARA public domain)
Jar preparation
Inspect jars for chips, cracks, or nicks on the rim — any defect will prevent a good seal. Wash jars, lids, and bands in hot soapy water and rinse thoroughly. Jars should be kept hot until filling to prevent thermal shock when hot food is added. The easiest method is to keep them submerged in the simmering canner water while you prepare the food.
Lids do not need to be boiled. Modern snap lids (the flat disc with sealing compound) just need to be clean; excessive heating can soften the sealing compound and reduce its effectiveness.
Filling jars and headspace
Headspace is the gap between the food surface and the top of the jar rim. The correct headspace for a given recipe is not arbitrary — it accounts for food expansion during processing and is part of what creates the vacuum seal as the jar cools. Using too little headspace can cause food to be forced under the lid, preventing sealing. Too much headspace leaves excess air that reduces shelf life.
- Jams and jellies: 6 mm (¼ inch)
- Fruits and tomatoes: 1.25 cm (½ inch)
- Pickles: 1.25 cm (½ inch)
After filling, run a thin spatula around the inside of the jar to release air bubbles trapped in the food. Wipe the jar rim with a clean, damp cloth — any residue will interfere with the seal. Apply the lid and band to fingertip tightness. Do not overtighten; the band needs to allow air to escape during processing.
Processing
Lower jars into the canner using a jar lifter. The water must be at a full rolling boil before the timer starts. Cover the pot and maintain the boil for the full processing time. For most tomato products, a full litre jar requires 85 minutes at sea level. Processing times are longer at altitude because water boils at a lower temperature as elevation increases.
Altitude adjustments for British Columbia, Alberta, and the Yukon — where many areas exceed 300 m — are required. Add one minute per 300 m above sea level for processing times under 20 minutes; for longer times, follow the USDA altitude adjustment table.
When a water-bath canner is not enough
Low-acid foods must be processed at 240°F (116°C). Boiling water at sea level reaches only 212°F (100°C). A pressure canner creates a sealed steam environment that raises the temperature above boiling. The risk with low-acid foods processed only in boiling water is Clostridium botulinum spores, which survive at 212°F but are destroyed at 240°F. The resulting toxin is colourless and odourless — there is no way to detect it without laboratory testing.
If you are canning green beans, corn, beets without vinegar, carrots, or any other low-acid vegetable, use a pressure canner and follow tested processing times. This is not a conservative interpretation — it is the minimum safe practice.
Loading vegetables into a pressure canner. Low-acid foods require pressure processing to eliminate botulism risk. (NARA public domain)
After processing: seals, cooling, and storage
Remove jars from the canner without tilting them and set them on a towel-covered surface, at least 2 cm apart, to cool. Do not push down on the centre of lids while they cool — the sealing pop will happen on its own as the jar contents contract. Leave undisturbed for 12–24 hours.
After cooling, test seals by pressing the lid centre. A properly sealed lid will not flex. Remove the bands and try lifting the lid by its edges; a sealed lid will hold. Any jar that does not seal should be refrigerated and used within a few days, or reprocessed within 24 hours with a new lid.
Label jars with contents and date. Store in a cool (below 18°C), dark location. Most canned goods retain best quality for 12–18 months, though sealed jars are safe beyond that if stored properly and show no signs of spoilage on opening.
Signs of spoilage
Before opening any home-canned jar, examine it. Discard without opening if the lid is bulging, the band has rusted through, or the seal appears broken. On opening, discard if there is spurting liquid, unusual smell, or visible mould. If botulism is suspected, do not taste the product and do not discard it in a way that could expose others (including pets) — double-bag and seal in a garbage bin.