Early August is peak tomato season across most of Canada, and if your plants are producing faster than your kitchen can keep up, water-bath canning is the most accessible way to preserve the harvest without a pressure canner. Search interest in home canning reliably spikes every summer as gardeners and farmers’-market shoppers look for a safe way to put up ripe tomatoes before they spoil. Done correctly, canned tomatoes keep for up to a year in a cool, dark pantry. Done incorrectly, low-acid canning is one of the few DIY kitchen tasks with a genuine safety risk (botulism), so every step below follows the tested method published by Bernardin, Canada’s home-canning brand, along with standard extension-service guidance on altitude adjustments.
1. Gather the right equipment
You’ll need a boiling water canner (a large pot with a rack works), mason jars in 500 ml, 750 ml, or 1 L sizes, new snap lids, screw bands, a jar lifter, and a non-metallic utensil for removing air bubbles. Reused screw bands are fine; lids should be new each time to guarantee a seal.
2. Prep your jars — but don’t preheat the lids
Wash jars in hot, soapy water and keep them hot in a simmering water bath (82°C/180°F) until ready to fill. Bernardin specifically advises against preheating the lids: the sealing compound performs better at room temperature, and preheating can actually reduce the vacuum seal you get later. Just wash lids in hot soapy water and set them aside.
3. Prep the tomatoes
Wash and blanch the tomatoes to loosen the skins, then slip the skins off and remove the cores and any bruised or discoloured spots. Leave them whole or halve them. Simmer the prepped tomatoes in tomato juice (enough to cover) for 5 minutes in a stainless steel pot.
4. Acidify every jar — this step is not optional
Tomatoes don’t reliably have enough natural acidity to can safely in a boiling water bath, so each jar needs added acid before you add anything else. Per Bernardin’s tested recipe: for a 500 ml jar, add 1 tablespoon (15 ml) bottled lemon juice or ¼ teaspoon (1 ml) citric acid; for a 1 L jar, use 2 tablespoons (30 ml) lemon juice or ½ teaspoon (2 ml) citric acid. Salt is optional and purely for taste. Never substitute fresh lemon juice — bottled juice has a standardized, verified acidity level that fresh juice doesn’t guarantee.
5. Fill, seal, and load the canner
Pack tomatoes into the hot jar to within ¾ inch (2 cm) of the rim, then add hot tomato juice to cover, leaving ½ inch (1 cm) of headspace. Run a non-metallic tool around the inside of the jar to release trapped air, wipe the rim clean of any residue, centre a lid on top, and screw the band down to fingertip-tight — not wrench-tight. Lower jars into the canner, keeping them covered by at least 1 inch (2.5 cm) of water.
6. Process for the full time — and adjust for altitude
Bring the canner to a full rolling boil before you start timing, then process for 85 minutes regardless of jar size (500 ml, 750 ml, or 1 L) if you’re at an elevation up to 1,000 feet. Higher up, add time: 5 extra minutes between 1,001–3,000 feet, 10 minutes between 3,001–6,000 feet, and 15 minutes between 6,001–8,000 feet, per standard extension-service altitude charts. Thinner air means water boils at a lower temperature, so the extra time compensates.
7. Cool, check, and store
When time is up, turn off the heat, remove the lid, and let jars sit 5 minutes before lifting them out upright onto a towel or cutting board. Let them cool undisturbed for 24 hours without retightening the bands. Once cooled, check that lids have sealed (they should curve downward and not flex when pressed); refrigerate and use within a few days any jar that didn’t seal. Store sealed jars in a cool, dark place and use within a year for best quality.
Sources: Bernardin Home Canning, “Whole or Halved Tomatoes” (bernardin.ca); standard extension-service altitude adjustment guidance for boiling-water canning (SDSU Extension, University of Minnesota Extension).
