If your tomato plants are producing faster than you can eat them, water-bath canning is the safest way to preserve a garden tomato harvest for winter — as long as you use a tested recipe and add acid to every jar. Mid-to-late August is peak tomato season across most of Canada, and “how to can tomatoes” is one of the most common preserving questions home gardeners search for at this time of year. Done correctly, canned tomatoes will keep safely in the pantry for up to a year. Done incorrectly, they carry a real risk of botulism, a rare but serious form of food poisoning. Here’s how to do it safely, step by step, using guidance from Bernardin, Canada’s leading home canning brand, and the Healthy Canning reference project.
1. Get the right equipment first
You’ll need a boiling water canner (a large pot with a rack works, as long as it’s deep enough to cover jars with at least one inch of water), 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. Note that tomatoes only require a boiling water canner, not a pressure canner — but the acid step below is what makes that safe.
2. Wash, blanch, and peel the tomatoes
Wash tomatoes and discard any with blemishes, cracks, or soft spots. Blanch them in boiling water for 30 to 60 seconds, then plunge into cold water — the skins will slip off easily. Remove the cores and any bruised or discoloured flesh, and leave the tomatoes whole or cut them in half.
3. Make a hot pack with tomato juice
Place the peeled tomatoes in a large stainless steel saucepan, add enough tomato juice (or crushed tomatoes) to cover them, bring to a boil, and boil gently for five minutes. This “hot pack” method helps the tomatoes hold their shape and colour in the jar.
4. Acidify every single jar — this step is not optional
This is the step people most often skip, and it’s the one that matters most for safety. Modern tomato varieties are bred to be sweeter and less acidic than older varieties, which means they can no longer be relied on to be acidic enough on their own to can safely in a boiling water bath. Add bottled lemon juice or citric acid to every jar before packing: 1 tablespoon (15 ml) of bottled lemon juice or ¼ teaspoon of citric acid per 500 ml jar, or 2 tablespoons (30 ml) of lemon juice or ½ teaspoon of citric acid per 1 L jar. Salt is optional and doesn’t affect safety. Never substitute fresh-squeezed lemon juice, since its acidity varies and can’t be relied on — use bottled juice, which has a standardized acid level.
5. Fill the jars and remove air bubbles
Pack the hot tomatoes into a hot jar (keep jars warm in simmering water beforehand so they don’t crack) to within 3/4 inch of the top rim, then add hot tomato juice to cover, leaving 1/2 inch of headspace. Slide a non-metallic tool around the inside of the jar to release trapped air bubbles, topping up with more tomatoes or liquid if the level drops. Wipe the jar rim clean of any residue, centre a lid on top, and screw the band down until fingertip tight — not forced.
6. Process in a boiling water bath for the full time
Lower the filled jars into the canner, making sure they’re covered by at least one inch of water, cover, and bring to a full rolling boil before you start timing. Process for 85 minutes, regardless of whether you’re using 500 ml, 750 ml, or 1 L jars, at altitudes up to 1,000 feet (add more time at higher elevations — check a current canning chart for your area). Don’t shorten the processing time to save time; it’s calculated to kill harmful bacteria all the way through the jar.
7. Cool, check the seals, and store
When the time is up, turn off the heat, remove the lid, and let the jars sit for five minutes before lifting them out (without tilting) onto a towel or cutting board. Let them cool undisturbed for 24 hours, and resist the urge to retighten the bands. Once cool, press the centre of each lid: if it’s concave and doesn’t flex, the jar sealed properly. Remove the bands, wipe the jars down, label them with the date, and store in a cool, dark place. For best quality, use home-canned tomatoes within a year.
A safety note on shortcuts
Skip any recipe or online tip that suggests oven canning, open-kettle canning without processing, or “just wing it” on the acid — these methods aren’t tested for safety and can leave jars vulnerable to botulism, which isn’t something you can see, smell, or taste. If a jar doesn’t seal, didn’t get the acid added, or shows any sign of spoilage (bulging lid, off smell, spurting liquid) when opened, discard it without tasting it. Stick to a source that publishes tested processing times, like Bernardin or a provincial agriculture extension office, rather than adapting a family recipe from before modern low-acid tomato varieties existed.
With the acid step covered and a full boiling-water processing time respected, canning tomatoes is a genuinely approachable weekend project — and a good way to turn an overflowing garden into a pantry that lasts well past the first snowfall.
