Safety first: Home canning is very safe when you follow a tested recipe exactly, but it’s one of the few kitchen skills where changing a step can matter. This post explains the process; always cross-check your specific processing time against a current, tested source like the National Center for Home Food Preservation (nchfp.uga.edu) or the USDA Complete Guide to Home Canning before you can.
Tomatoes are a favorite first canning project, but they’re technically a “borderline” acid food, meaning their natural acidity varies by variety, ripeness, and growing conditions. That’s why every tested tomato recipe requires you to add a measured amount of acid before processing. Skipping this step is the most common mistake beginners make.
What you’ll need
- Fresh, ripe (not overripe or bruised) tomatoes
- Bottled lemon juice or citric acid, not fresh lemons, since bottled juice has a standardized, reliable acidity
- Canning jars, new lids, and bands, all inspected for chips or dents
- A water-bath canner, or any large, deep pot with a rack to keep jars off the bottom
- A jar lifter, canning funnel, and bubble remover / headspace tool
The process, step by step
1. Prep your tomatoes
Wash, then blanch tomatoes in boiling water for 30–60 seconds and transfer to an ice bath; the skins will slip off easily. Core and quarter or crush, depending on your recipe.
2. Acidify every jar
Add bottled lemon juice or citric acid to each jar before adding tomatoes. The standard tested amounts are 2 tablespoons bottled lemon juice (or ½ teaspoon citric acid) per quart jar, and 1 tablespoon bottled lemon juice (or ¼ teaspoon citric acid) per pint jar. This step is non-negotiable, regardless of how acidic your tomatoes taste.
3. Pack the jars
Fill jars with tomatoes, leaving the headspace called for in your specific recipe (commonly ½ inch for tomatoes). Run a bubble remover around the inside of the jar, wipe the rim clean of any residue, and center a new lid before screwing the band on fingertip-tight.
4. Process in the water bath
Lower jars onto the rack, making sure they’re covered by at least 1–2 inches of water, and bring to a rolling boil. Processing time depends on jar size, whether it’s a raw or hot pack, and your altitude; higher elevations need longer processing times. Always look up the current time for your exact recipe and jar size rather than guessing or reusing an old family time, since guidance is occasionally updated.
5. Cool and check the seal
Turn off the heat, let jars sit in the water for 5 minutes, then move them to a towel-lined counter and leave undisturbed for 12–24 hours. Press the center of each lid; it shouldn’t flex or pop. If a lid didn’t seal, refrigerate that jar and use it within a few days instead of storing it on the shelf.
Printable: Canning Day Checklist
- Jars, lids, and bands inspected, no chips or dents
- Acid added to every jar (lemon juice or citric acid)
- Correct headspace left in every jar
- Rims wiped clean before lidding
- Processing time confirmed for your altitude and jar size
- Seals checked the next day; unsealed jars refrigerated
Next in this series: turning these canned tomatoes into a simple, shelf-stable marinara using a tested recipe.
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