Canning Plain Tomatoes: The One Acid Rule and a First-Season Recipe

August is when the question always comes up, and this year you might actually have the tomatoes to answer it. You can dry them, freeze them, or pickle them, but the oldest and most satisfying move is plain canning: juice, sauce, crushed, or whole. No vinegar bath, no big flavor decisions, just tomatoes in a jar.

There is one catch, and it is the whole reason this article exists. Tomatoes used to be considered high-acid food, and old recipes canned them with nothing added. Modern varieties, over-ripe fruit, and late-season tomatoes from tired vines are often not acidic enough to can that way. The fix is a small, exact amount of added acid. Follow the rule below and plain canned tomatoes are a safe, beginner project. Skip it, and you are leaving a food-safety decision to chance.

Why plain tomato jars need added acid

The safety line in water bath canning is a pH of 4.6. The spores that cause botulism cannot grow in food at or below that acidity, and a rolling boil in a water bath canner is enough heat for acidified food. Below that threshold, tomatoes sit on the wrong side of the line.

The tested rule from the USDA and university extension programs is specific:

• Quart jars: add 2 tablespoons of bottled lemon juice, or 1/2 teaspoon of citric acid, per jar
• Pint jars: add 1 tablespoon of bottled lemon juice, or 1/4 teaspoon of citric acid, per jar

A few details that matter:

• Use bottled lemon juice, not fresh. Fresh lemon juice varies in acidity from lemon to lemon, and the tested amounts assume a known starting acid level.
• Add the acid to the empty jar before filling. It is much easier to measure per jar than to divide it across a big pot.
• Sugar is fine. A small amount softens the sharp taste the acid adds. Salt is optional.
• Vinegar can substitute, but it changes the flavor more than lemon juice or citric acid do, so most people skip it.

What you need before you start

• Mason canning jars, checked for cracks and chips, with new flat lids for every jar. Bands can be reused; lids cannot.
• A boiling water bath canner, or a very large pot with a rack and a lid
• A jar lifter, a wide canning funnel, a long spoon or ladle
• A clean damp cloth or paper towel for wiping rims
• A timer
• A tested recipe from the USDA Complete Guide to Home Canning or your state extension service

Choose the right fruit and prep it

Canning shows off what you put in it, so the fruit choice matters as much as the process.

• Pick tomatoes that are ripe but firm and unblemished. No soft spots, cracks, or scarring. Canning is not the place for your best salads and not the place for your worst fruit.
• Rinse and core them.
• To peel: lower them into boiling water for about 30 to 60 seconds depending on size, then move them to an ice water bath. The skins should slip off in pieces.
• Cut or crush them to match your recipe, and keep the product hot in a big pot while you fill jars. Hot product into warm jars is part of the process working.

Filling the jars

Work one jar at a time and keep it orderly:

1. Add your measured acid to the empty, hot jar.
2. Ladle in the hot tomatoes, leaving 1/2 inch of headspace. That means the product sits half an inch below the rim, in both pints and quarts of tomato products.
3. Run a knife or thin spoon through the jar to release trapped air bubbles, and add more product if the level drops.
4. Wipe the rim clean with a damp cloth. A speck of tomato on the rim can keep the lid from sealing.
5. Center a new flat lid and screw the band on finger tight, snug but not torqued. Over-tightened lids can leak or prevent a seal.
6. Lower the jar into the canner. Water should cover the tops of the jars by 1 to 2 inches, with about an inch between jars.

Process the right time, at your altitude

Start your timer when the water reaches a full rolling boil, not when the jars go in. At sea level, a standard tested recipe for tomato sauce calls for about 35 minutes for pints and 40 minutes for quarts; whole, crushed, and juice products are in the same neighborhood. Use the time printed on the recipe you are actually following.

Water boils at a lower temperature as you climb, so higher elevations need longer processing:

• 1,001 to 3,000 feet: add 5 minutes
• 3,001 to 6,000 feet: add 10 minutes
• 6,001 to 8,000 feet: add 15 minutes
• Above 8,000 feet: add 20 minutes

During the process, keep the water at a gentle boil and above the tops of the jars. If it boils down, add more hot water. When the timer runs out, turn off the heat, wait a minute or two, and lift the jars out with the jar lifter. Set them on a towel or rack on a counter, away from a draft, and leave them alone for 12 hours. Do not re-tighten bands while the jars cool. You will hear and feel lids popping as they seal, and that is the good news.

Check the seals and put them away

After 12 to 24 hours, run your finger over each lid. It should be flat and stay down when you press the center. It should not dome, dent, or pop.

• Sealed jars: remove the bands, wash the jar, dry it, and store it in a cool, dark, dry place. Canned tomatoes keep their best quality for about 12 to 18 months, so you will be fine if you get through them before then.
• Unsealed jars: the food is not ruined. Refrigerate those jars and use them within a few days, or freeze the contents.

Write the contents and date on each lid with a marker before you shelve them. A year from now, "jar of red stuff" will not tell you if it was juice or sauce, and dated jars are easier to rotate.

What not to change about the recipe

Plain canned tomatoes are simple, but "plain" is doing safety work in that recipe. Keep the recipe as written:

• Do not add basil, spinach, or other low-acid vegetables to water bath tomato jars. Herbs are fine to stir in after opening.
• Do not can tomatoes in oil or "store tomatoes in oil" style. The tested guidance does not support it, because oil can trap spores around it.
• Do not halve or double recipes across different sources, and do not swap one tested recipe's times with another recipe's ingredients.
• Do not use oven canning, open-kettle canning, or microwave "canning" methods. They are not tested and they are not safe for this food.

One last note

The scariest part of this whole project is the acid rule, and it takes ten seconds to follow. If you keep the acid amounts exact, the times right for your elevation, and the lids new, plain canned tomatoes are about as close to guaranteed as canning gets. Start with a half dozen pints of sauce this August, check the seals on Sunday, and you will have a pantry shelf that pays for the whole afternoon.

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— C. Steward 🍅