How much lemon juice for canning tomatoes

The amounts, by jar size

Acid goes in the empty jar first, then the tomatoes. Any one of these three works — you don't add more than one:

Note the amount tracks jar size, not how much tomato you packed in. A loosely filled quart gets the same 2 tablespoons as a tightly packed one.

Does salt count? No — it's flavor only

This trips people up because salt preserves so many other foods. In canned tomatoes it does no preserving work at all. USDA lists 1 teaspoon per quart "if desired" — you can halve it, or leave it out completely, and the jar is exactly as safe. The acid is the part that isn't optional. If you're on a low-sodium diet, skip the salt and keep the lemon juice.

Why bottled lemon juice, not fresh

Bottled lemon juice is standardized to a known acidity. Fresh lemons aren't — acidity swings with variety, ripeness, and how long the fruit sat around. Since the entire point is to reliably push the jar below pH 4.6, a variable ingredient can't do the job. This is the single most common substitution people make, and it's the one that quietly removes the safety margin. Buy the cheap bottle; it's for chemistry, not flavor.

Which tomato products need it

NCHFP's acidification direction covers whole, crushed, or juiced tomatoes — so plain packed tomatoes, diced, crushed, and tomato juice all get the treatment. For mixed products like sauces, salsas and ketchup, follow the specific tested recipe you're using: those recipes include their own acid, and the amounts are calculated for that exact mix of ingredients. Don't add extra vegetables to a tested tomato recipe either — peppers, onions and garlic are low-acid and they move the math.

Why tomatoes need added acid at all

Tomatoes sit right at the borderline. The dividing line for botulism safety in a boiling-water bath is pH 4.6, and many modern tomato varieties hover close to it — ripeness, variety, and even overripe or frost-damaged fruit can nudge a batch to the wrong side. Rather than have you test each batch, USDA moves every jar safely under the line with a measured amount of acid. Tomatoes were reclassified this way decades ago; a pre-1990s family recipe that skips the acid reflects the old guidance, not current safety.

Does pressure canning let you skip it?

No. USDA's tested pressure processes for tomatoes assume the same added acid — pressure canning buys you a shorter process time, not an exemption. Whichever method you use, the jar gets its lemon juice or citric acid. If you're deciding between the two, see water bath vs. pressure canning.

It tastes too sour — can I fix that?

Yes, and there's an official answer: add sugar to balance the taste, but never leave the acid out. A teaspoon or so per quart takes the edge off without touching safety. If the lemon flavor itself is the problem rather than the sourness, switch to citric acid — same job, most neutral taste of the three options.

What if you forgot the acid?

Treat those jars as not shelf-stable. Don't count on re-processing to rescue them — the safe move is to refrigerate and use them within a few days, or freeze them, and to keep them out of the pantry. It's a frustrating outcome, which is exactly why the acid goes into the empty jars before filling: lining up the jars and dosing them all first makes it very hard to lose track of which ones you've done.