Tomatoes were long considered high-acid fruit, but their pH is right at the borderline and can exceed the threshold depending on the variety and ripeness. That is why a measured amount of acid is added to the jar: 2 tablespoons (15 ml each) of bottled lemon juice or 1/2 teaspoon of citric acid for a 1-quart (about 1-liter) jar. For a 0.5-liter jar, use half that amount.
Tomatoes have a special place in canning because they are right on the borderline. The safety threshold is pH 4.6, and depending on the variety, ripeness, soil where they were grown, and harvest time, tomatoes can fall either below or above this value. Overripe, cracked, or softened tomatoes left on the vine have noticeably lower acidity. Since you cannot tell by sight which side of the threshold a jar is on, the decision is left not to individual tomatoes but to the rule of making a measured addition. Even two crates of tomatoes picked from the same garden can have different acidity.
The amount is given per jar and should not be estimated by eye. Add 2 tablespoons (15 ml each) of bottled lemon juice to a 1-liter jar and 1 tablespoon (15 ml) to a 0.5-liter jar. If using citric acid, add 1/2 teaspoon to a 1-quart (about 1-liter) jar and 1/4 teaspoon to a 1-pint (about 0.5-liter) jar. Add the acid to the bottom of the empty jar before filling it with tomatoes. These amounts are the same for strained tomatoes, chopped tomatoes, and whole tomatoes.
Choosing bottled lemon juice instead of freshly squeezed lemon is not just a matter of convenience. The acidity of fresh lemons varies from fruit to fruit and by season, so the same amount can contain different amounts of acid. The acidity of bottled juice is standardized, and recipes are calculated using that consistent value. The same applies to vinegar: if using it, choose one with at least 5% acidity, and do not use homemade vinegar for this purpose. Also write down when the bottle was opened, because the concentration of a product that has been open for a long time decreases.
The added acid changes the flavor, and that is expected. If the recipe allows it, you can add some sugar per jar to balance the sourness; do not reduce the amount of acid. Salt is not part of this calculation either: salt does not preserve food in canning and is only for flavor. Do not transfer this amount as is to tomato paste, a product made into a sauce, or a mixture containing low-acid ingredients such as onions, peppers, garlic, or zucchini; that mixture is no longer canned tomatoes.
Acidified tomatoes can be processed in a boiling-water canner; that is what acidification makes possible. In a mixture that includes other vegetables, however, the acidity cannot be calculated, and the product is considered a low-acid food, meaning it requires a pressure canner. There is no recommended method for canning unpickled vegetables in a boiling-water canner, and this rule does not become more flexible just because tomatoes are included in the mixture. Never open or taste a jar of tomatoes with a bulging lid or an off odor. A jar containing mixed vegetables should not be evaluated using guidance for canned tomatoes, even if it contains tomatoes.