Most of us think of tomatoes as acid food. However, with home canning, the acid level, or pH, of tomatoes is very close to the borderline for being considered low-acid. Low-acid foods like vegetables and meats need to be processed in the pressure canner to be safe.
As a measure of safety, experts recommend that home canned tomatoes that are typically processed in the boiling water canner be acidified. This includes whole, crushed and juiced tomatoes, and tomatillos. Recipes for these products will sometimes have directions for processing in the boiling water canner and pressure canner. The product should still be acidified even if processed in the pressure canner.
Bottled lemon juice or citric acid can be used to acidify tomatoes. Use the following proportions of acid:
per quart - 2 tablespoons
per pint - 1 tablespoon
per quart - 1/2 teaspoon
per pint - 1/4 teaspoon
Add the acid directly to the jar before adding the product. A small amount of sugar can be added to offset an acid taste.
Be sure to use bottled lemon juice not fresh. The bottled juice has a standardized acid level, but fresh may vary. Citric acid is usually found where canning supplies are sold.
Vinegar of 5% acidity can be used instead of lemon juice or citric acid. Use four tablespoons of vinegar per quart or two tablespoons per pint. However, vinegar may cause undesirable changes in flavor so lemon juice or citric acid is preferred.
Visit the National Center for Home Food Preservation for up-to-date recipes for canning tomato products, including salsas. http://www.uga.edu/nchfp/how/can3_tomato.html