Why Acidify Tomatoes?
Tomato varieties have been bred for ease in harvesting; as a result many now have milder flavor and lower acidity than the ancestral tomatoes. Testing has shown that some current tomato varieties have pH values at or above pH 4.6; a few have values of pH 5 or even higher. Adding the recommended amount of lemon juice (or citric acid or vinegar) lowers the pH of all tested varieties enough to allow for safe boiling water bath canning. Acidifying all tomatoes now is recommended because it allows for safe processing in a boiling water bath canner (and for a safe short process in a pressure canner).
Some procedures from the USDA Complete Guide to Home Canning offer both boiling water and pressure canning options for tomatoes. For examples, see canning procedures for crushed tomatoes, whole or halved tomatoes, tomatillos, tomato juice, and tomato sauce (So Easy to Preserve, pages 51-56). Both the boiling water and pressure canning options require acidification. The boiling water and pressure alternatives calculated for canning these products are different time and temperature combinations that yield equivalent killing rates for molds and yeasts. These particular pressure canning options require acidity to ensure a safe product.
How About Salsas?
Salsas typically combine low acid foods (onions, peppers) and acid foods (tomatoes, fruits). Adding the recommended amount of acidity in the form of vinegar, bottled lemon juice or bottled lime juice is essential to produce salsas that can be safely processed in a boiling water canner. To produce shelf-stable salsa, follow the proven, tested recipes provided at the Home & Garden Information Center (HGIC), at the National Center for Home Food Preservation, or in So Easy to Preserve (pp. 66-76). If your friends want to make a family salsa recipe, ask them to refrigerate it or freeze it.
And How About Tomatoes with Okra or Spaghetti Sauce?
Neither vinegar nor lemon juice are included in recipes for canning tomatoes with okra (p.54) or canning spaghetti sauce (pp. 63-64) in So Easy to Preserve. These recipes require pressure canning because of the low acid ingredients. Adding meat to spaghetti sauce lengthens the required pressure canning process. The recipes for tomato-vegetable mixtures in So Easy to Preserve (pp. 54, 63, 64) or on the HGIC were tested for pH and heat penetration and safely include shorter processing times. However, the recipes and preparation steps must be followed precisely. If tested recipes for tomato-vegetable mixtures are not used, then the mixtures should be pressure canned according to instructions for the vegetable in the mixture with the longest processing time.