Salsa contains both high-acid and low-acid ingredients, which need to be combined in specific proportions, along with an acidic liquid like lemon juice or vinegar, to make a finished product safe for canning. Although you can make some minor adjustments to a canning recipe, the proportions called for shouldn't change. When canning, use only recipes scientifically tested by an authority, such as an Extension service, as botulism is a potential hazard with untested recipes.
The main ingredient in salsa is tomatoes or tomatillos. Select fresh Italian or paste tomatoes for firmer salsa or slicing tomatoes for more watery salsa. Discard soft, moldy or bruised tomatoes, and choose mild or spicy peppers and some onions. While you may change the type of peppers or onions called for in your recipe, you cannot change the proportion. For example, you may substitute mild peppers for jalapeno peppers or yellow onions for red onions, but not the amount of either type of ingredient called for in your recipe. You may safely adjust the amounts of dry seasonings in your recipe.
Prepare the Ingredients
Boil the tomatoes for up to a minute and place them in a bowl of ice water to loosen their skins before peeling them. Squeeze the excess juice from your tomatoes, core them and remove the seeds. Broil hot peppers for as long as eight minutes at 400 degrees Fahrenheit until their skin blisters, making it easier to remove. Once cool, peel and seed them, or leave some seeds for spicier salsa. Coarsely chop the tomatoes, peppers and onions. Add bottled lemon or lime juice or vinegar containing 5 percent acetic acid. Never use homemade vinegar or freshly squeezed juice, because the amount of acid in each varies.
Mix and Fill
Combine the salsa ingredients in a pot and bring them to a boil over medium-high heat. Reduce the heat and simmer the ingredients for 20 to 30 minutes. Preheat canning jars in a boiling water canner for 10 minutes, and wash and dry the lids and rings in hot, soapy water. Spoon the hot salsa into the hot jars, leaving 1/2 inch of headspace. Wipe the jar rims and secure the lids and rings on the jars.
Can the Salsa
Place the cans of salsa in the boiling water canner and process them for 15 to 20 minutes, depending on your elevation. Allow the canner to cool for five minutes before removing the jars of salsa and placing them in a cool, dark pantry for 24 hours. Check their seals -- the tops of the lids shouldn't pop up when you press on them. Reprocess any improperly sealed jars. Store salsa in the pantry for up to one year. Once opened, store salsa in the refrigerator set to 40 F or below for up to two weeks, recommends the EatByDate website.
Never substitute vinegar for lemon or lime juice in a salsa recipe, because it's less acidic. Use canning salt, not regular salt, to prevent discoloration of the finished product. Never use cornstarch or flour to thicken salsa before canning, as it can change the acidity of the product; use canned tomato paste instead. Boil any spoiled cans of salsa for 30 minutes before discarding them.