Canning salsa is one way to use up the garden bounty, but you might be surprised at the results. During the canning process, the water in the tomatoes separates from the fruit. Home-canned salsas are often thin and runny. Cook the salsa first to stabilize the ingredients and add a can of tomato paste to thicken it. You can try commercial thickening products, but don't use cornstarch.
One reason you shouldn't use cornstarch when canning salsa is food safety. Cornstarch is an alkaline substance, and when canned, any alkaline foods can create an environment where the bacteria that cause botulism can grow. Most salsa recipes call for vinegar, which provides enough acidity to eliminate the risk of botulism. However, cornstarch can thicken salsa enough that heat won't penetrate the inside of the jar during the canning process. When this happens, harmful organisms can survive and cause spoilage or food-borne illness.
Cornstarch is nothing more than the starch from the endosperm of the corn. When added to foods and heated, it causes the product to thicken. Cornstarch is desirable in many situations because it has no flavor and it creates a clear, translucent finish. When subjected to high heat, though, such as during canning, the starches break down. If you use cornstarch to can salsa, you'll end up with a runny product.
Clearjel is the only thickener recommended by the U.S. Department of Agriculture for canning. Clearjel is a modified cornstarch that doesn't break down during canning and doesn't thicken foods to the point of creating a safety issue. If you'd like to use Clearjel instead of cornstarch, use just a little less than double the amount of cornstarch required. Heat all the salsa ingredients to a boil except the Clearjel. Stir Clearjel into water first before adding it to salsa.
Many of the ingredients in salsa, including onions and peppers, are alkaline and could potentially increase your risk of food-borne illness. Using a pressure canner is one way of ensuring a safe food product, but if you use a water bath canner to can salsa, use only an approved and tested recipe. Never alter salsa recipes for canning or add ingredients, including thickeners, unless the recipe specifically calls for them.
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