In my experience, it is almost impossible to boil down pureed tomatoes to a consistency that can accurately be referred to as “paste”, and yet, that is what the published canning experts tell us to do. Here is a much better alternative:
Peel tomatoes and puree them until smooth. For every 4 cups of puree, use 1 teaspoon salt. Place the tomatoes and salt in a large pan and bring to a boil over high heat. Reduce heat to low and simmer, stirring occasionally, until very thick, about 2 hours. Spread the mixture on non-aluminum baking sheets* and place in a 250F oven for 4 to 5 hours. Stir, using a spatula, at least every hour. When the paste has become the consistency of peanut butter, place in sterilized 125 ml or 250 ml jars and process in a boiling water canner for 45 minutes.
*Non-aluminum baking sheets are recommended because the acid in the tomatoes can react with the aluminum, possibly resulting in bitter-tasting tomatoes and potentially discoloured cookware.
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