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A: While initially one would think that you could, the answer is NO, and involves a clearer understanding of the science behind that answer.  According to Elizabeth Andress at the National Center for Home Food Preservation, citric acid does not acidify all types of foods in the same amounts.  For example, just because a certain amount works for X amount of tomatoes, for acidification, does not mean it acidifies peppers and onions in the same ratios.  Different foods have what is called buffering capacities, or components that help them try to resist a change in pH.  So, the composition of different foods matters as to how they will acidify.  In salsas, we are having to acidify peppers and onions as well as tomatoes.  Therefore, you CANNOT make a substitution of citric acid for lemon juice in our canned salsa recipes, or any other mixtures of vegetables or foods.
Also, the LIQUID phase, or amount of liquid, for canning salsas matters to the process time.  If you left out the lemon juice, you would have a much thicker salsa and the process time may not be sufficient for the salsa without it.
USDA and the National Center for Home Food Preservation salsa recipes for canning are intended to be made per the recipes.  Citric acid was not tested as an acidifier for those mixtures. The type of vegetable being acidified does matter and all do not react to different acids in the same way.

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