Adding acid to salsa

The acid ingredients used in salsa help preserve it. You must add acid to canned salsas because the natural acidity may not be high enough. Commonly used acids in home canning are vinegar and lemon juice. Lemon juice is more acidic than vinegar, but has less effect on flavor. Use only vinegar that is at least 5% acid and use only commercially bottled lemon juice.

If you wish, you may safely substitute an equal amount of lemon juice for vinegar in recipes using vinegar. Do not substitute vinegar for lemon juice. This substitution will result in a less acid and potentially unsafe salsa.

Reference:  National Center for Home Food Preservation