PRESERVED TOMATO PURéE

Steps:

• Peel, core and roughly chop tomatoes. Put in a large nonreactive pot, cover and simmer 10 minutes. Purée through medium disc of a food mill to remove skins and most seeds.
• Put a rack in a large stockpot or line pot with a folded kitchen towel, then fill with water and bring to a boil. Add quart or pint jars and boil 10 minutes. Jars may be left in warm water until ready to fill. Alternatively, sterilize jars by running them through a dishwasher cycle, keeping them warm in the machine.
• Place canning rings in small saucepan, cover with water and bring to a boil. Turn off heat and add flat lids to soften rubber gaskets. Rings and lids may be left in water until jars are filled.
• Bring purée to a boil, then down to a sturdy simmer and cook to reduce by one third, about 30 minutes. Ladle hot purée into warm jars, leaving a little more than 1/2 inch head space to accommodate lemon juice. If using citric acid, fill to 1/2 inch head space.
• Into every quart jar, add 2 tablespoons lemon juice or 1/2 teaspoon citric acid. For pints, use one tablespoon lemon juice or 1/4 teaspoon citric acid. If using salt, add 1 teaspoon to each quart or 1/2 teaspoon to each pint jar.
• Wipe jar rims clean with a damp towel. Place lids on jars, screw on rings and lower jars upright into pot of boiling water. Return to a full boil and process for 45 minutes for quarts or 35 minutes for pints. If there are mixed sizes, process for the longer time. Transfer jars to a folded towel and let cool for 12 hours. Jars will ping as they seal.
• Once cool, test the seals by removing rings and lifting jars by their flat lids. If a lid releases, the seal has not formed. Unsealed jars should be refrigerated and used within one week or reprocessed. Rings and jars may be reused, but a new flat lid must be used each time.