Spread a kitchen towel on the counter. Use the jar lifter to remove a jar from the canner. Drain the water back into the canner, and place it on the towel. Keep the remaining jars in the canner so they stay hot.
Add 1/2-teaspoon citric acid or 2 tablespoons lemon juice to each quart sized jar (1/4-teaspoon citric acid or 1 tablespoon lemon juice for pints).
If using salt, add up to 1 teaspoon for each quart, and 1/2 teaspoon per pint.
Use the canning funnel and ladle to fill the jar with the tomato puree. Leave a 1/2-inch headspace.
Wipe the rim to remove any residue, center a lid on the jar, and screw on a band until it is fingertip tight.
Use your jar lifter to place the jar back into the canner, and repeat with the remaining jars. Try to leave a little space in between the jars.
Once all the jars are in canner, adjust the water level so it is two inches above the jar tops. If you need to add water, pour hot water around the jars and not directly on them.
Cover the canner and bring to boil over high heat. Once water boils vigorously, continue boiling quart size jars for 40 minutes (35 minutes for pint jars) at altitudes of less than 1,000 feet. Adjust processing time for your altitude if necessary (See note below).
When processing time is complete, turn off the heat, remove the cover, and let the canner settle for about 5 minutes.
Spread a dry kitchen towel on the counter. Remove the cover by tilting lid away from you so that steam does not burn your face.
Use the jar lifter to remove the jars from canner and place on the towel. Keep the jars upright, and don't tighten bands or check the seals yet. Let the jars sit undisturbed for 12 to 24-hours to cool.
After 12 to 24-hours, check to be sure jar lids have sealed by pushing on the center of the lid. The lid should not pop up. If the lid flexes up and down, it did not seal. Place the jar in the refrigerator and use up within a week, or place the puree into freezer containers and freeze for up to 6 months.
Remove the screw on bands and wash the jars. Label, date, and store your jars in a dark, cool place (50 to 70 F). Use within 12 to 18 months for the best quality and flavor. Once the jar is open, refrigerate and use up within a week. Yields about 7 quarts of tomato puree.