Fresh tomatoesfigure on roughly 2-3/4 lbs tomatoes per quart jar
Bottled lemon juice or citric acid: 2 tablespoons bottled lemon juice per quart jar or 1 tablespoon per pint jar OR 1/2 teaspoon citric acid per quart jar or 1/4 teaspoon per pint jar
Saltoptional: 1 teaspoon per quart or 1/2 teaspoon per pint jar
Bring a large pot of water to a boil. Using a knife, cut a small "x" in the bottom of each tomato and drop the tomatoes in the boiling water, a few at a time, for 30-60 seconds. Lift them out with a slotted spoon and put them directly into a bowl of ice water so they can cool quickly.
Once cool enough to handle, peel the tomatoes. Having blanched the tomatoes, the skins will now slip off easily (removing the skins is the standard recommendation because it's the skins that harbor potential bacteria). Discard the skins. Cut out the stem part of the tomato and dice the tomatoes.
Place the diced tomatoes and tomato juices into a large pot and bring it to a boil. Boil the tomatoes for 5 minutes to release their juices.
Have the canning jars ready (sterilized and hot). Place one tablespoon of bottled lemon juice (not fresh) OR 1/4 teaspoon citric acid in each pint jar or two tablespoons lemon juice OR 1/2 teaspoon citric acid in each quart jar.
Pack the hot diced tomatoes and their juices in the hot sterilized jars, leaving 1/2 inch headspace from the top of the jar.
Optional, Salt: Place 1/2 teaspoon of kosher salt in each pint jar or 1 teaspoon in each quart jar.
Use a slim utensil like a butter knife to slide down into the jars to remove any air bubbles. Wipe the rims of each jar and screw on the canning lids/bands.
Place the jars in the boiling water bath canner and process for 35 minutes for pints and 45 minutes for quarts, adjusting for altitude. Remove the jars, being careful not to tilt them. Let them sit undisturbed for 24 hours then check the seals.
These diced tomatoes will keep in a cool place for at least a year.