Slice persimmons into small chunks and remove stems and seeds.
Add persimmons and lemon juice to a small saucepan and cook on low for a few minutes until persimmons begin to release their juices. Use a potato masher to help speed the process along.
Once they've fallen apart, add sugar and cook over medium-low heat until the jam thickens. Test for consistency on a plate in the freezer.
When the consistency is right, pour into prepared canning jars leaving 1/4 inch headspace. Process in a water bath canner for 10 minutes, and then turn off heat and leave the jars in the canner for an additional 5 minutes. Remove jars to the counter and allow them to cool.
Check seals, and store any unsealed jars in the refrigerator for immediate use.
Notes
Acidity ~ If you're canning this persimmon jam recipe, you MUST use at least 1/4 cup lemon or lime juice. Persimmons are not quite acidic enough for safe canning without added acidity. The citrus also really brings out their flavor, so I choose to use 1/2 cup for a brighter finished flavor.Sugar ~ I prefer low sugar persimmon jam, and for this reason, I only use 1 cup of sugar to 4 pounds of fruit. For a standard sweet jam, feel free to add the full 2 cups, or anywhere in between. The recipe makes 4-5 half-pint jars (8oz) of jam.