Wash jars and bands in hot soapy water and keep warm. Fill water bath canner with water and put on medium heat.
Mash berries with a potato masher, blender, or immersion blender to desired consistency. I prefer mine chunky, but my husband likes it more pureed. (Note, liquid or pureed berries take longer to reach the gelling point)
Place berries, sugar, lemon juice, and lemon zest (or grated apple if using in place of lemon) into large pot. Stir until well combined. Bring berries to a boil. Stir frequently to keep sugar from scorching.
Simmer on a low boil for 20 minutes.You can test the set of the jam by the sheeting test. Place a metal spoon in the freezer when you begin making your jam.
After the 20 minutes of boiling, use the chilled metal spoon to ladle out a spoonful of jam. Hold the spoon and watch the way the jam drips off of the spoon. If its little individual drops, jam is not set, if it’s big goops, it’s almost there. If it comes off the spoon in a sheet or doesn’t really drop off at all, then jam is set, yank that baby off the heat.
Place jars on a dish towel. Fill jars with a ¼ inch from the top with jam. A canning funnel will be your best friend during this part. With a clean damp towel, wipe down rim of jar. Place lids on, then bands, and screw down to finger tight.
Immerse jars in water bath canner inside the canning rack, making sure water covers the tops of the jars by 1 to 2 inches. Once water is boiling, set timer for 10 minutes and allow jars to process.
When time is up, turn off heat. Wait 5 minutes and then remove jars from canner. Place on a towel folded in thirds in a draft free area. Allow to cool and set overnight or for at least 12 hours.
Check seals. If the center of the lid gives, then store in the fridge and eat soon.
If jars are sealed, wipe down with a damp cloth and store in the pantry out of the light for up to a year