Instructions:
- Sterilize jars if necessary for long-term storage. Place a small plate in the freezer to chill for the setting test later.
- Macerate: Combine blueberries, sugar, lemon zest, and salt in a large, heavy-bottomed saucepan. Let sit for 10 minutes.
- Initial Cook: Place the pan over medium-high heat. Stir gently until the sugar is completely dissolved and the blueberries begin to release their juices (about 5 minutes).
- The Hard Boil: Increase the heat to high. Add the lemon juice. Bring the mixture to a rolling boil that cannot be stirred down, as this activates the pectin.
- Skim & Stir: Boil vigorously, stirring frequently to prevent sticking. Use a spoon to skim off any surface foam that forms.
- Setting Point Test: After about 15–20 minutes of boiling, test for the set by dollopping a teaspoon of hot jam onto the chilled plate. Return the plate to the freezer for 1 minute. Push the jam with your finger; if the surface wrinkles, it is ready. If runny, continue boiling and test again every 3 minutes.
- Jarring: Remove the pan from the heat immediately once the set point is reached. Let the bubbling subside for about 2 minutes. Carefully ladle the hot jam into your sterilized jars, leaving about 1/4 inch headspace.
- Sealing & Cooling: Wipe the rims clean, seal tightly with lids, and allow to cool completely on the counter until sealed.