Instructions:
- Sterilise jars and lids. Place a small saucer in the freezer to chill—this is your testing surface.
- Combine blueberries and lemon juice in a heavy-bottomed large saucepan. Cook over medium heat, stirring occasionally, until the berries have burst and released their juices (about 5–7 minutes). Lightly mash some berries if a smoother jam is desired.
- Add the granulated sugar to the hot fruit mixture. Stir continuously over medium-low heat until all the sugar has completely dissolved. Do not proceed until crystals are gone.
- Increase the heat to medium-high. Bring the mixture to a rolling boil—one that cannot be stirred down. Boil vigorously, stirring frequently to prevent scorching the bottom.
- Test for the set (wrinkle test): Remove the pan from the heat. Spoon a teaspoon of jam onto the frozen saucer. Return the saucer to the freezer for 1 minute. Push the surface of the jam with your finger. If the surface wrinkles, the jam is ready. If runny, continue boiling for another 3–5 minutes and test again.
- Once set, use a flat spoon to gently skim off any foam or scum from the surface. Carefully ladle the hot jam into the sterilised jars, leaving about 1/4 inch headspace.
- Wipe the rims clean, affix the lids tightly, and process or refrigerate as necessary. Allow jars to cool completely before checking the seals.