Slow Cooker Blackberry Jam with Lemon
Blackberry jam made in a slow cooker is a hands-off way to preserve summer fruit without constant monitoring or the heat of a stovetop. This version balances tart lemon with sweet blackberries and jam sugar to create a glossy spread with a naturally set texture. The slow cooker method takes 4-5 hours total but requires minimal intervention beyond an initial stir and gentle mashing. The result is thick, intensely flavored jam perfect for toast, pastries, or baking. This recipe suits anyone intimidated by traditional jam-making or seeking a lower-heat alternative. Serve it fresh from sterilized jars as a homemade gift or pantry staple. The wrinkle test and thermometer methods ensure you'll know exactly when it's ready, taking the guesswork out of setting point.
Ingredients
Instructions
- 1
Wash the blackberries and remove any stalks or leaves.
- 2
Put the blackberries, jam sugar and lemon juice in the slow cooker. Cook on low for 1 hour.
- 3
After 1 hour, stir to mix the fruit and sugar together, replace the lid and cook for a further 1 hour on low to ensure the sugar dissolves fully.
- 4
After the two hours, mash the fruit gently using the back of a fork then switch to high.
- 5
Cook for 2-3 hours until the jam reaches 221°F.
- 6
Check for setting point using a food thermometer or the wrinkle test: drop a small spoon of hot jam onto a chilled saucer, let cool for a minute, then push with your finger. If it wrinkles, setting point is reached. If not, continue cooking for another hour.
- 7
When setting point is confirmed, pour the jam into sterilised jam jars and seal.
Tips
Use a chilled saucer for the wrinkle test by placing it in the freezer from the start of cooking. This ensures accurate setting point detection without guesswork.
If your slow cooker runs hot, check temperature after 2 hours (4 total) rather than waiting the full 2-3 hours on high to avoid overcooking.
Sterilize jars by running them through a hot dishwasher cycle or boiling them in water for 10 minutes before filling, ensuring a proper seal.
Good to Know
Sealed jars store in a cool, dark place for up to 1 year. Once opened, refrigerate and use within 3-4 weeks.
Make up to 2 weeks in advance. Jars must be sterilized and sealed the day of cooking for food safety.
Serve on buttered toast, with yogurt, over ice cream, or as a pastry filling.
Common Mistakes
Skip the initial low-heat stage to avoid crystallized sugar. The two hours on low allow sugar to dissolve evenly, preventing grittiness.
Use a lid during cooking to avoid excessive evaporation. Remove only when mashing and checking temperature.
Pour jam into hot jars, not room-temperature ones, to reduce thermal shock and ensure a proper seal.
Substitutions
FAQ
Can I use frozen blackberries instead of fresh?
Yes. Thaw them completely and drain excess liquid before adding to the slow cooker. The cooking time may remain similar, but fresh juice loss could affect final yield slightly.
What if my jam doesn't reach 105°C after 4 hours on high?
Continue cooking in 30-minute increments, checking temperature each time. Some slow cookers run cooler. If it won't set after 6 hours total, use the wrinkle test instead or stir in pectin as directed on the package.
How long does homemade slow cooker jam keep?
Sealed jars last up to 1 year in a cool, dark cupboard. After opening, refrigerate and consume within 3-4 weeks. If mold appears, discard the jar entirely for safety.