Jam is fruit, sugar and acid boiled together until the fruit's natural pectin sets into a gel. Once you understand how those three work together, you can make jam from almost any fruit without needing a specific recipe.
Pectin is the gelling agent, found mostly in the skins, cores and pips of fruit.
High-pectin fruits set easily. Low-pectin fruits need a helping hand: use jam sugar (which has pectin added), add extra lemon juice, or mix in a high-pectin fruit such as grated apple.
Sugar helps the pectin gel, and at high concentrations it preserves the jam (see the science section below). The traditional British ratio is 1:1 fruit to sugar by weight, which gives a firm set and keeps for a year or more. You can drop to roughly 2 parts fruit to 1 part sugar for a fruitier, softer jam, but it won't keep as long and is best stored in the fridge.
Acid helps the pectin set and brightens the flavour. Most fruit benefits from a squeeze of lemon, and low-acid fruits (strawberries, blueberries, cherries, pears) really need it. Around 1 tbsp lemon juice per 500g fruit is a good starting point.
Setting point is around 104 to 105°C, so a sugar thermometer makes life easy.
Without one, use the wrinkle test: put a few saucers in the freezer before you start, drop a little jam on a cold saucer, wait a minute, then push it with your finger. If the surface wrinkles, it's ready. If it runs, keep boiling and test again every couple of minutes. Take the pan off the heat while testing so you don't overshoot.
Makes roughly five 340g jars. Swap in any fruit and adjust the lemon and sugar type based on its pectin level.
Ingredients