Finely grated zest and juice of 4 large limes or 3 lemons
1.25kg jam sugar (with added pectin)
1. Put the berries, lime/lemon zest and juice into a large stainless steel pan and cook over a low heat, stirring now and then, until the berries are soft and plenty of juice has been released. Press gently with a potato masher to release even more juice and break up some of the fruit, but don’t reduce all the berries to pulp.
2. Add the sugar, then stir for about 10 minutes until the sugar dissolves. (This is important to prevent the jam from crystallising during storage.) Increase the heat, then boil steadily for 7 minutes until setting point is reached (105°C, or see p100 if you don’t have a sugar thermometer). Remove the pan from the heat, skim any scum off the surface, then leave for 5 minutes for the fruit to settle down into the syrup. Stir well, pour the hot jam into warm sterilised jars (see p100 for how to sterilise jam jars), then immediately cover with waxed discs and lids. Label, then store in a cool, dark place.