A delicious recipe to turn grapefruit into a red grapefruit marmalade!
Peel the grapefruit zest with a peeler and cut them into shreds. Juice the grapefruits and the lemon and strain the juice into a large pan, keeping back any flesh and seeds. Cut the empty fruit shells in half, then peel away the squeezed-out flesh from the pith. Discard the pith and put the fleshy membrane, along with the flesh and pips strained out of the juice, into a piece of muslin. Wrap this up and tie it with kitchen string. Add this to the pan along with the shredded zest and 1.25 litres of water. Bring to the boil, then turn down to a simmer, put on the lid and cook for 1 hour, or until the zest is soft. Put a small plate in the freezer to get cold.
Remove the muslin bag from the liquid, put it in a sieve over the pan and, once it is cool enough to handle, squeeze out as much juice as you can, before discarding the bag. Add the sugar to the pan and cook over fairly low heat until the sugar is dissolved. Once dissolved, bring to a boil and bubble until it reaches a temperature of 105C or test for setting by dropping a little of the mixture onto the chilled plate and seeing if the set mixture wrinkles when you press your finger into it. Let the mixture cool for 10 minutes, then stir in the Aperol. Divide between sterilised jars and seal.