To use the grapefruit’s natural pectin, you put all the membranes and seeds in a cheesecloth bag and boil that with the juice.
Put the grapefruit peel, raspberries, water, and grapefruit juice in a large, heavy pot and boil for 40 minutes.
While the juice is boiling, you can prepare sterile jars if you want to store your marmalade outside the refrigerator.
Place jars in a pot of boiling water that is at least 2 inches above the level of the jars.