Cut off the tops and bottoms of the grapefruit, and then cut each grapefruit into quarters. Using a sharp knife, cut the fruit from the rind and pith. Then, working over the pot you'll be using to cook the jam, separate the fruit into segments. Use your fingers to gently peel each segment from its surrounding membrane so that you are left with only the fruit.

Squeeze the membrane over the pot in order to collect any juices before discarding (or, collect the membrane, pith, and seeds in cheesecloth, bundled and tied at the top, to boil with the jam for some extra natural pectin). Prepare your canning supplies. Bring the temperature of the glass jars up by processing them in hot water for several minutes, and heat a few cups of water in a small saucepan for the lids.

Add the sugar to the grapefruit and over high heat, bring the mixture to a boil. Boil hard, stirring constantly, until the mixture reaches the gel stage, about 20 minutes longer. When you reach the gel stage, skim off any foam and ladle the hot jam into the hot jars, leaving 1/4″ headspace. Place the lids and bands on top, screwing on the bands just until fingertip-tight. Place the full jars back into the boiling water and process 10 minutes. Remove from the water and place the jars on a towel. Let the jars cool. The seals should suck down (you’ll hear a popping noise as they do). Makes about 4 eight-ounce jars.