- 6 1/2 cups sugar
- 4 medium or 3 large pink grapefruit
- 1/8 teaspoon baking soda
- One (1.75-ounce) package regular powdered fruit pectin
- 1/4 teaspoon unsalted butter
Measure the sugar into a large bowl and set aside.
Wash the grapefruit and remove the rinds in quarters. Discard 4 of the quarters (the rind of 1 grapefruit). Thinly slice the remaining quarters lengthwise, then roughly chop them crosswise into smaller pieces. Transfer the rinds to a large pot and add 2 cups of water and the baking soda. Bring to a boil over medium-high heat, cover, lower the heat, and simmer until the rinds are soft and beginning to turn translucent, about 15 minutes.
Meanwhile, roughly chop the grapefruit and remove any seeds. Transfer the grapefruit to a food processor and pulse until coarsely chopped. Add the fruit to the pot with the rind and return the mixture to a simmer. Simmer, covered, until the fruit has softened a bit, about 15 minutes.
Measure 5 cups of the fruit mixture, making sure to get a good combination of rind, fruit, and juice. Discard the remaining fruit mixture. Return the reserved 5 cups of the fruit mixture to the pot. Stir in the pectin and the butter and bring to a boil over high heat, stirring constantly. Add the sugar all at once and return the mixture to a full rolling boil, stirring constantly. Boil hard for one minute.
Remove the pot from the heat and skim any foam from the surface of the marmalade with a cold metal spoon. Ladle the marmalade into hot, sterilized jars and process them in a hot water bath for 10 minutes.