-
1
Measure the sugar into a large bowl and set aside.
-
2
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.
-
3
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.
-
4
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.
-
5
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.
