Peel the grapefruit, and remove all the white membranes, too.
Put the fruit pulp and sugar in a pan, mix lightly, and leave for about 5 minutes.
When some moisture comes out of the pump and the sugar starts to melt, add the lemon juice and start cooking over medium heat.
When it comes to a boil and some scum rises to the surface, skim it off 2 to 3 times. (The white stuff in the middle of the photo is the scum.)
After simmering for 15 to 20 minutes, when the orange color darkens and the mixture thickens, it's done. Even if it's quite liquid, it will thicken when it cools down. Pour into a sterilized jar while it's still hot close the lid.
If you prefer to have a bitter taste: finely shred about 1/2 of the peel, and simmer for about 5 minutes to get rid of some of the bitterness. Strain and add it to Step 3 and continue.
Story Behind this Recipe
When I see fruit in season, I just can't resist the urge to turn it into jam. Add it to yogurt, or spread on bread. I love the sweet smell in the kitchen when I'm making jam, too.
Since both the peel and membranes are removed, not a lot of scum will come out. Be sure keep skimming when it starts to simmer. I don't like the bitterness of marmalade so I leave the peel out, but if you do like that bitter flavor, it's probably better to add the peel for a bitter flavor accent.