Though the white pith and membrane of citrus fruits are edible, they're also quite bitter. For recipes like grapefruit terrine, you'll want to supreme the grapefruit, removing the sweet flesh from its bitter surrounds. Here's how.
1 Trim top and bottom; set cut side up. Slice lengthwise between flesh and peel, following fruit's contour; remove peel and pith.
2 Hold fruit over bowl to catch juice. Slice lengthwise between 1 segment and the membrane until you reach the center of the fruit.
3 Make a similar slice on the other side of the segment. Use the knife blade to remove segment. Repeat.
4 Once all segments are removed, squeeze juice from membrane into the bowl. Discard membrane; reserve juice for another use.