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1
Cut 1/2 inch off the top and bottom of one of the grapefruits.
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2
Place grapefruit cut side down on a cutting board and, using a sharp knife, slice downward, following the curve of the grapefruit, to remove the peel and about 1/2 of the pith (white portion) in large pieces.
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3
Slice peel lengthwise into 1/4-inch-wide strips.
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4
Repeat with remaining grapefruits.
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5
Blanch strips by placing in a large saucepan, covering with about 3 inches of cold water, and placing over high heat.
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6
As soon as the mixture boils, remove from heat, drain off water, and repeat process 3 more times.
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7
After strips have been blanched the fourth and final time, drain and set aside.
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8
Add 6 cups of the water and 6 cups of the sugar to a large saucepan and stir until sugar is moistened.
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9
Add strips and bring mixture to a boil over high heat.
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10
Boil, stirring occasionally, until syrup is thick, surface of liquid is covered with large, opaque bubbles, and zest is completely translucent (the strips will look like gummy worms), about 40 minutes.
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11
Pour mixture through a strainer set over a large heatproof bowl; reserve syrup for another use and set zest aside to cool slightly, about 2 to 3 minutes.
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12
Place the remaining 2 cups sugar in a medium bowl and, working in small batches, toss warm strips in sugar.
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13
Coat zest thoroughly, then place in a single layer on a rack to cool completely, about 10 minutes.
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14
Once dry, place zest in a tightly covered container and store at room temperature for up to 2 weeks.