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1
Quarter the grapefruits lengthwise and remove the pulp, reserving it for another use.
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2
Scrape as much of the pith from the rind as possible with a spoon, cut the pieces of zest in half crosswise, and cut them lengthwise into 1/4-inch-wide strips.
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3
In a kettle cover the zest with 4 cups of the water, bring the water to a boil, and simmer the zest for 10 minutes.
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4
Drain the zest, refresh it under cold water, and repeat the simmering, draining, and refreshing process 2 more times.
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5
Sprinkle each of 2 jelly-roll pans with 1 cup of the sugar.
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6
In the kettle combine the remaining 2 cups sugar, the remaining 1 cup water, and the corn syrup and boil the mixture, washing down any sugar crystals clinging to the side with a brush dipped in cold water, for 3 minutes.
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7
Stir in the zest and simmer it, stirring occasionally, for 15 minutes, or until the syrup is thickened.
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8
Continue to simmer the syrup, stirring constantly and being careful not to let it burn, until a candy thermometer registers 230F.
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9
Transfer the zest with a fork to the prepared pans, roll it in the sugar to coat it well, separating the strips, and let it cool.
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10
Transfer the zest to racks and let it dry for at least 4 hours or overnight.
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11
The candied zest keeps, separated by sheets of wax paper in an airtight container lined with wax paper, at room temperature for 2 weeks or chilled for 1 month.
