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1
Quarter fruit lengthwise and remove peel (including white pith) in 1 piece from each quarter, reserving fruit for another use.
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2
Cut peel lengthwise into 1/3-inch-wide strips (if using grapefruit, cut peel in half crosswise first).
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3
Put peel in a large bowl and cover with cold water, then soak 1 hour.
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4
Drain in a colander.
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5
Transfer peel to a wide 4- to 6-quart heavy pot.
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6
Add cold water to cover by 1 inch and bring to a boil.
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7
Reduce heat and simmer 10 minutes, then drain in colander.
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8
Repeat blanching process 2 more times.
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9
Cover peel once more with cold water and simmer until skin side is tender, about 30 minutes, then drain in colander.
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10
Return pot to stovetop and add 6 cups sugar and 6 cups water.
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11
Bring to a boil, stirring until sugar is dissolved.
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12
Wash down any sugar crystals clinging to side of pot with a pastry brush dipped in cold water.
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13
Add food coloring (if using) and boil syrup, uncovered and undisturbed, until it registers 220F on thermometer, about 30 minutes.
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14
Add peel and simmer over low heat until translucent, about 45 minutes.
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15
Remove from heat and let peel stand in syrup, uncovered, at room temperature 8 to 12 hours.
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16
Return syrup with peel to a boil and boil, uncovered and undisturbed, until it registers 226F on thermometer, about 30 minutes.
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17
Remove from heat and let peel stand in syrup, uncovered, at room temperature 8 to 12 hours.
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18
Return syrup to a boil once more and boil, uncovered and undisturbed, until it registers 228F on thermometer, about 30 minutes.
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19
Remove from heat and let peel stand in syrup, uncovered, at room temperature 8 to 12 hours.
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20
Reheat syrup with peel over low heat just until syrup has liquefied (peel will have absorbed most of syrup), then drain peel in colander.
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21
Immediately spread peel, separating pieces, on 2 metal racks set in 2 shallow baking pans and dry, uncovered, until just barely sticky, 8 to 24 hours.
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22
Spread 1/2 cup of remaining sugar on a plate and roll each piece of peel in sugar to coat well, then transfer to a sheet of wax paper.
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23
Add more sugar as needed to coat remaining peel.
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24
(Pour sugar through a medium-mesh sieve occasionally to remove bits of peel and clumps of sugar.)
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25
Dry sugared peel on wax paper 1 hour.
