Blood oranges owe their startling color to a genetic mutation that encourages the development of anthocyanins, compounds that lend a reddish-purple color to whatever they are present in. Blood orange marmalade is crimson and tart, a feast for the eyes as much as the mouth. Be warned that this marmalade is time-consuming, but we think the results are well worth the extra effort.

Prepare a water-bath canner, gather all your canning equipment, place a couple of saucers in the freezer, and have ready 5 clean, hot half-pint jars. Using a vegetable peeler, remove the zest in strips from:

Stack the strips of zest on top of one another and cut them crosswise into very thin slivers. Place in a large, wide pot and set aside.

Cut off and discard the very top and bottom of each orange so they stand upright on the cutting board. Following the curve of the oranges with your knife, cut off the white pith to expose the fruit inside. Discard the pith. You will now be able to see thin, vertical lines of membrane that show you where the segments of the oranges are. Cut along the membranes to release the fruit sections, leaving the membranes behind (see the illustration). Discard any seeds. Place the orange sections in the pot with the slivered zest, and squeeze any juice remaining in the orange membranes into the pot as well.

Add just enough water to the pot to cover the fruit and zest (3 to 4 cups). Simmer the fruit, uncovered, until the water has almost completely evaporated and the zest is completely tender. If needed, add a little more water and simmer longer to achieve tenderness.

When the zest is tender, gradually stir in:

Cook the marmalade over high heat, stirring to dissolve the sugar, until the mixture reaches the jelling point. Ladle into prepared jars, leaving ¼ inch headspace. Wipe the rims. Place lids on jars and screw on rings until fingertip-tight. Process for 10 minutes. Let cool completely and store as directed.