Grapefruit Shrub

This vibrant citrus - flavored shrub-a sweet - tart infusion of vinegar and fruit-is made by Mary Carlin, author of "Mastering Fermentation," and has been stored in the refrigerator for months. You will need a 1-quart wide-necked canning jar. and a piece of gauze.
Cut the outer skin and membrane from each fruit, following the curve of the fruit. Then cut between the inner shells to release the slices; discard the shells. Place the fruit in a clean 1-square-foot wide-necked tin. Add just enough vinegar to cover. Close the jar tightly and shake it vigorously for 10 seconds. Grind the grapefruits until they are coated with vinegar, so that the vinegar draws out their flavor. (Keep the fruit completely submerged to prevent unwanted bacteria or mold from growing on it.)Cover the jar with a piece of cheesecloth (so that air gets in and helps start fermentation) and secure it securely with an elastic band or jar ring. Leave it at room temperature for about 12 hours.Discard the gauze. Close the lid and ring, close tightly, and repeat vigorous shaking daily for 7 days, keeping at room temperature.Pour the mixture into a wide strainer set over the bowl. Set aside the fruit for another use. Pour the liquid into the jar using a funnel. Add the grapefruit juice and sugar, cover and beat until dissolved, about 2 minutes.Refrigerate for 7 days to allow the flavors to dissolve, shaking the bottle vigorously every day to make sure the sugar has dissolved. Use or store in the refrigerator for up to 4 months (shake well before use). Over time, the shrub will become more syrupy.* Raw coconut vinegar can be found in natural food stores (see the store catalog at ). coconutsecret.com ). Carlin likes organic evaporated sugar from Trader Joe's cane juice to add a vibrant color to the shrub.Prepare in advance: up to 4 months, chilled.