Sugared grapes make a lovely decorative garnish for holiday platters, and although painting the fruit is a bit time-consuming, the finished product makes your effort worthwhile. For best effect, you need to use superfine sugar and a small watercolor paintbrush (#2 or #4). Using different colors of grapes makes for an attractive display, although the frosting effect is most dramatic on dark grapes. If you plan to eat the sugared grapes, use pasteurized liquid egg whites if you have concerns about salmonella.
With scissors, carefully cut the bunch of grapes into 2-, 3-, and 4-inch clusters. A variety of sizes will create a more interesting display.
Line a rimmed baking sheet with parchment or wax paper. Set a wire rack over the pan. Place the egg whites and 2 teaspoons of water in a small teacup and stir with a fork to blend. Holding a cluster by the stem, dip the grapes into the egg whites. Use a small brush to carefully paint any part of the grapes that did not get dipped into the whites.
Place the grape cluster on the wire rack and immediately sprinkle with some of the sugar, turning the grapes in order to coat all sides of the fruit. Pick up the cluster and gently shake off excess sugar. Repeat with remaining grapes.
Tip: vary the amount of sugar on some of the grapes to create a natural variety effect, rather than have all the grapes look the same.
Allow the grapes to dry completely at room temperature, about 2 hours. The sugared grapes will keep at room temperature for up to 2 days. Do not cover or refrigerate, or the coating will be ruined.