At least 3 hours and up to 7 days before serving, remove the grapes from their stems and place in the freezer. When ready to serve, whisk the egg whites with the lemon juice in a large bowl until frothy. Put the sugar in another bowl.
Drop the grapes into the bowl of egg whites, then pour the contents of the bowl through a strainer to drain the liquid. Dump the grapes onto a paper towel and roll them around until most of the excess egg white has been absorbed. Working in batches, add the grapes to the sugar and shake them around to coat. Shake off any excess. Transfer to serving glasses and serve immediately.
*RAW EGG WARNING
Food Network Kitchens suggest caution in consuming raw and lightly-cooked eggs due to the slight risk of Salmonella or other food-borne illness. To reduce this risk, we recommend you use only fresh, properly-refrigerated, clean, grade A or AA eggs with intact shells, and avoid contact between the yolks or whites and the shell.
Recipe courtesy of Gale Gand's Just A Bite by Gale Gand and Julia Moskin: Clarkson N. Potter Publishers, 2001