Traditionally, people use raw egg whites to frost fresh fruits. If you are not afraid of getting Salmonella, you can find lots of recipes online. I am offering an alternative that works well, makes great looking frosted grapes and are safe to eat.
2 Tablespoons of corn syrup
2 Tablespoons of water
1/3 cup of granulated sugar
In a small container, mix syrup and water together. Roll the grapes in the syrup to coat with the mixture. Then roll the grapes on a plate of sugar and sprinkle some all around it. Gently set the grapes on a wire rack to set. Grapes left on a bunch, are very easy to handle. Use them for centerpieces, cake decorations or any to make a cheese tray or a platter more attractive.