Two egg whites plus three cups of powdered sugar plus a squeeze of lemon juice. That’s the standard formula that I learned years ago, though these days, because of concern over the use of raw eggs, more and more people are using meringue powder to make royal icing. I’d need to consult with an expert on salmonella, but I personally have a hard time seeing how any bacteria would be able to survive in so much sugar, which is as lethal to microbes as salt. Perhaps the concern is that if by some extraordinary means a few bugs survived in the icing, they might thrive on/in a cake. I find that a stretch as well, but do as your conscience dictates. I’ll include both recipes. Here’s the conventional:
2 egg whites (pasteurized is ideal for people living in the US)
12 ounces (3 cups) powdered sugar
1/2 teaspoon of lemon juice (or a few drops of lemon or vanilla extract)
Put the whites in the bowl of a mixer fitted with a whip. Whip on high until they’re foamy, then with the machine running add the powdered sugar and juice/extract. Whip about five minutes until the icing is thick and glossy. OR
3 tablespoons meringue powder
16 ounces (4 cups) powdered sugar
4 ounces (1/2 cup) water
1/2 teaspoon lemon juice or a few drops of lemon or vanilla extract
Combine the meringue powder and powdered sugar in the bowl of a mixer fitted with a whip. Mix the two on low speed, then turn the mixer up to high and add the water and juice/extract. Whip about five minutes until thick and glossy. Adjust the consistency with more water if necessary.
However you do it, transfer the icing immediately to a pastry bag or an airtight container until you’re ready to use it, because this icing will harden quickly with exposure to air.