Yesterday there was a question on Facebook, whether there is a danger of salmonellae infection by using Royal Icing on cookies, cakes etc. And as the question arises really often, I sat down and tried to read more into this subject and thought you also might like this information.
Royal Icing is the main medium to decorate cookies and the classic choice for cakes, but many have concerns, because of the use of egg white, without baking or boiling. As it is used fresh from the shell, the fear of salmonella infection is then quickly obvious. Anybody who has been through such a disease, knows that all this is not funny and I speak from experience here, too (thanks frozen yoghurt's in Florida 1997 ;) )
For eggs the bacteria are found on the outside of the eggshell. If the egg is „infected“ and cracked the bacteria have the chance to get into the egg/egg whites. In addition, eggs from battery hens are less affected than in free-range ones. So if you choose to use fresh egg white pay attention, as hopefully you will always do, to common cleanliness and cleaning equipment, bowls, table tops etc. thoroughly. And as I just learned: don't wash your eggs! If you try to „clean“ them, you will damage the natural protective cover and the pores will be open for the microorganism to get into the egg.
Pasteurized or dried egg whites are the alternative, if you want to play it safe (and for me dried egg whites are much easier to use, as you can weigh it more accurate). Normally it is made of normal eggs white by being spray dried by heat. It is pasteurized simultaneously, which kills most of the microorganisms. But even if you use pasteurized or dried egg whites: if the environment during processing is not clean, salmonella can get back into the egg whites!
To multiply, salmonella need the appropriate living conditions. Their growth is limited when moisture is removed, by acid or also when a lot of sugar is added!
Sugar, as well as salt, is one of the oldest preservative in human history. It is bacteristatic, i.e. It inhibits the growth of bacteria. By using a lot of powdered sugar to make royal icing from egg white, the salmonella my not reproduce properly. Furthermore, once royal icing is properly dried, w the moisture will be missing, which the bacteria need as well. Often lemon acid or cream of tartar is added (though more to avoid cracking), so the living conditions are extremely difficult for salmonella.
If using dried egg whites now instead for fresh ones, the risk of Salmonella disease from decorated cookies, gingerbread houses, ornaments etc. o cakes is very low. However, when the already dried royal icing gets wet again (for example on a cream cake), left for a length of time and PREVIOUSLY was affected, then the Salmonella can multiply again. So if you are mindful of the preparation and try to avoid getting bacteria into the icing while preparing it, you won't have any in your finished product
The very high sugar content is really important and as well as the complete through-drying, for a safe use of Royal Icing. Macaroons, meringue etc., for example, don't have such a high level of sugar, so they need to be heat-dried in the oven, to make more durable.
And please allow me a little historic addition: royal icing has been used for centuries. The traditional English wedding cake (rich fruit cake, covered with marzipan and royal icing) were embarked at colonial times to be shipped from London to India (just an example) to show the standing in society of the brides family, by presenting a wedding cake from 1000 miles away. If over the decades/centuries serious illnesses have resulted from eating cake, humanity would certainly not be so stupid to use this kind of method over and over again.
I hope this information will help you a bit in your decision of using royal icing. It isn't something dangerous, as long as you work and prepare in a healthy and clean way.