Swiss Meringue (Cooked)

Swiss meringue is simply a cooked French meringue. It consists of combining sugar, egg whites, and cream of tartar in a mixing bowl, and then placing this bowl over a hot water bath. The mixture is constantly whisked until it reaches a temperature between 57°C / 135°F and 60°C / 140°F. Do not exceed 60°C / 140°F to prevent over-coagulating the egg whites (remember that egg white proteins cook at 62°C / 145°F), but do not stop at a temperature lower than 57°C / 135°F, because bacteria will not die below those temperatures, and part of the purpose of cooking egg whites like this is to pasteurize them. Of course, if you are using pasteurized egg whites you do not need to worry about bacteria, just overcooking. If using pasteurized egg whites, heat them to 60°C / 140°F, which will warm up the egg whites and dissolve the sugar at the same time.