Pastry student Nick writes in with this very good question about Italian meringue buttercream:
When making the IMBC, does the heat from the syrup at 245F fully pasteurize the meringue? Or do standard undercooked food warnings apply?
Friends, this is what gives me confidence in the pastry world of tomorrow. A student sees a delicious-looking egg-based recipe and the first thing he thinks to ask is: is it safe? Bravo.
The answer is that IMBC is a safe recipe — or at least so my references tell me — one that can be stored at room temperature for up to two days. Though I haven’t measured it myself, I believe the reason is in fact because the hot syrup brings the temperature of the meringue up to 160 degrees Fahrenheit, the temperature at which Salmonella bacteria are destroyed (the sugar in the meringue keeps the egg proteins in the whipped whites from “cooking”, which would ordinarily start to occur at about 140 degrees).
Sugar is another bug-killer that works to the advantage of the buttercream maker, and Italian meringue has quite a bit of it. Then there is all the fat, which while it doesn’t kill bacteria, doesn’t provide a hospitable environment in which it can grow. Taken together, I believe this is what renders IMBC a “safe” preparation. However I am by no means the last word on these kinds of things, and you should take this up with a true food safety expert if you have more questions.