I haven't been doing this as long as some of you, so I'm going by what "The Cake Bible" says in terms of the Mousseline Buttercream. Rose Levy Beranbaum has a master's degree in food science, so I pretty much trust her not to poison me. She says this will keep two days at room temperature".
The recipe for Mousseline buttercream in this book says the syrup should be between 248 and 250 degrees. The adaptation on baking911 says the same thing--not surprising since it is credited to The Cake BIble.
Just the same, I don't want to use raw egg whites. I haven't had any trouble using dried egg whites which are not the same as meringue powder, but are simiolar enough that you can use them interchangably most of the time. The difference is that meringue powder has some sugar and maybe cornstarch in it. I don't have any around, so I'm not sure what is added, I just know some things are. The dried whites are just . It contains only pasteurized dried egg whites. It doesn't dissovle really well, but once it has, it works great. I think powdered whites are the same thing as dried--just different names.
I buy my egg whites at Whole Foods Markets. It is a natural foods supermarket. I have seen them at Trader Joe's. I haven't actually checked at the regular supermarket. (I never thought to look) If your grocery store doesn't have them, I'd say check health food stores.
Pasteurized eggs in the shell are available at at least one supermarket here here but the whites do not whip well. I've tried it a couple of times and decided the dried ones are less expensive, easier to keep around, and yield better results--at least for me.
While it is true that substituting any ingredient for another will not always yield the exact same results, some of us have various constraints we have to, or want to, work with, so we do the best we can. As obsessive as I am, I have to be willing to be a little flexible and strive for the best tasting results I can possibly get.