Well....Meringue powder as you know from looking at it is a fine white powder. It’s made primarily from dried egg whites…..it does have cornstarch in it, to keep it from clumping while stored and some food gums. Food gums, what the heck? The food gums help the meringue bind together easier when your whipping it up. Once reconstituted with water and beaten at high speed, you get fluffy meringue.
The primary advantage of meringue powder is that it is pasteurized. Pasteurized equals less potential for food borne illness such as salmonella, or "sammynella" as my little sister use to call it growing up. This trait makes it ideal for recipes which call for uncooked egg whites.
The process of creating meringue powder starts by drying egg whites, then mixing them with cornstarch and gum, creating the white powder we know as Meringue Powder. Flavorings like vanilla are often added, partially to cut down on the slightly starchy taste of the meringue powder. If you don’t like the flavor of meringue powder you should play around with how much flavoring you add because sometimes depending on the brand you are using the flavor of the meringue powder can be very strong.
With regards to your class buttercream recipe, we are adding meringue powder because it acts as a stabilizer, keeping the shortening in the recipe (or half shortening/half butter depending on how you made it) as well as the other liquids in the recipe from breaking down. You can also use the meringue powder to add body to boiled icing and regular meringue (think pie...mmmm pie sounds good right now! Blackberry cream pie....). Oh, sorry! Back to meringue powder...
It can also be used in place of egg whites in most recipes, but only when the recipe calls for egg whites; it cannot replace a whole egg. You can also add a tablespoon or two of meringue powder to your cake mix or a cake batter recipe to add volume to the cake.
I was asked about a “natural” substitution for the meringue powder....there is no substitution for meringue powder it's self. However, you could use fresh egg whites or pasteurized egg whites but you would have to make a boiled icing recipe to use that method. Boiled icing....that's a whole other topic! One of which I will delightfully cover at some point! ;-) But for now, let's stick to the basics!