The more fat in the recipe that less it will crust. So you will want to look for recipes that have a higher fat content. A recipe with a cup of crisco to a lb of sugar will crust. A recipe with 3 cups of fat to 1.5 lbs of sugar will be slow to crust if it ever do.
Also from my understanding, butter in icing can be left out for a few days.
True buttercreams like SMBC or IMBC do not crust over, where "frosting" or "icings" like you are describing (lack of butter hence cannot be buttercream) do crust over when exposed to air and left to sit.
Try making a SMBC or IMBC and see the difference...yes more costly, but yummy!
I just read in my baking manual that you are not supposed to leave this type of icing out of the fridge if made with milk being the liquid due to milk being parishable.
It is the high sugar content (as stated above) that causes the crusting over..the moisture dries out on the surface causing very small sugar crystals to form and thus makes the firm exterior finish.
This was from baking911 website...
Question: Why doesn't buttercream made with butter, powdered sugar and a small amount of milk need refrigeration?
Answer: Because of its high content of sugar and fat. Micro-organisms need water to grow. When the sugar content is so high, the sugar binds the water in such a way that micro-organisms cannot utilize it. Technically its called "water activity control
JoAnn
http://members.nuvox.net/~zt.proicer/message/saved/RECIPES.htm#Chef
try the one chefleah's version. GREAT
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