Powdered sugar is not that same as granular sugar and it has cornstarch mixed in with it to keep it a free flowing fine powder. To process amylose (corn starch) into the sugar units that make up the starch the bees do indeed have an enzyme to do this - it is known as diastase or amylase. The powder version of granulated sugar is more of a product attribute due to how the sugar was handled and transported (texture) resulting in the sugar being of a much finer consistency. If you're talking about the sugar having corn starch in it, confectioner's sugar, I would not feed that alone but in a mixed supply with granulated sugar it could be OK.
Erik
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