The costco sugar has beet sugar in it instead of being pure cane sugar. From what I understand from my instructor you don't want to use beet sugar in a BC brcause it doesn't hold up as well as cane sugar. I forgot what she said precisely would/wouldn't happen if you use beet sugar. I just stored it and moved on. lol!
My Wilton instructor told us that powdered sugars actually vary in their cornstarch content based on the areas to which they are shipped - so the stores in our humid South Texas climate, for instance, would receive sugar with more cornstarch in it than, say, the stores in Arizona.
I would love to be able to verify that!
The difference between cane sugar and beet sugar, as far as I can tell, is that cane sugars will give a smoother, creamier icing. Beet sugars are a little more gritty.
I don't know what costco sells, I know Sam's club used to sell a 50 lb beet sugar bag and a 20 lb (i think) cane sugar bag.
But the original question about corn starch. Corn starch is added to all the powder sugar I have ever seen. It's to prevent caking. It's what helps the sugar stay in a fine powder and not react to humidity.
I'm not a pro either - but I use costco powder sugar all the time - I am on my 3rd 50# bag. The only thing that I don't like about it is that after you open the bag you always have to sift the sugar before you use it or its lumpy.
if you buy the 2# bags from wal-mart - you just open and dump - I really HATE sifting!!
It is much cheaper and I never noticed a difference in my buttercream/fondant or anything.
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