Here's an interesting article:
http://www.sfgate.com/cgi-bin/article.cgi?file=/chronicle/archive/1999/03/31/FD91867.DTL
If your powdered sugar label doesn't state that it is pure cane it is most likely beet. However, generic or store brands will get their sugar from wherever it is cheapest so if cane sugar is cheap that week thats what they will use. So you could get a store brands that works great for months and then all of a sudden it is beet sugar and you will have issues. The only way to know for sure is to use the brands that state that they are pure cane sugar.
Beet sugar does not carmelize at the same stage as cane. I had the most problems when making candy, it would never really carmelize and get to the hard stage that I needed, I would heat it way hotter than the recipe suggested and it would just crystalize. I also notice a difference in my baking. When I make certain cookies with beet sugar they go flat. In icing it just tastes a little chalky. After trying the cheap stuff and having intermitten results I finally looked into it and found out that it was because beet and cane sugars were being offered interchangably as the same product. So now I buy only pure cane sugar. On the West coast that is C&H and costco carries it in bulk so it's not too expensive.
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