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I was looking up how to make my own powdered/confectioner/icing sugar. Some 'recipes' say that you should add a bit of cornstarch while others just leave this out.

So what is the role of cornstarch? Does it act like a filler (since it's cheaper than sugar)? Is it to prevent lumps? Does it help with texture? Does it do something else?

If this question is too broad, assume I'm only talking about frosting, since that's a frequent use of this sugar.

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FYI, I am sure that you know already. But not all powdered sugar has cornstarch. Maybe the question should be more like why is corn starch often added to powdered sugar. Perhaps I am just nitpicking. –  jeffwllms Apr 4 '12 at 16:44
    
@tastefive you're right. I'm interested in what it does, so that I know the difference between powdered sugar with and without cornstarch. –  Mien Apr 4 '12 at 17:05
    
I understand powdered sugar with and without cornstarch. It is on my list of things to do powdered donuts and make American buttercream frosting with "homemade" powdered sugar. I swear I can taste the cornstarch in typical American powdered sugar. One of these days I will whirr my own. The major purpose of cornstarch in powdered sugar is to keep it from clumping in the bag. If I can get granulated or "superfine" sugar to powder in my food processor, I doubt I'll ever go back. –  Jolenealaska Nov 8 '13 at 6:43
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2 Answers 2

up vote 15 down vote accepted

It's to prevent caking. See, for example, the second FAQ on Domino Sugar's website:

It is not recommended to substitute confectioners sugar for granulated sugar. Since confectioners sugar has a much finer texture, and it contains a small percentage of cornstarch to prevent caking, substituting can give you unexpected results.

Many shredded cheeses include corn starch for the same reason.

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I'm sure you are correct. I grinded some regular sugar and it was one hard piece two or three days later. I cut it up again, added some cornstarch and now (a week later), it's still powdery, like it should be. –  Mien Apr 15 '12 at 19:13
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The cornstarch does indeed prevent the extremely fine grained sugar from caking, but it also serves a purpose beyond that. Since cornstarch forms a non-Newtonian fluid when water is added, adding it to powdered sugar allows you to use it to make glazes and icings. Without the cornstarch, you'd just be pouring sweet water over your pastries, but with cornstarch you have a glaze that will coat and set.

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