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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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Other starches might be used, such as Tapioca starch. This might make a difference in baking certain recipes, see here: seriouseats.com/2016/03/… – Agos May 27 at 16:30
up vote 17 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

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 (makes liquid more viscous) 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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Syrup becomes viscous all on its own without adding a thickener, AND the kind of thickness you get from cornstarch would usually be not the kind you want in a cake glaze - you might want it in a filling, but then you will add cornstarch anyway. – rackandboneman May 25 at 9:49
    
@rackandboneman Syrup can become viscous on its own, but not at concentrations that can be achieved via stirring at room temperature. AND Glazes are one of the primary uses of powdered sugar. – SourDoh May 25 at 12:39
    
Oh OK, I'm used to making such glazes the hot way - and also, the thickening effect from cornstarch isn't very strong now if you don't heat it? – rackandboneman May 25 at 15:57

A better anti-caking agent for icing sugar is tricalcium phosphate. Cornstarch gives a gritty texture to buttercream or other icing (frosting), whereas tricalcium phosphate keeps it much more smooth. In the UK, Silver Spoon icing sugar, or Sainsbury's own brand - I don't know about other supermarkets - is made from sugar beet rather than cane sugar, and has this better anti-caking agent in it. Tate and Lyle has cornstarch. A lot of people have complained online about the gritty texture of the cornstarch kind, and I found out the hard way after making lemon cake filling and wondering why it was gritty. Had to buy the better sugar and start again! I have never had any problem with the watery icing suggested above: using the right amount of sugar to water will set the icing properly, and it will coat things successfully.

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