Cornstarch isn't recommended during the reduction phase (nor is any form or byproduct of corn), so you may wish to try arrowroot powder instead. You'll find it bottled in the spice section of many markets, or you can purchase it in larger quantity in some Asian markets, in some Whole Foods Markets, on web sites of spice companies such as www.penzeys.com, www.thespicehouse.com, www.penderys.com, www.frontiercoop.com/products/spices.html or from companies such as www.bakerscatalogue.com.
Here is some information about arrowroot from Cook's Thesaurus:
This starch thickener has several advantages over cornstarch. It has a more neutral flavor, so it's a good thickener for delicately flavored sauces. It also works at a lower temperature, and tolerates acidic ingredients and prolonged cooking better. And while sauces thickened with cornstarch turn into a spongy mess if they're frozen, those made with arrowroot can be frozen and thawed with impunity. The downside is that arrowroot is pricier than cornstarch, and it's not a good thickener for dairy-based sauces, since it turns them slimy.
To thicken with arrowroot, mix it with an equal amount of cold water, then whisk the slurry into a hot liquid for about 30 seconds. Look for it in Asian markets and health food stores. Equivalents: One tablespoon thickens one cup of liquid.
A final note worth mentioning about arrowroot is that because its thickening properties tend to break down with prolonged heating, it's not advised to add the arrowroot slurry until the last few moments of cooking. If worse comes to worse and the mixture ends up thinning, you'll need to mix up a bit more slurry and try again.
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