Sauces like Port and Cranberry Sauce and Tangy Rosemary and Tomato Sauce are thickened with cornstarch, which gives them a translucent, glossy finish. Made from the endosperm of the corn kernel, cornstarch has a thickening power similar to that of flour, but its neutral flavor means it doesn't need to be cooked as long as flour does. It's easy to use, but there are a few things to keep in mind.
- Dissolve the cornstarch in water or another cold liquid to make a slurry; if it's added directly to a hot liquid it will create lumps.
- Restir the slurry just before adding it to the sauce; it will separate as it sits.
- Whisk it into simmering liquid-cornstarch reaches its full thickening power at around 200°F--a little at a time until you get the right consistency. It takes 1 tsp. to 1 Tbs. cornstarch to thicken 1 cup of sauce.
- Don't continue to whisk vigorously once the sauce has fully thickened (which takes about 1 minute), and avoid simmering the sauce for a prolonged time after adding the cornstarch. In both cases, the gelatinous starch molecules may break apart, which will thin the sauce again. If this happens, bring it back to a simmer and add more cornstarch slurry.