Thickeners are used to add body to dishes such as pie fillings, gravies, sauces, soups and puddings.
Starches are popular thickeners because they thicken without impacting the fat content or affecting the flavour (if properly used.) There are four main starch thickeners: wheat flour, cornstarch, tapioca and arrowroot. The last three are purer starches than flour, and so are more efficient thickeners, but this doesn't mean they are better to use -- they will give a sheen to food, which makes them great for things like fruit pie fillings, but not for gravies, soups or stews, where such a sheen would just look bizarre.
When you thicken a liquid with a starch, it seems you stir it forever, nothing happens, but then the instant you leave the stove for 1 second, all hell breaks lose in the pot and you have lumps for days. Here's what's happening. Starch granules are held together and surrounded by molecular bonds. At around 150 F / 60 C, these molecular bonds break, and admit water into the granules. The granules swell up as the water surges into them, and "gelatinize" (become goopy.) When your liquid hits this magic temperature range, this all happens at once, but not before.
- Cornstarch works well for thickening sauces with cream or dairy in them;
- Cornstarch doesn't work well in acidic sauces;
- Cornstarch doesn't freeze well; sauces and fillings made with it can turn spongy when frozen.
- Arrowroot doesn't work well combined with dairy; it becomes slimy;
- Arrowroot isn't affected by acidity.
- Arrowroot isn't affected by freezing;
- Arrowroot has a very neutral taste, making it good to use with delicate flavours.
- Tapioca thickened dishes freeze well;
- Tapioca starch thickens quickly at a lower temperature than other starches. Useful for last-minute correcting of sauces (if the glossiness wouldn't be off-putting.)Skim any fat off a sauce or gravy before you thicken it. After you've added the thickener, the fat will be harder to remove because it will be blended in.
If something goes wrong and you get lumps in your sauces, don't despair: haul the blender or the food processor out of the cupboard and blend it smooth (or pass it through a fine strainer).
For making gravy using flour, see entry on Gravy.
For tips on thickening soups, see the entry on Soups.