For centuries cooks and bakers thickened their sauces with everything from eggs, to nuts to breadcrumbs. Today, there are many pure starches available for professionals and home cooks. In America, cornflour is the most common, while chefs in Europe often use potato starch. Both have their advantages, though the differences between them are significant. It is important to understand how the starches work, and to choose the correct one for a given usage.
About Refined Starches
Starches are made up of simple sugar molecules, joined up by the thousands to form chains. There are two kinds of starches. Amylopectiin makes bushy chains that resemble pom-poms, while amylose makes longer, straighter chains. When heated in water, they form a sort of 3D mesh that traps water and holds it in place. The resulting texture, called a gel, is neither solid nor liquid. Starches high in amylopectin make soft gels; those higher in amylose make firmer gels.
Potatoes are both a popular food and a very starchy one, so it isn't surprising that potato starch was the first to be refined for market. Potato starch makes relatively coarse granules, and may sometimes be detected in the texture of a finished sauce. Like flour, potato starch makes a sauce that is opaque, rather than clear, and has a distinct body. This makes it excellent for pan gravies and similar sauces, but less suitable for more refined applications. Potato starch is the most powerful of the common thickeners, and resists solidifying as the sauce cools because it is high in amylopectin.
Cornflour is processed into a very fine white powder, high in amylose. It is also an excellent thickener, about twice as powerful as wheat flour. Cornflour is more neutral in flavour than potato starch. Cornflour does not make sauces opaque, as flour and potato starch do, and so is valued for the clarity and brilliance of the sauces it can make. Cornflour must be brought to the boiling point before it will thicken, so its sauces have a "cooked" flavour. Too much cornflour will give a sauce an unpleasantly gluey texture.
Cornflour is a versatile and widely used thickener, prized for its many virtues. However, it is unsuited to use in products which will be frozen. When frozen and thawed, a conventional cornflour gel will begin to break down and release, or "weep," liquid. Many home bakers are familiar with this, from freezing and thawing fruit pies that have been thickened with cornflour. Modified cornflour is produced from special varieties of "waxy" corn, and has the ability to retain its texture when frozen and thawed. It is used primarily in commercial food manufacturing.