Several readers have written in with questions about starch thickeners: wheat flour or cornstarch (corn flour) in particular. Several have told stories about failed puddings which thickened at first but later thinned out after several minutes of simmering. They key thing to remember when it comes to starch-thickened mixtures is that they’re a as thick as they’re going to get as soon as they come to the boil. Starch-thickened puddings, creams or sauces can handle maybe 2-3 minutes of boiling, but that’s all. Beyond that they start to become thinner rather than thicker.
Why? The reason has to do with the way starches behave when they get wet and hot. A particle of ground wheat or corn endosperm is like a bundle of reeds (the “reeds” are the individual long-chain starch molecules). Immerse that bundle in water and apply heat and the bundle starts to come apart. Water gets in between the starch molecules on the outside of the bundle and starts to pry them off. Still the main bundle stays more or less intact.
That’s good because you need both loose starch molecules and larger particles to create thickening. To use Harold McGee’s excellent analogy, the long, stringy starch molecules become a net which trap and hold the starch bundles (the fish). The more large bundles that get caught up in the tangle, the less the liquid around them flows, so the mixture thickens.
But you can overdo this process by applying too much heat. For as long as the water is simmering, the endosperm bundles continue to shed starch molecules. Eventually they become so small that they slip out of the “net” and — because the individual starch molecules aren’t bonded to one another — the entire lattice collapses and washes away. The mixture becomes thin again.
I should add that vigorous stirring can also thin out a starch-thickened mixture as it simply wrecks the starchy lattices. So go easy on the whisk!