Reader Patti writes:
I made your roux-and-butter frosting last night and it didn’t work at all. The flour and milk didn’t thicken even after refrigeration even though I simmered it for ten minutes. Once I added the butter I just got soup. Can you tell me where things might have gone wrong?
I certainly can, Patti! It’s the detail you included about simmering the flour and milk for ten minutes. The flour mixture did “gel”, the problem is that afterward it thinned back out again. To explain why I have to amend my analogy from down below where I compared gelling starch molecules to a “net.” The analogy wasn’t terribly far off. Starch molecules do indeed act like a net, just one with fish in it.
Allow me to explain. A particle of ground wheat or corn endosperm is like a bundle of sticks, the “sticks” being individual starch molecules. Immerse that bundle in water and apply heat, and the bundle starts to come apart. Water gets in between some of the starch molecules and pries them off the bundle. The endosperm particle itself, because it contains hundreds or thousands of individual starches, stays more or less intact.
And that’s good because you need both loose starch molecules and large endosperm particles to create a good starch “gel.” To use Harold McGee’s excellent analogy, the molecules become the net and the particles are like fish caught in the net. The more of the larger particles that get caught up in the tangle, the less the liquid around them flows.
But you can overdo this process by applying too much heat. For as long as the water is simmering, the endosperm particles continue to lose their molecules. Eventually they become so small that they slip out of the “net” and — because the individual starch molecules aren’t chemically bonded to one another — the entire lattice collapses. The liquid thins back out again.
It happens all the time to flour-thickened gravy if it’s left on the boil, and it’s what probably happened to your flour-milk mixture, Patricia. Next time, just simmer for a moment or two. That’s all the heat the mixture needs. Thanks for the terrific comment!