How does flour thicken?

Good question, reader Luther! It’s commonly said that flour granules “pop” in a hot, wet environment, releasing their starches and creating thickening. But that’s not quite right. Think of a flour granule as a tightly-bound bundle of reeds and branches (amylose and amylopectin [starch] molecules), a broken-off chunk of wheat berry endosperm. Immerse that granule in hot water and the water molecules start to intrude into the mass, prying off the individual starches. Those molecules then disperse into the liquid.

Gelatin proteins behave in a similar way. However the big difference between starches and proteins is that starches don’t bond with one another like proteins do. Thus the “net” that they make is really more like a tangle, a logjam as it were, that keeps the water within from flowing. And when water molecules can no longer flow, you get thickening.

Continuing with the logjam analogy, it’s the combination of lots of little twigs and branches (the individual molecules) and big logs (the mostly-undissolved granules) that causes thickening. If the logs all broke down into little twigs, the current of the river would wash them all away. This is essentially what happens when you heat a flour-thickened sauce for too long. It reaches optimal thickness just as it’s coming to a boil, where the proportion of logs and twigs is just right for stopping liquid flow. Beyond that point the sauce begins to thin out again as the logs begin to break into pieces and are carried away in the stream.

2 thoughts on “How does flour thicken?”

  1. Just to be sure I’m clear about this, I believe you’re saying this applies to both corn starch and white/wheat flour when you cook to thicken? I know when making pastry cream I’m supposed to bring the milk, yolks and corn starch just to a brief boil, and now I’m learning it’s the same for thickening the milk and flour for heritage frosting. Is it the same exact principal of the logs/twigs analogy and boiling method in both cases with corn starch and flour? Are there any other flours used for thickening that you know of that this would also apply to? (last question just out of curiosity).

    I’ve had thin custard/pudding/pastry cream problems with not boiling the pastry cream base due to fear of curdling the yolks, and in another woefully misguided attempt to correct my errors, I’ve cooked the corn starch/milk separately to thicken, then added the yolks and cooked to 175° which is the advised temperature to thicken yolks – little did I know that left their amylase intact and broke down the starch, turning my custard runny by the next day. SMDH. Between that and overcooking my heritage frosting base and having a similar thinner/gooey result, I am full of chagrin for all the years of attempting to improve the thickening powers instead undoing them by my efforts. FINALLY I expect I’ll be able to have good results next time I make either recipe, thanks to your various scientific explanations.

    1. Hey again Julia!

      Yes, you’ve got it, this principle applies to any starch thickener: wheat flour, corn flour, tapioca, the list goes on.

      And I can sympathize with your errors. I myself have done similar things. Remember the nice thing about sugars and starches is that they get in the way of those clumping proteins, so they can’t curdle as easily. Something like a custard is really a system, not a collection of discreet ingredients all doing their own thing. If you have a good quality recipe, just watch your temperatures and move forward with confidence. The rest should take care of itself!

      Cheers,

      Joe

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