Kitchen Science: How Flour Thickens Sauces

One of the kitchen miracles that I always used to find the most astonishing, and bewildering, was the ability of flour, or corn flour, to thicken a sauce. At first when you stir in your floury thickener, nothing happens. The mixture is runny and liquid, barely viscous, it appears to be merely flavoured water, tasty but thin… until then you add heat. Oh, adding heat changes everything, although not straight away. You stir, and stir and stir, you begin to believe nothing will ever happen. If you’re me, you think about adding more flour, or turning the heat up, or somehow making it DO SOMETHING, because you’re impatient. Then, finally, the magic happens, the sauce begins to thicken, you can feel resistance increasing as you stir. If you’re making certain corn flour sauces, you see the mixture begin to turn from opaque to translucent. This is my favourite part of making a lemon meringue pie, actually, apart from the eating it bit.

So what’s going on? What is the heat affecting? The answer is starch. Sauce thickening with flour, be it wheat flour or corn flour, is all about the starch.

Starch is made up of long chains of glucose sugar molecules joined together, to form a huge string. There are actually two types of starch; amylose and amylopectin. In amylose, all the glucose molecules are joined together in one long line. Amylose is sleek and streamlined; the racing car of the starchy world. Amylopectin, however, isn’t so smooth. The glucose molecules attach to one another at different angles, producing shorter bulkier chains, with branches and bristles sticking out; more of a 4×4 than a racing vehicle.

The most important thing about starch, though, is the way in which it reacts to heat. Starch from corn or wheat, or indeed any plant, is stored in the form of tiny solid granules, containing a mixture of amylose and amylopectin (the precise ratio depends on the plant). These granules are starch molecules packed together in a tight rigid structure. When mixed into cold water, the granules absorb a little bit of water, but not much; they are rigid, they don’t like outsiders, thank you very much. At this point, if you add too much water the granules simply clump together at the bottom of the pan, rejecting the water entirely. I think we’ve ALL been there. Lumpy gravy anyone? Gradually adding water distributes the granules throughout the liquid, but they’re still distinct granules just floating about in water, they don’t want to socialise, and they have no effect on the thickness of the liquid.

When you add heat, though, you see changes. Gradually the water molecules gain energy from the heat; they start to jump around in excitement, leaping and bounding around the starch granules. Soon, this jumping and pouncing starts to affect the granules, the weaker sections start to break down, they want to play, and they start to think about interacting with all that energetic water. As soon as one region of the granule breaks down and interacts, this creates room for more water to be absorbed. The more water that the granule absorbs, the more strain it is under to give way and interact with the water. Starch molecules begin to leak out into the water, and flap around joyously. If they happen to bump into another starch molecule, they hold hands and skip together. More and more chains bump together, and hold hands; they start to form a meshy network of chains throughout the liquid, this is when your sauce starts to thicken.

If your starch is mainly made up of amylose, it will thicken faster at this point. The sleek long structure of amylose means that it stretches across a greater area; it is more likely than compact, sturdy amylopectin to bump into other starch molecules, hold hands and form a network.

Eventually, the granule just can’t resist any longer; its rigid structure breaks down entirely. Starch molecules burst into the water, shouting “PLAYTIME”, ready to engage and have some fun. Chains bash together, and hold tight, and the network of interlocking chains brings the sauce to its maximum thickness. Smooth and lovely. Perfect.
So, that’s how starch thickens your gravy, or your béchamel sauce. Or any other sauces you add it to, in fact. Different types of flour and starch contain different amounts of amylose and amylopectin, this means that they may thicken faster, or that you may need more starch to get the same effect. Potato starch, for example, contains a lot of long amylose chains, so you need less potato starch than you would wheat flour to get the same effect. Either way, it’s all down to water molecules breaking down the antisocial barriers of the starch granules, and having a great big sauce-thickening, tasty tasty love-in.

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