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Kitchen Science: the many wonders of humble flour

Have you ever wondered why you have so many different types of flour in your pantry? You might have cornflour and arrowroot, self-raising, and plain flour. And if you like to bake bread, possibly strong bakers’ flour.

The reason for such diversity is that each of these flours has different chemical and physical properties that help them do their job in recipes.

No lumps

Cornflour has an interesting property that makes it perfect for making smooth custard and sauces. This is because it does not “whet” very readily and typically forms clumps. Cornflour starch is a large chain-like molecule that is tightly wrapped up in starch granules, so it doesn’t dissolve in water, as do salt or sugar.

These large molecules tend to clump together as they are “hydrophobic”, which means they have a tendency to avoid water. This is likely caused by tiny mutually repelling charges on the water and starch molecules.

The action of mixing a small amount of water with starch to make a paste helps to prevent the clumping, and helps to disperse the starch (the colloid) to form an emulsion, which is a solid dispersed within a liquid, but not fully dissolved.

Although, as soon as you stop mixing the paste, the starch starts separating from the water. So make sure you mix the paste just before you are about to put it in the sauce.

Pastes of cornflour in water are often described as a non-Newtonian fluid. These are remarkable substances that bend our conception of how a fluid normally works.

A non-Newtownian fluid can be poured, but if you strike it quickly, it will go stiff and hard. This is because the colloid changes the surface tension of the fluid to make it behave as if it were solid when struck. This tends to work only when the starches are uncooked.

So what happens when starches are cooked?

If you add starch to a heated sauce, the sauce thickens. This is how we thicken gravies and sauces, and can set a blancmange.

This effectively is forming a plastic, although not quite in the way we normally think of plastics.

The effect of heating starches in water helps to breakdown the bonds in the starch molecules. This starts to form hydrogen bonds with water in what is called gelatinisation. In this process, water is acting as a plasticiser.

The result is a semi-permanent solution as the starches dissolve. This is why cooked starches go transparent, unless you add other things to the sauce, of course.

To make the gel more permanent, starches can be modified to help stabilise the hydrogen bonds that form with water. If you don’t do this, the starch can start to retrograde and collapse, releasing water. You can see this if a blancmange or custard has been left for a few days.

These retrograde starches are found in pasta, unripe bananas and rice that has been cooled. These are hard for us to digest, but can be good source of food for the bacteria in our large intestine. This may be good for us but can result in a little flatulence.

Strong for light bread

Cornflour and arrowroot are very rich in starch and low in protein, so they are great for thickening sauces.

However, if you want to bake bread, you are often told you need strong bakers’ flour. Why? What is wrong with normal flour?

The answer is simple: not enough gluten. While it is trendy in some quarters to avoid gluten, unless you have an intolerance to it or coeliac disease, consumption of gluten is perfectly fine.

If you mix flour with water, and then rinse until the water runs clear, this removes all the starch, leaving behind the gluten. ilovebutter/Flickr, CC BY

In fact, developing the bonds between the two proteins that make up gluten (gliadin and glutenin) gives a dough the stretch and potential to capture bubbles of gas that can give bread its structure. Stronger bakers’ flours simply tend to have more of these proteins (around 11 to 13g of protein per 100g, compared to nine to ten grams in normal flour). So, when choosing a flour to make bread, if you want a light airy bread, a strong flour is needed.

What’s in a bad pie?

However, if you want to make a light crumbly pastry, gluten can be your enemy. Most of us have eaten a pie where the pastry was a little leathery, so what went wrong?

In terms of protein, this time less is more. This is also why fat is used, which helps the gluten to form. The more the pastry is worked, the more gluten is developed, and the tougher the pastry.

Flours have many fascinating properties. This is why you need a range of different flours in your pantry if you want to cook a range of different things. So, when choosing the flour to cook with, it is important to make the correct choice and work it properly.

This article is part of the Kitchen Science series, exploring the amazing physics and chemistry going on in our kitchens every day. If you’re an academic with an idea for a Kitchen Science article, get in touch!

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