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Who doesn’t want a light and fluffy, airy, donut? Well, maybe not everyone, but I sure prefer a light and airy donut over a dense greasy one. So when reading that a water roux (also referred to as tang zhong) can help you achieve exactly that, my curiosity was peaked. Why would a water roux create a lighter donut? And does it really work?
So we got to work and made a (way too large!) batch of donuts, with and without the water roux. Of course, we also had a look at our earlier donut repertoire (our sourdough donuts, cake donuts, and ‘regular’ yeasted donuts). And naturally, we dug into the science of this phenomenon. Bringing together bechamel sauce science, starch thickening agents, and donuts!
What is a water roux?
Before digging into the effect on donuts, let’s zoom in on the water roux. As the name says, it’s a type of roux. A roux is a mixture of flour and liquid (e.g. water, milk, stock, but also oil or butter) that has been heated. A water roux is a very simple type of roux. It’s made of just water & flour. In this instance, when making donuts, we’re using a roux with a flour:water ratio of roughly 1:5 (see full recipe below).
Note on the name tangzhong: Here we’re referring to water roux since that is a pretty good english description of what it is. However, you’ll often find it being referred to as tangzhong. This is an Asian (likely Chinese, see comments below) term for describing this same concept. Water roux is often used in Japanese breads to make them light and airy! We’re no Chinese nor Japanese language experts so will stick with the english description here.
Why does a water roux thicken liquids?
The heating process initiates a series of transformations in the mixture. The important one being that the resulting roux will have become a lot thicker in consistency than the original mixture. As such, rouxs are often used for thickening soups and sauces (e.g. in a bechamel sauce). In some cases, as is the case for a dish called gumbo, a roux is heated even further. This results in the formation of a range of delicious flavors and aromas but the thickening power will be less.
Thickening of the water roux is caused by starch molecules, which wheat flour contains a decently large amount of. Starch is a type of carbohydrate, a polysaccharide. Starch molecules are long, complex molecules that easily get entangled. Starch doesn’t dissolve well in water, instead, it floats in the water and absorbs just a little water.
When you heat up the starch though it will start to absorb more water. From approx. 50-60°C (122-140°F), the starch loses its structure, the starch granules (which contain the starch molecules) burst. All these individual molecules will leak out and bind water. As a result, it thickens the liquid. We call this process the gelatinization of starch. It’s a very common process. It also happens when you cook potatoes, make another type of roux, a creamy creme patissiere or boil bagels.
Making a water roux
So, in order to make a water roux, you start by mixing together flour and water. It’s important the water is not warm at this point. You want to make sure you can disperse the flour in the water first, before initiating the gelatinization process (which can cause clumping at this point). You then gently heat up the mixture in a pan on the stove while stirring. Gently, because you don’t want it to burn on the bottom, starch burns easily. As it heats up you will notice that the mixture starts to thicken. This starts at approximately 65°C. When the resulting mixture then cools down it thickens even more, resulting in a gel-like texture.
In our donut recipe we used a 1:5 ratio for our water roux, but this ratio is not set in stone. You need enough water to properly hydrate the flour (based on our experiments we’d recommend at least twice the amount of water to flour). If you have a low moisture content recipe though, you might want to use a ratio of 1:3 or 1:4. That will allow you to incorporate more flour into the dough, without overshooting the water content.
So why use water roux in donuts?
It is commonly said that using the water roux method to make breads will result in lighter, fluffier breads with a longer shelf life. Even though that seems to be quoted commonly, proper evidence for this fact is actually very limited. So that leaves us to hypothesize what’s going on.
As we discussed, when you make a water roux, the starches in the flour will gelatinize. This process would normally also happen when baking bread. However, if would only happen during the final bake and not all starch would gelatinize. The amount of starch to gelatinize seems to depend, among other factors, on the water content of the dough. A higher water content will result in more gelatinized starch.
You want the starch to gelatinize to create a softer and bigger (thus lighter, less dense) bread. Some research has shown that pregelatinized flour (which is essentially what a water roux is) can help improve the quality of the bread. However, evidence is quite limited and not very convincing. As a matter of fact, simply changing the moisture content of a bread dough was shown to have similar effects as using the water roux.
Comparing donuts with & without water roux
Since the scientific literature wouldn’t tell us what was going on. We decided to test it ourselves. We made two batches of donuts, one contained a water roux (see recipe below). The other donut was identical in overall flour and water content, however, did not contain the water roux.
First of all, we noticed that the water roux dough needed more water to come together than the dough without a water roux. This made sense since the gelatinized flour was holding onto that water which then wasn’t accessible during mixing. Also, it was in agreement with previous research who found that water roux doughs do need more moisture.
After frying the donuts, those made with water roux were lighter and airier than those made without. The dough itself was also a little tougher to handle. However, since the two doughs needed adjusting for water content further optimizing of both recipes might have improved both. Suffice to say, there is a difference when using a water roux. But, you might be able to achieve similar outcomes by adjusting a non-water roux based donut recipe. The water roux donuts tasted great though, so worth a try, even if the evidence for them being softer isn’t as strong as we might have expected.
Water roux in other doughs
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Of course, you can use the water roux method in more products than just donuts. Research into water roux has looked into whole wheat breads for instance. By making a water roux out of some of the whole wheat flour you can pre-gelatinize the starch, but also pre-hydrate the whole wheat flour. Whole wheat flour contains the bran of a wheat kernel and this tends to make it harder to make it into a soft extensible dough. By making a water roux first though, you can help hydrate this bran.
We tested it out by making a 50% whole wheat flour (the rest is regular white bread flour) bread with two identical recipes. One used a water roux (made out of 100g of flour + 300g water), the other one did not. both breads turned out delicious but they clearly were different. The water roux dough was stiffer, it had clearly absorbed more moisture. As a result it was a little easier to shape into a ball. However, both breads tasted pretty much identical and apart from some visual differences couldn’t be distinguished.
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