Learn the science behind:
Why a Water Roux Improves Your Donuts
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
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.
Harold McGee, On Food and Cooking; find our review here
Baking Industry Research Trust, Starch, link
Jenni Field, Tangzhong Bread Recipe | Perfect Pain au Lait Sandwich Bread, Pastry chef online, 2013, link.
Shigehiro NAITO+, Shinji FUKAMI,, Yasuyuki MIZOKAMI,, Rieko HIROSE-, Koji KAWASHIMA.,
Hiroyuki TAKANO/, Nobuaki ISHIDA+, Mika KOIZUMI+ and Hiromi KANO, The Effect of Gelatinized Starch on Baking Bread, Food Sci. Technol. Res., 11 (2), 194-201, 2005, link
Ottavia Parenti∗, Lorenzo Guerrini, Valentina Canuti, Giulia Angeloni, Piernicola Masella,
Bruno Zanon, The effect of the addition of gelatinized flour on dough rheology and quality
of bread made from brown wheat flour, LWT – Food Science & Technology, 106 (2019), 240-246, link
Hi there! Loved your scientific analysis of tang zhong (aka water roux). This is a technique we often use in making Japanese or Asian style “milk breads” to make them soft and fluffy (without going stale too fast).
Hi Mika, thanks for coming by! Yes, it’s an interesting technique, I hadn’t used it before, but it worked out well :-).
Glad you found my article useful. I love the Tangzhong method. And now I *need* to give it a try with doughnuts, although just like with you, mine would be gone too quickly to see if Tangzhong extends shelf life! lol
Great that you came by :-). I would sure advise trying it out, I’ll have to try out other recipes with Tangzhong in the future as well!
Hi i just did your recipe using water roux.. Is it normal? Cause my dough is soft and sticky? Thank you
Hello, thank you for coming by! The dough is slightly more sticky and wet than a normal bread dough due to the water roux and high moisture content. However, you should be able to roll it out. Here are a few tips that might help: make sure that when you make the roux it has thickened up properly (it shouldn’t flow freely anymore). If it still feels too wet after it has risen, feel free to add some extra flour to knead and form it (it’s a pretty delicate dough, so be generous with flour to prevent it from sticking to the surface). Last but not least, I’ve noticed that flours from different regions require more of less water, if your dough hasn’t come together into a ball before adding the water roux but is still very wet, add some extra flour to compensate. Hope this helps!
Hi, is the milk 60 gE in liquid or Powder ? Sorry This is My first time trying water Roux donut .
Thank you !
Hi Ann, that will be liquid milk! Thanks for asking and good luck 🙂
Tang zhong is chinese not japanese
Thanks for coming by! The majority of sources I have found state it is Japanese so I based my statement on that. That said, it is also used by the Chinese and I found statements for both, a few examples for Japanese are this one or this one) or this one, for Chinese I found this one.
As usual with food history, origin stories of food tend to be messy and not super clear, so it might have been either. If you have a reliable source for me to quote on the Chinese origin, I’d be happy to include that as well!
Regardless of who originated the technique, “tangzhong” is not a Japanese term. Japanese doesn’t even have a “zh” sound, so if you see a foreign word romanized with a “zh”, you can be sure it’s not Japanese.
As far as I can tell, the actual Japanese word for water roux is “yudane”:
and the word “tangzhong” is in fact Chinese:
Thank you Josh! So it seems the technique originated in Japan, but that the word we’re describing to use it is Chinese.
Is there any specific ratio, I have my own doughnut recipe and it already has water, more butter and eggs than yours per cup of flour.
I already use a roux but I feel you use about 12.5%( roux) flour weight compared to the flour used in the actual dough recipe(20g/160gx100=12.5%). Is this a good ratio or will it be affected by my increased butter/water/milk levels so I will need less of the roux.
Hope that makes sense.
There is no ‘perfect’ roux:flour ratio, there’s some flexibility there. A few watch outs and tips, hope that helps answer your question:
Hope that helps, and if it doesn’t answer your questions fully yet, please let me know!
Hello, I was wondering how you would adapt any donut recipe to include the roux. I have one that I really like but would love to incorporate a roux to extend shelf life. Is there some sort of formula?
Yes! You can definitely incorporate a water roux into an existing recipe. You might need to experiment a little since the roux does tend to soften your dough so handling may be slightly different. Some tips for a first try:
Hope that helps! Good luck 🙂
Alright! Thank you for your message. I will do just that, follow your instructions and test with a 1/4 of the recipe. I’ll be doing some math this weekend
hello, i’ve tried this recipe for the first time and its just amazing! the best one so far!
but i do have one question, can you freeze the dough? and if yes how long can you keep it in the freezer before it goes bad?
Yes, you can freeze this dough. I would recommend freezing it right after you’ve cut the dough into circles, just before the last raise. That was, when you thaw them, you can wake up the yeast and ensure that last proof happens for a nice and fluffy donut. From a food safety perspective, you can freeze it almost indefinitely (assuming your freezer is at -18C/0F). However, the dough can dry out over time because of freezer burn. As such, I would recommend freezing the rings on a tray in the freezer until they’re solid. Then put them in a sturdy plastic bag together and get rid of as much air as possible and wrap as tight as possible. That way you should at least be able to store them for 2-3 months.
Good luck and glad to hear you’ve enjoyed the recipe!
Plain flour not s. W. B. Flour.
Roll out to 1/4 to 1/2 inch thick
3 inch diam cutter and 1 inch cutter for centre.
Hi, i just tried this recipe and it utterly failed! after the batch fermentation, it didn’t rise AT ALL, and the dough was so wet it didn’t stick together at all. When i look at the hydration percentage, taking into account water and milk, it’s at 100% hydration, which seems wrong. Any clue what I could have done wrong?
This was quite an old recipe (>5years) so we had to re-do the recipe to figure out what might have gone wrong. You were right, we noticed that there was an error in the recipe, it did not contain enough flour at all. We’ve upped the flour content and that made it work perfectly again! Our apologies for the error, but hope you’ll give it a try again. That water roux really does make a difference :-)!
Hello. I came across your excellent site when searching for a way to make “starchy water” and was curious if you think your water roux might answer this slightly off topic question! Many chefs recommend reserving some water in which pasta is cooked so it can be added to the sauce to help bind and thicken the sauce. Additionally it helps with buttery or oily sauces to emulsify it into a creamy, non-greasy coating. To create a really starchy water, some utilize the “risotto technique”. This works well for smaller pasta shapes but I have found it to be more difficult with long spaghetti although not impossible. Others suggest saving pasta water and adding it to fresh water which will increase the the starch content, although that runs the risk of inadvertently also increasing the saltiness of the water. And in even another technique I came across, the “French Guy Cooking” concentrates starchy water in advance which he uses for cooking cacio e pepe. My question is how to make starchy water without cooking pasta. On line a number of options are given including microwaving water and corn starch. But if dry pasta is made with just flour and water, would making a really dilute water roux work? If so, then I could make it in advance and have “really starchy water” on hand whenever I wanted to create a lovely pasta sauce (eg freeze in ice cube tray to then be added to the sauce.) Or is there something different about the starchy water created when cooking pasta vs. raw flower cooked in water.
This is a fascinating question! Not sure I have a straight-off answer but I’ll throw in some thoughts and hypotheses :-).
I did some digging into the scientific literature and it seems that most of what leaks out of the pasta into the cooking water is amylose. Amylose is one of the two main components of starch in flour. There isn’t that much protein that’s lost. That means that the composition of the leaked materials in the water is very different from that of the original flour.
That seems to agree with the fact that the water is indeed ‘starchy’. It does mostly contain starches that have leaked out of the pasta dough during cooking, even though it is only part of the overall starch. (I should also put a disclaimer here, there’s surprisingly little knowledge in this field, so it’s just a handful articles that state this, most of them quite old and actually focussed on a slightly different research topic.)
That would mean that if you’re trying to replicate the composition of this water, you’d need to find a way to also just get this amylose. Somehow, you need to bind the protein and other starch components that are present in flour during cooking. That’s where I think using the water roux might not do the job. A water roux will still contain all the components of the flour, you haven’t ‘filtered’ out the amylose specifically. It will also contain the proteins. As such, the composition will be quite different. It seems that making a thin dough (which pasta is) is actually quite a good way to ensure those proteins stay put (as most of the amylopectin, the other part of the starch) while the amylose gets a chance to leak out.
What about trying to make a paste with a starch though, corn, tapioca, or potato starch for instance. That way you might be a little closer to the actual composition of the starchy water?
But that does bring me to another question. I don’t think we really know what the ideal composition for such a ‘starchy water’ is. It may well be that it’s not a problem if it also contains the gluten and other components of flour, and can still do the job? I suspect there might be a lot of ways to do this. Traditionally, it definitely made sense to just use the pasta water, since you have it at hand anyway. But if you’re trying to use the same principles, for something completely different, I’m pretty sure there are other ways to get there.
Another aspect to consider is just how much water you use to cook the pasta. If you use a lot of water (which is advised traditionally), the concentration of the starch in the remaining water is quite low. However, you can cook pasta with significantly less water. I found research saying that the loss of starches etc. was very similar regardless of the amount of water used. So, I’d suggest cooking up a pot of pasta with enough, but not too much water. Then, save this pretty concentrated water with a lot of starch and use it in other dishes. You might even have to dilute it, since it’s so concentrated.
Just a short aside, in your question you mention that some suggest saving pasta water and then adding it to fresh water as it should increase the starch content. It will increase the starch content of the fresh water, but it will dilute the starch content of the pasta water, so not exactly sure what the benefit is?
Hope that makes sense and do let me know if you do any further experiments!