shortbread, caramel chocolate bars

Role of flour, butter and sugar in cookies – Shortbread science

Take some butter, mix it with sugar, add some flour and you’ve got a cookie going. Three ingredients is all you need to bake a decent cookie. Of course, you add more ingredients, nuts, fruits, some chocolate, the options are endless, but you don’t go anywhere without your three basic ingredients.

That said, your cookies can still fail miserably with these three ingredients. Ending up with a puddle instead of a crisp cookie, or a cookie that just doesn’t hold together. One the other hand, you can also vary your cookies on purpose, making them more or less crispy for instance.

And if you didn’t know, that’s what we call cookie science. There’s so many experiments you can do with your cookies and this quick guide to the role of these three basic ingredients in your cookie, should get you rolling!

Science starts with a recipe

Good cookie science, starts with a good cookie recipe to start with. Start your science from there. Increase your butter, decrease your sugar. Feel what you think is good to do. But if you want some guidance, continue reading because we’ll be discussing the role of all three ingredients next.

Shortbread cookies

Shortbread cookies

Prep Time: 10 minutes
Cook Time: 15 minutes
Total Time: 25 minutes

Two styles of shortbread, both work, but slightly different.


Dutch 'zandkoekjes'

  • 100g flour
  • 50g sugar
  • 75g butter
  • sprinkle of salt

Shortbread for Millionaire's shortbread

  • 150g flour
  • 50g sugar
  • 100g butter


Both recipes work the same:

  1. Mix all ingredients together and knead into a coherent ball of dough. Leave to cool in the fridge to firm up slightly (not too long or it will be impossible to roll out flat).
  2. Roll out dough and cut into shapes or fill the bottom of a baking tray (make sure to layer the trays with baking paper, it will make it a lot easier to take out the cookie!).
  3. Bake in the oven at 180C for approx. 15 minutes. The overall required time depends on the thickness of the cookies and on the colour you'd like. Slightly brown cookies require a little longer baking time, but make sure they don't burn, towards the end a few minutes more or less can make quite a difference in colour.

Imagine mixing sugar & butter and baking that in the oven? Any idea what will? Chances are you’l end up with a puddle buttery burnt sugar. As soon as the butter-sugar mixture gets in the oven the butter will melt, flatten out and you certainly don’t have a cookie… That’s why flour is in your cookie. Flour holds it all together.

How flour does the job? You’ll have to look at starch and gluten, the two main components in flour.

Gluten are proteins in flour. You’be probably heard of developing gluten when baking bread. In cookies you don’t want to develop gluten (which is why you don’t knead the dough extensively). But gluten still play a role. When you heat your cookie dough in the oven and the gluten proteins will denature and solidify in their structure. It helps to make the cookie, but it isn’t as essential as the other ingredient: starch.

Starch isn’t a protein, instead it’s a carbohydrate. The carbohydrates in starch though are pretty long chains of carbohydrates. These molecules do two things when making a cookie. First of all, starch absorbs water and holds on to it creating a firmer dough. Then, when the cookie gets cooked the starch will ‘cook’ as well. It will swell up and hold on to even more water, thickening the cookie. Think of making a bechamel sauce, where you make a roux in which the starch does a similar thing. Since there’s only little water in shortbread, the main role isn’t necessary to hold on to a lot of water, but it will do enough to create that solid structure.

Butter – Binds & softens

Another cookie though experiment: what happens if you make a cookie with just flour & sugar? It probably won’t be the cookie you’re hoping for. Instead, it will just be a heap of powders! The flour and sugar will brown in the oven, but they certainly won’t hold together. That’s where butter comes in, butter binds the flour and sugar together, but not just that, it also softens and enriches your cookie.

If you remember, butter is made up of approx. 20% water and 80% fat. That starch we talked about before will absorb some of that moisture.

So let’s do another thought experiment. What happens if we mix flour, sugar and water? Again, it won’t be the shortbread cookie you’re looking for. It will be more bread or crisp like. It won’t have that richness and smoothness and crumbliness.

That’s where the fat comes in. The fat will mix nicely between the flour and the sugar and stick everything together. Fat also prevents gluten from forming a network. That way, your cookie is more crumbly and it will be less dry.

Butter is interesting also in how it affects baking. Butter is solid in the fridge, soft at room temperature but it will turn liquid in the oven. If there’s too much butter in a cookie if will spread out too much. Too little and it won’t hold together well. Cooling dough before it goes in the oven will help it keep its shape.

Last but not least, butter specifically (a but some other fats as well) contributes to flavour and appearance. Butter isn’t purely fat and water, it also contains some proteins, a little sugar and various flavour molecules. The flavour molecules impact the flavour of course. The proteins on the other hand really impact the colour of the cookie. The proteins can react with the sugar (the Maillard reaction) forming brown colour.

Sugar – Crispy & brown

You’ve guessed correctly, yes, sugar contributes to the sweetness of your cookie. More sugar will give a sweeter cookie. Again there’s more though.

Sugar contributes to browning of the cookie. Sugar caramelizes but also participates in the Maillard reaction which browns the cookie of nicely.

But sugar also helps to create a crispy cookie. Just butter and flour (yes, another thought experiment) make a softer consistency. Since a shortbread cookie only contains very little moisture, most of the sugar will not dissolve. Instead, it will remain crystalline. In the oven, some more sugar will dissolve though (at higher temperature more sugar dissolves in the same quantity of water). Upon cooling though, it will recrystallize again, giving the cookie it’s crisp. This is part of the reason why cooling cookies properly is so important.

Tweaking a shortbread cookie

Now that you know your science, it’s start to do some science. Take a look at the cookie recipes at the top of this post and start experimenting. Want a less crispy cookie? You might want to use less sugar. Want to have your cookie spread out, maybe add some more (warmer) butter!

If you want you cookie to stay a shortbread cookie, there’s still ample opportunity to tweak. There’s recipes for shortbread with 100:75 ratio of flour:butter, but there’s also recipes that use a 100:100 ratio.

Of course, there’s a chance some of your recipes will fail. But use the guides above to improve it again and get where you want it to go to!

Advanced cookie science

The science of cookies has more to it than just understanding these three ingredients. You can start decorating your cookies (shortbread is very well suited for that!), vary your oven baking process, start adding your ingredients in a specific order, make one on the barbecue, or have a look what else is out there online.

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