Role of flour, butter and sugar in shortbread cookies – Cookie science

One of the more popular posts on this website is one explaining the role of ingredients in choux pastry. Which I completely understand, it’s a bit of a strange process to make choux pastry and a little harder to understand. It’s also something I really like, I like understanding what and why I do stuff when cooking, so I decided to take a similar approach for cookies!

The challenge for cookies though is that there are so many different types of cookies. They can be crunchy, chewy, crispy, thick, thin, round, Christmas tree shaped, etc. So whereas I was able to describe quite well what happens in a choux pastry and how to make it, it’s a little different for cookies. Different recipes have different ratios of ingredients and all give a slightly different cookie.

So I split it up, there’s recipes on my blog (such as these for peanut butter cookies, pepernoten or macademia cookies) and there’s deep dive posts of which this is an example! Interested to read more (cookie) science, have a look at my round up on cookie science on the internet or the introduction to food science.

Shortbread cookies

Explaining theory is a lot easier if you’re got some actual practical examples. So when looking into the role of flour, sugar and butter in cookies (these ingredients are part of just about every cookie) we need an example. To make sure no other ingredients interfer we’ll use a cookie recipe which is actually made up of just those ingredients: flour, sugar and butter. These cookies are commonly called: shortbread cookies (in Dutch ‘zandkoekjes’).

It’s easiest cookie recipe I know. As you can see below, it’s just three ingredients, mixed together in a dough:

Shortbread cookies
 
Prep time
Cook time
Total time
 
Two styles of shortbread, both work, but slightly different.
Author:
Ingredients
Dutch 'zandkoekjes'
  • 100g flour
  • 50g sugar
  • 75g butter
  • sprinkle of salt
Shortbread for Millionaire's shortbread
  • 150g flour
  • 50g sugar
  • 100g butter
Instructions
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.

Since there’s only three ingredients (even the salt is optional!), we’ll zoom in on the role of each of these one by one:

Flour

Flour is the ingredient that makes a cookie into a solid cookie that doesn’t fall apart or form one flat puddle. This is due to its two main components: gluten and starch.

Gluten are proteins and these proteins can form a structure within a cookie. However, when making a shortbread you don’t want too much influence of the gluten (which is why this cookie doesn’t need a lot of kneading) as you do with bread for instance. This will give a more crumbly structure. Read more on gluten in a separate post on the topic.

Starch is the bulk ingredient here that strongly influences the cookie, together with the gluten, it will cook and transform the dough into something crispy. Starch will absorb moisture and swell up when heated (it’s what happens when boiling potatoes as well for instance). However, there is only very limited water available in a shortbread recipe, thus the starch won’t absorb that much water.

Butter

Butter has several functions in these cookies. Butter is made up of approx. 20% water and 80% fat. Each play a different role in this cookie.

As mentioned when discussing flour, starch can absorb water. And when this starch + water mixture is heated the starch granules will swell up. Without water this process will not be able to occur, that is the first role of butter.

Second, the butter provides fat to the cookie. Fat will enrich the cookie, it will become softer. Making a cookie of only water, sugar and flour tends to make a dry tasting cookie. Butter is also the ingredient that will melt during baking. This contributes to spreading out of cookies, although in the case of shortbread this effect tends to be minor. The butter is held back by the flour.

Last but not least butter contributes to flavour and appearance. Besides the two main ingredients water and fat, butter also contains some proteins and little sugar as well as some milk flavour molecules. Oil definitely smells different than butter when heated in a frying pan and that is partly due to this effect. The protein molecules also contribute to browning of the cookie in the so-called Maillard reaction (which we describe in greater detail in various other posts on this website).

Sugar

Last but not least, sugar. Without sugar your cookie would taste quite bland (more like a plain crust crust). The sugar gives it that sweet taste.

Also, without sugar the shortbread cookie wouldn’t be as nice and brown. Browning of the cookie is mostly due to caramelization of the sugar as well as the Maillard reaction (for which we also need the proteins to make it happen).

Another role of sugar is less obvious though, it’s the impact of sugar on the structure of the cookie. A shortbread contains very little water, therefore, most of the sugar won’t dissolve during mixing. However, when the cookie is heated in the oven, some additional sugar will dissolve. This is due to the phenomenon that solubility generally increases at higher temperatures. Upon cooling of the cookie though, the sugar will crystallize again, this will give a cookie its distinct snap. It’s also a reason why warm cookies generally don’t snap, yet, their sugar has not yet crystallized (nor has the fat solidified).

Discussing the ratios (tweaking a shortbread cookie)

The ratio of ingredients of the recipe we just discussed are as follows:

  • 1 part of flour
  • 3/4 part of butter (note that butter is at least 80% milk fat, and approx. 16% water)
  • 1/2 part of sugar

However, even within shortbread recipe world there is variation. This sits mostly in the varying ranges of these three ingredients. Some recipes have a ratio of 1:1 flour and butter. I’ve also seen recipes with both higher as well as a lower sugar content.

The ratio of these ingredients has to be chosen such that:

  1. The cookie doesn’t fall apart completely when baking (imagine using a lot of butter & sugar, it will just spread out!)
  2. The dough sticks together and forms a ball (imagine only using sugar and flour, no way will that form into a ball)
  3. The cookie tastes good, some like it sweeter, some prefer it more plain (especially if you’re still planning to use a super sweet icing on top)

However, all in all, shortbread cookies are quite hard to fail completely. In most cases you’ve got quite some freedom to lower or increase one of the ingredients to have the cookie the way you like it!

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