The Science of Shortbread - Role of Flour, Butter, Sugar (2024)

Feel like eating a cookie? But want something simple, with only a few ingredients? Shortbread cookies are what you should be looking at. You can make this cookie style with as little as three ingredients. Mix flour, butter, and some sugar together, bake the rolled out dough and you’ve got some simple, but delicious cookies!

And with just three ingredients, you can make a range of different cookies. From very crispy to soft and crumbly to extremely buttery. Once you know how the ratios for these ingredients impact the final cookie texture, you can make your cookie exactly like you want it. Crispier, or more buttery, whatever you prefer!

Science starts with experiments

In order to test the impact of all the different ingredients, you will have to make several batches of cookies. Each running the risk of failing miserably. But, failing is good here because it will teach you what not to do!

When comparing these different recipes you’d want to consider which judging criteria to use on these cookies. A good scientist will come up with these before doing the actual test. In the case of shortbread cookie science, there are a few factors you can consider:

• Do the ingredients form a coherent dough?
• Does the cookie turn a nice colour in the oven?
• What is the shape of the cookie coming out of the oven? Does it form a puddle, or hold its original shape quite well?
• How does the cookie taste? What is the texture like (hard, brittle, crumbly)?

After making several batches of cookies we could conclude that the ratio of just these three ingredients has a major impact on how your shortbread cookie turns out. Let’s have a look at them one by one.

Butter – Binds & softens

Have you ever tried making a cookie with just flour and sugar? Probably not. You can easily imagine that just mixing these two ingredients together won’t make a dough. Instead, you’ll be left with just powder. Sure, you can bake it, but it won’t magically form a coherent cookie once it is in the oven.

Binds ingredients together

This brings us to the first important role of butter in this simple shortbread cookie: it binds the ingredients together! You need butter to make a dough in these cookies.

Butter consists of about 80% fat with 20% water. Both the fat and the water help bind the ingredients together. The moisture allows the flour to hydrate. Flour needs to hydrate to properly ‘cook’ as we’ll discuss further on. The moisture allows the starch to gelatinize and bind the dough, similar to how it does so in a roux for a bechamel sauce.

Fat sits around the different powder particles and sticks to itself. However, this fat all melts once the cookie goes into the oven. As this point butter will spread into a puddle if there isn’t enough flour and sugar to hold it all together.

Color

Aside from the water and fat, butter also contains a small amount of proteins. These proteins can drastically impact the color of your cookie. Proteins and sugar react together in the Maillard reaction which is what causes browning of the cookies. Using more butter can help more of those reactions to occur.

If you’re using another type of solid fat instead of butter, e.g. margarine, you may notice less browning of the cookie. This is because those fats do not contain these extra proteins.

Flavor & texture

Butter contains a lot of flavor. And with only three ingredients in this cookie, butter is an important part of the flavor. What’s more, butter also contributes richness to the cookie. A cookie with very little butter (assuming you replace the butter with water) will give a drier, crisper cookie.

Summarizing butter

Using too much butter will:

• Cause your cookie to spread out way too much during baking
• Cause your cookie to brown

Using too little butter will:

• Not allow you to make a dough, the dough will either not come together at all or be very crumbly and hard to roll out.
• Give a drier, crisper cookie.

Flour – Holds & keeps it all together

Ever tried making a cookie with just butter and sugar? Again, you probably haven’t, but try to imagine what would happen though. They will form a ball together, however, it will be quite greasy and sticky. Also, once in the oven, the dough will into a puddle with nothing to hold it together.

This is where the flour would come in. Flour absorbs some of the moisture of the butter, drying the dough. The butter will cover the flour particles in the dough. Once the cookie is in the oven, the butter will melt, but the flour will continue to hold its strength, holding the cookie together.

Most of the work in the flour comes from the starch in the flour. Starch is a large carbohydrate. The starch absorbs water while making the dough. Once the starch gets hot in the oven though, it will undergo a process called gelatinization. During this process the starch swells up and is able to hold onto even more moisture. Cooked starch can thicken sauces, but also hold onto a cookie structure.

Flour forms the base of a cookie but it also contributes to flavour and colour. Starch contains some sugars that will also participate to some extent in browning reactions.

Sugar – Crispness & browning

You might have tried to reduce the sugar content of a cookie. Finding the previous recipe too sweet or just wanting to cut down on sugar in general. And whereas there are ways to replace sugar, they will likely impact the final cookie outcome. Sugar isn’t just in your cookie for sweetness, which is an important role, it also greatly impact the final cookie texture.

Colour

First of all, sugar also greatly impacts color. Just have a look at the cookie on the photo below that doesn’t contain any sugar. It is by far the blandest of the four.

Increasing sugar content increases browning, but only to a certain extent! In the cookies on the tray below the highest sugar cookie isn’t the darkest. This is likely because this recipe also contain relatively less moisture. The Maillard reaction proceeds faster within certain ranges of moisture content. Once there isn’t enough moisture anymore it will slow down!

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

Crispness and size

Did you also notice that the cookies with most sugar are the largest ones? To be clear, all these cookies were cut to the exact same round size. The high sugar cookies simply expanded more during baking. This is because there was relatively less flour that helps to hold it all together. Sugar does not help to hold the cookie together.

But, notice that these cookies are less spread out than those with a lot of butter. In fact, their outer edges are even a little thick. This is because sugar does help increase the viscosity of the cookie. Sugar dissolves in water and more dissolves at higher temperatures. This prevents it from flattening out completely.

That cookie on the far right, with 3x the amount of sugar, that was also by far the crispiest cookie. It was almost too hard for a cookie and was very crisp. This is an important role of sugar, it creates a harder, crisper and crunchier cookie.

So why does it do that? The sugar that partially dissolves in the oven, recrystallizes into strong crystals when the cookie comes out of the oven. Not all crystals even dissolved in the oven, but they will still be hard when the cookie comes out of the oven.

Summarizing the impact of sugar

Using too much sugar will:

• Give a very crisp and hard cookie
• Make a sticky dough (sugar pulls on moisture from the surrounding)
• Make the cookie too sweet

Using too little sugar will:

• Make a very bland tasting cookie, it’ll be more like a savory pie crust.
• Give a very whitish colored cookie
• Make the sweet not sweet enough

Finding your favorite cookie

What’s so great about these simple cookies is that there aren’t really a lot of ways to truly fail. As long as you stay within a certain space (see below for our experimentation range) there’s a lot you can vary.

If you’re uncertain, take a recipe you know and start varying the sugar content. You’ve got quite some freedom here without completely ruining your cookie!

Shortbread cookies

Prep Time: 10 minutes

Cook Time: 15 minutes

Total Time: 25 minutes

There are a lot of basic shortbread cookies out there. These are two starting points for you to experiment with. Both give a crumbly cookie that doesn't greatly expand in the oven.

Ingredients

Dutch 'zandkoekjes' / Basic shortbread

• 100g flour
• 50g sugar
• 75g butter
• sprinkle of salt (optional)

Shortbread for Millionaire's shortbread

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

Instructions

Follow the same instructions for both recipes

1. Mix all ingredients together and knead into a coherent ball of dough. Leave to cool in the fridge to firm up slightly, max. 30 minutes. If you left it in for several hours or if it turned really hard, leave it on the counter until it has softened enough to flatten it again. (If it's not that warm in your house you can skip the fridge step, cooling in the fridge does help to get a nice flat cookie.)
2. Take your baking trays and cover them with parchment paper or a baking mat.
3. Roll out dough and cut into shapes or fill the bottom of a baking tray depending on whether you can to make individual cookies or a base of a bigger bake.
4. Bake in the oven at 180C (350F) for approx. 15 minutes. The overall required time depends on the thickness of the cookies and on the colour you'd like. Slightly browner cookies require a little longer baking time, but make sure they don't burn! Also, if you prefer your cookies extra crisp, just leave them in for an extra minute!

Want to do our experiment?

All the cookie photos come from an experiment we did with these recipes. If you’re int

Shortbread Cookie Experiment

Prep Time: 1 hour

Total Time: 1 hour

Want to do our cookie experiment as well? We've got 7 recipes for you (of which you'll use one twice) to repeat our experiment. You'll end up with 6 cookies of which 2 aren't great by themselves but work perfectly fine re-used in other recipes (don't just throw them out!).

Materials

• 275g all-purpose flour
• 160g regular sugar
• 235g butter

Tools

• Baking trays with parchment paper/baking mats
• Oven
• Mixing bowl
• Rolling pin
• Cookie cutter (you can use a glass!)
• Pen and paper to make notes!

Instructions

Recipes

Every cookie will be made with a slightly different recipe, all using just those three ingredients.:

1. 50g flour + 17g sugar + 33g butter
2. 50g flour + 33g sugar + 17g butter
3. 50g flour + 0g sugar + 33g butter
4. 25g flour + 25g sugar + 50g butter
5. 50g flour + 33g sugar + 50g butter
6. 50g flour + 33g sugar + 33g butter
7. 50g flour + 50g sugar + 33g butter

Making the Cookies

1. You make every cookie type in the same way (see notes for recipe 2 and 7 below). Weigh the ingredients in a bowl and knead the dough together by hand. You can use an electric mixer but then you'll need to at least double your recipe and for experiments we prefer working with smaller quantities.
2. For each dough, roll it out in a sheet of about 5-8mm thickness. Using your cutter (or glass) cut out shapes of your cookie.
3. Place the cookies on a baking tray. Take care to keep the different types apart.
4. Bake them all in a pre-heated oven at 180C (350F) for 15 minutes. Your oven might be a little different,so if you need more or less time just adjust. But, be sure to bake all of them in the exact same way to determine differences between the cookies!

Notes

Recipe 2 will not come together in a dough. It is added to help you see the impact of using too little butter. After you've tried, use this batch of dough to make recipe no. 5, just add some extra butter and see how it now comes together!

More cookie science

The role of ingredients is just one part of the science of cookie baking. There are so many other aspects you can tweak and improve upon. Here are just a few suggestions:

• The importance of cooling cookies
• Investigation of what really happens in that oven
• Tweak your ingredients further, e.g. browning your butter!

References

King Arthur Flour, How to reduce sugar in cookies and bars, March-15, 2017, link ; a great piece of work looking at the effect of sugar in cookies