Learn the science behind:
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
- Butter – Binds & softens
- Flour – Holds & keeps it all together
- Sugar – Crispness & browning
- Finding your favorite cookie
- Want to do our experiment?
- More cookie science
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.
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.
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:
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- 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.
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!
Want to do our experiment?
All the cookie photos come from an experiment we did with these recipes. If you’re int
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!
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