Baking cookies – What happens in the oven

Cookies + science = happiness, isn’t it? We’ve discussed cookie science quite a few times here on the blog. Whether it’s the role of different ingredients, the role of food colouring or just to share recipes. We haven’t discussed though what happens in the oven, where the cookie actually gets transformed from a soft dough into a crunchy, chewy, crispy cookie. If you’ve ever stood in front of an oven while cookies are baking, you will literally see the transformation occurring, right in front of your eyes!

Just in case I’m the only crazy person who stands in front of ovens with cookies baking inside, I’ll show you what happens.

The steps of cookie baking

When cookies are placed in an oven there’s one big change: the temperature around the cookie will increase. Come to think of it, that temperature does all the remaining work to transform the dough into the cookies. More happens then you might think. Let’s start of by summarizing the main processes and we’ll take a deep dive in each of them as we go.

  1. Melting of fats (this occurs at pretty low temperatures, butter is mostly liquid at 40°C)
  2. Activating of leavening agents such as baking soda & baking powder
  3. Denaturation of proteins (e.g. those in eggs, but also those in flour, the gluten)
  4. ‘Cooking’ of starch
  5. Evaporation of water
  6. Browning of the cookie due to caramellization & Maillard reactions

Each of these processes influences how the final cookie will look like. If there is a lot of fat, a cookie might spread out a lot, if there’s no leavening agents, no expansion will take place, no evaporation of moisture (or only little) will make for a softer, less crunchy cookie. So let’s dig into the theory.

what happens in the oven - cookies just went in
The cookie dough has just been placed in the oven, the dough has become glossy, this is the fat that has just started to melt on the outside.

Step 1: Melting of the fats

The cookie dough needs some time to come up to temperature, so the first process that will occur is that which happens at the lowest temperature. In most cases that is melting of the fats. If only oils are used this is not relevant, they are already liquid. But, if butter or margarine is used in a cookie recipe, these will only melt in the oven. Both are almost fully melted at 40°C.

Melting of the fats makes the cookie dough very soft and flexible. If there is not enough flour to hold the cookie together it might run completely flat, whereas higher flour content cookies will keep their shape a little better. The cookies used in this post contained quite a bit of flour do didn’t flatten out that much. Other cookies though can flatten out quite a bit.

The increased flexibility of the cookie caused by the melted fats will help the next few steps to occur. In those next steps the final shape of the cookie will be determined, because at this point, nothing has really changed permanently yet. If the oven would be turned off, you would be left with a warm dough.

Step 2: Leavening agents come into play

The leavening agents also do their work early on in the baking process. The leavening agents expand the cookie, add some more air bubbles and create the ‘craters’ in the cookie. Once the cookie starts ‘cooking’ it will not be able to change shape that much anymore. So, any expansion (which is what leavening agents do) has to take place at the start of the baking process.

In a previous post we’ve discussed leavening agents to a greater extent. We’ll skimp through them quickly though. Both baking soda and baking powder work in a very similar way. Both contain a carbonate. This carbonate can react with acid and upon the reaction with an acid carbon dioxide, a gas, is formed. It’s this gas that causes the expansion of our cookies.

If baking soda is used in a cookie, there will always be another acid ingredient in the cookie dough for it to react to. For baking powder this isn’t necessary since the acid already sits in the baking powder (in the dry form). If you mix baking powder and baking soda into a dough (or batter, it’s easier to see in a batter) you might already see bubbles forming. This is some of the active ingredient reacting and forming carbon dioxide. However, the majority of the component will react once the cookie is placed in the oven, the heat will accelerate the reaction.

what happens in the oven - cookies getting their shape
If you look closely you can see the cookies have slightly expanded compared to the previous photo. You can also see they’ve got flat bottom which is the fat that has sunk to the bottom.

Denaturation of proteins

Whereas the first two processes occuring can be called 1 and 2, it becomes a little more vague from this point onwards. Several processes will occur at the same time. At this point in time the leavening agents are probably still doing their job, some fat might still melt. However, at this point other chemical reactions set in as well, one of which is the denaturation of proteins.

Most proteins cannot stand heat, from approx. 70 degrees Celsius they will have trouble keeping their original shape. In order to understand what’s happening you have to know that a protein is a very long chain of building blocks (amino acids) that are organized in a very complex 3D structure. This structure is not made up of covalent bonds between atoms, instead, there are all sorts of interactions going on here. Since they are less permanent than an actual bond between two atoms, relatively mild heat can mess them up. In denaturation the proteins lose their original 3D structure. In the case of egg proteins they will set (hence the boiled egg).

Keep in mind that it’s not just the egg proteins that denature, it’s also the gluten proteins in the flour. However, the gluten proteins do not cause as much thickening as the egg proteins do. Gluten proteins again work slight different.

Even though most proteins set in some way when heated, there’s one big exception: milk proteins!

Cooking of the starch

Starch is one of the main components of flour. When brought into contact with water it will absorb the water. When heated though, the starch will actually swell and absorb even more water. This gelatinization as it’s called is a very common process when baking bread, muffins, pizzas, making a roux or pie filling, but also when baking cookies.

Once the starch has swelled and gelatinized this process cannot be reversed. So when cooling down the starch will not be able to revert to its original structure.

what happens in the oven - cookies almost finished
The cookies are almost ready to leave the oven.

Evaporation of water

Water evaporates at 100°C, so once the cookie dough has reached that temperature moisture will evaporate and leave the cookie. Moisture might not be able to leave easily though if it’s trapped inside the dough. In that case it might either take longer for the moisture to evaporate or it might not evaporate at all!

The amount of moisture evaporation greatly determines the crispiness of your cookie. The moisture moisture evaporates during the cookie baking process, the crunchier it is. Less moisture evaporation gives a more moist and chewy cookie. Since moisture evaporation tends to take a while to get going (and takes even longer at the center than at the outside), baking time will strongly influence the amoint of moisture evaporation. Longer baking times, means more evaporation of water, means crispier cookie.

Browning of the cookie

In most cases we’d like our cookies to turn a little more brown in the baking process (though not in all!). Browning of the cookies is caused by two reactions:

  • Caramellization: which is solely caused by sugar and only occurs at very high (well above 100C) temperatures
  • Maillard reaction: a reaction between both proteins and sugar

To which extent browning occurs mostly depends on the recipe and the time & temperature of the oven. The Maillard reaction is sped up by a higher content of sugar as well as proteins (e.g. milk, butter, egg). Both are sped up by higher temperatures. However, they also require a certain level of moisture to occur well. Too much moisture (as well as too little) will slow down these reactions.

This is why browning generally occurs more towards the end of the baking process. By that time most of the moisture has evaporated and browning can set in.

Hazelnut cookies oven experiment three different temperatures
In the photo below you can see an experiment with different baking temperatures for cookies and the effect on browning (and blackening).

Cookie recipe

Curious to know what recipe I used when making the photos for this post? They’re a chocolate type cookie, using cocoa powder (make sure you use the right type of cocoa powder) and turn out pretty soft, not that crunchy.

Baking cookies - What happens in the oven
Prep time:
Cook time:
Total time:
Serves: 30 cookies
  • 115g butter
  • 150g brown sugar
  • 50g granulated sugar
  • 1 egg
  • 1 tbsp vanilla extract
  • 1 tbsp orange zest extract
  • 1 tbsp lemon zest extract (if you want to leave the orange & lemon extracts out you might want to add 1-2 tbsp of water)
  • 250g flour
  • 20g cocoa powder
  • 1,5 tsp baking powder
  • pinch of salt
  1. Mix the butter and sugar. Add the egg and the extracts.
  2. Add all the dry ingredients (flour, cooca powder, baking powder and the salt) and mix them all through in one go. You can either do this by hand or using an electric (stand) mixer.
  3. Shape the dough into little balls, the exact size doesn't matter, but will influence your baking time. I made at least 30 cookies out of this dough.
  4. Place the balls on a tray and bake in a pre-heated oven at 180C for 12 minutes.
  5. Leave to cool before packing.


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