Learn the science behind:
Baking cookies – What happens in the oven?
Are your cookies in the oven (or did they just come out)? And are you wondering what it is that transforms the uncooked raw dough into crispy or chewy cookies? Of course, the heat causes these transformations, but what’s the chemistry behind the changes? We’ll do a deep dive into just exactly what happens in the oven, some entertainment for you to read while waiting for those cookies to come out!
What happens in the oven
The biggest difference between being in and out of the oven: temperature. As soon as that cookie enters the oven, the surrounding temperature jumps by over 140°C (250°F). The outside of the cookie will heat up very quickly, but the inside, depending on how large the cookie is, will take a while to heat up (we’ve discussed this phenomenon before, using pie as an example). However, once the heat has touched your cookie, even if it’s just the outside, the baking begins!
A cookie dough…
There are a lot of different cookies out there, with a lot of different ingredients. Just exactly what happens to a cookie depends on what’s in that cookie. Generally speaking though, most cookies will contain most if not all of these ingredients:
- Fat: this can come from oils, butter, margarine, chocolate, tahini, nut flours, etc.
- Leavening agents: most often this is baking soda or baking powder
- Protein: eggs and milk are full of protein, but wheat flour contains them as well (gluten) as do nuts as well as aquafaba (a commonly used egg white replacer)
- Starch: often come from flour (wheat might be most common, but can also be a wide range of other sources such as rice or tapioca)
- Water: eggs, milk (whether plant- or dairy-based), fruit, they all contain water
- Sugar: this can be ‘regular’ granulated sugar, lactose (in milk), or syrupy sugars such as corn, maple, or rice syrup
…starts to transform
Once the cookie dough enters the oven, each of these ingredients starts to transform in one way or another. What’s more, they’ll likely interact with one another as well. Some will simply change phase (e.g melt or evaporate), others will participate in a chemical reaction or transform permanently. We’ll dig deeper into each of the mechanisms mentioned below, trying to figure out how they’ll impact your final cookie texture and taste!:
- Fat: if your fat is solid before entering the oven (e.g. coconut fat, butter, margarine), it will melt in the oven. This starts at quite low temperatures, by 50°C (122°F) most has gone liquid.
- Leavening agents: are activated by the heat, causing the formation of carbon dioxide, lifting up the cookie
- Protein: every protein behaves differently, but most will denature, for some that helps the cookie to set, others don’t impact texture as much
- Starch: the starches will cook, similar to what happens when cooking potatoes
- Water: evaporates once it’s above its boiling point (100°C / 212°F), if it evaporates within the cookie but can’t escape, it might help it to rise slightly
- Sugar: browns the cookie due to caramelization & Maillard reactions (the second also requires proteins to happen)
Melting of the fats
If your cookie contains some solid fats, think of (non-melted) butter, margarine, or coconut fat, melting of these fats is one of the first things that will happen in the oven. Since solid fat gives the cookie dough firmness (it’s why you might be asked to put your butter cookie dough in the fridge before cutting or rolling it), melting of the fat does the reverse. The cookie becomes softer.
If your cookie contains a lot of formerly solid fat, it might soften so much that the dough starts to spread out. Whether this happens depends on the ratio of your ingredients, as we noticed when testing it out on shortbread. For instance, flour helps a cookie to hold together. If your cookie uses less of it compared to the amount of fat, chances are it will spread more (good example: these brown butter cookies, which are supposed to spread a lot).
The molten fats can also make your cookie glossy. This is caused by the liquid fats, which reflect light differently than the solid ones did.
Melting of most fats used in baking is mostly completed at 50°C (122°F). It is as such one of the first major changes you’ll see in your cookie.
Leavening agents come into play
If you’re looking for an airier cookie with a slight lift, you’ve probably added some baking soda or baking powder to your cookie. Baking powder and baking soda work in a very similar way. Both contain sodium carbonate. When sodium bicarbonate gets into contact with acid it will react to form carbon dioxide gases. These are what expand your cookie dough. These reactions do occur at lower temperatures (especially for baking soda), however, they go much faster and to a further extent when happening at elevated temperatures.
This is where the oven comes in. Once the dough heats up, the leavening agents will start to expand your cookie dough. This reaction starts at quite low temperatures, well below the actual temperature of the oven. This is a good thing. All the expanding of the cookie has to happen before the cookie sets (to what extent this happens in the oven depends on the cookie). Once proteins for instance have started to set, they become less inflexible.
Denaturation of proteins
Both melting of the fats and the expansion caused by leavening agents start at quite moderate temperatures, just above room temperature. The remainder of the processes start later, once the cookie has heated up more. Also, processes will become messier and most of them will occur simultaneously, while also interacting with one another.
Denaturation of proteins is one of those processes. Proteins are complex folded structures of long chains of amino acids. These 3-dimensional structures are important for their functionality, but most proteins can’t stand high temperatures. Increased temperatures (roughly starting at 60-70°C, but this is very dependent on the specific protein) will cause proteins to unfold in a process called denaturation. This process causes changes in the proteins behavior.
Just how denaturation impacts the proteins’ behavior depends on the protein. If you’ve seen an egg cook, you will have seen that it stiffens and changes color, there’s a big change going on. In general, a large percentage of egg protein will ‘set’ because of denaturation. This helps your cookie to firm up, especially if it is based on an egg white foam structure.
Other proteins on the other hand have less of an impact. Milk proteins for instance, don’t firm up in a similar way, nor do gluten proteins. Nevertheless, their denaturation will impact the cookie’s texture and structure.
Cooking of the starch
So far, most changes that we’ve discussed haven’t actually ‘baked’ the cookie, that is irrevocably changed the cookie. But, as we all know, once a cookie is baked, there’s no way you can transform it back into that initial cookie dough.
Melting of fat and the action of leavening agents, neither permanently change the cookie. If the cookie dough can’t hold onto that gas that is being formed, it will just dissipate again over time, The denaturation of proteins is irreversible and it’s mostly important for egg based cookies. the other major permanent transformation is the cooking of the starch.
Starch is one of the main components of most flours (e.g. wheat, rice, or cassava flour). Starch is a large, complex carbohydrate. When brought into contact with water starch absorbs water. When you then heat this mix, the starch swells up, to absorb even more water. At some point the starch granules will break, releasing individual molecules. This process is called gelatinization. If you’ve ever made a roux based sauce (e.g. bechamel), you will have seen this happen in front of your eyes.
This gelatinization process, the cooking of the starch, helps set the cookie. If you don’t have enough flour (or egg, which has a similar role) in your cookie, your cookie might collapse later on in baking, not being able to hold onto all those components.
Evaporation of water
As long as the surrounding air is dry enough, water evaporates. At room temperature, this happens slowly. But, if you leave a bowl of water out for long enough, it will dry out over the course of several days. As you might intuitively know, this process happens a lot faster at higher temperatures. When water boils, at 100°C at sea level (212°F), water is continuously evaporating.
In a hot oven, water within the cookie dough will evaporate as well. In some cases, the water can easily escape. In other instances, the water evaporates but remains trapped within the cookie. This can help a cookie to expand slightly (though is less strong than the effect of baking soda and baking powder).
If enough water evaporates your food can become crunchy and crispy (see our example of crispy chicken skin). It’s why crispy foods have very little, e.g. potato chips or crackers.
As such, control of the evaporation of water is important to get your ideal cookie texture. If you want your cookie to be chewy, you don’t want to evaporate too much water. However, if you’re looking for a crispy cookie, you might want to leave that cookie in for a few extra minutes so more water can evaporate. Chewiness/Crispiness is influenced by the recipe itself, but a few more (or less) minutes in the oven can make a huge difference for sure.
Browning of the cookie
Lastly, once the cookie has gotten nice and warm, a ton of chemical reactions occur simultaneously that contribute both flavor and color to your cookies. Most of these reactions involve some sort of sugar.
Depending on your cookie type, the sugar within can caramelize. Caramelization only happens at temperatures well above 100°C (212°F). The exact temperature depends on the type of sugar in your cookie, sucrose (the chemical name for the sugar in granulated sugar) caramelizes at a different temperature than glucose and fructose (present in invert sugar) for instance.
Since caramelization happens at quite high temperatures, it won’t happen in every. In order for it to happen there needs to be very few water and quite a lot of sugar (preferably on the dry outside).
A more common chemical reaction that happens in almost every cookie is the Maillard reaction. The Maillard reaction is a reaction between reducing sugars (e.g. glucose) and proteins (e.g. those in butter). Even though the same describes it as just one reaction, it is actually a series of complex reactions, all happening simultaneously. Along the way a lot of different flavor molecules are formed, as well as a brown color.
The Maillard reaction occurs at room temperature, but only very very slowly. It can take months before you see a change in color. At higher temperatures though it speeds up considerably, especially when the amount of moisture is not too high (which is often the case in cookies).
Once you’re happy with the progression of all these processes, it’s time to take your cookies out of the oven. You may judge so by the color of the cookies, their shape or just based on experience of the required baking times.
Don’t be surprised though if your cookie doesn’t yet look like what you had envisioned. It’s probably still a lot softer in texture for instance. Remember, the cookie is still hot, so all those fats are still liquid. Also, the starch structures are softer and will thicken up upon cooling. Only once your cookie has cooled down completely, will you be able to judge whether it’s what you hoped it would be. And, even during cooling you still have some influence over what happens. We’ve got another post for you, covering all you need to know about cooling down your cookies :-).