Chemistry is A Piece of Cake – The Science of Baking

baked cake looks yummy

When you think about it, baking involves a ton of precision: measurements, timing, a certain elegance in the kitchen that all come together to yield a delectable product. All this seems remarkably similar to a chemistry experiment, yet few pastry chefs deem it necessary to understand what happens at a molecular level throughout the baking process. Also, what in the world is gluten? Welcome to the chemistry of baking.

It’s almost like doing a high school chemistry lab practical, you throw in a bunch of compounds under the right conditions and bam! You get acetylsalicylic acid. But real chemists know that in order to take the next step, to be able to identify improvements to the recipe – or even create a new one yourself – we must develop a deeper understanding of the ingredients and processes involved. Enough talk, let’s apply some chemistry to the art of baking!

First, Flour

Our first focus is flour, as it makes up the bulk of a cake. Gluten is what makes this ingredient special, consisting of proteins called gliadins and glutenins. They are large proteins with molecular weights between 20,000 to 100,000 grams per mole1!

While gliadins are monomers, glutenins tend to aggregate into extremely large structures (mw over 10 million) by interchain disulfide (S-S) bonds. When water is added, these structures are joined by hydrogen bonds, ionic bonds and hydrophobic bonds in order to give the dough its characteristic structure and elasticity.

Without gluten, the starchy bits don’t allow for gases to be trapped. Here’s some dense gluten-free bread I made earlier… I’m kidding, of course


Water, Water, Everywhere

Just as in our bodies, water works by dissolving all the other components, providing them a medium to perform their required chemical reactions. Water, having a high heat capacity, also controls the internal temperature during baking, while keeping the dough nice and moist.

It’s essential in all pastries and yet many recipes don’t call for water – that’s because all the water the dough needs comes from wet ingredients such as melted chocolate, milk and eggs. The interesting thing about eggs is that though their main role is to ‘bind’ the other ingredients, egg whites also protect the air bubbles during the baking process, forming a cooked layer around them and preventing them from bursting. This ensures your delicious cake is also free from lumps and unevenness!

No Substitute for Yeast

Hydration of the dough activates amylase, which breaks down the starch into simple glucose units. Yeast can be added at this point, which converts glucose to ethanol and carbon dioxide, which causes expansion (leavening) of the dough:

C6H12O6 → 2 C2H5OH + 2 CO2

In cakes, baking soda (sodium bicarbonate) is preferred to yeast, due to time constraints. Yeast takes a little while to ramp up production, requiring a few hours of being left alone to do its dirty work. It does almost the same thing when heated, producing carbon dioxide and causing expansion of the cake:

2 NaHCO3 → Na2CO3 + H2O + CO2

The problem here is that sodium carbonate (Na2CO3) has a bitter taste and stains the batter yellow, while turning the batter alkaline or basic (it has a pKb of 3.67). It can be neutralized by acids such as those found in vinegar (acetic), lemon juice (citric) or the more popular cream of tartar (salt of tartaric acid).

In order to bypass this whole process like a lazy chemist/baker, acid (H+) is carefully added to baking soda in the form of monocalcium phosphate, forming ready-to-use baking powder, which reacts in the presence of water:

NaHCO3 + H+ → Na+ + H2O + CO2

Problem solved, thanks chemistry!

A Fatty Environment

Fats play an important role in controlling the chemical environment of a bake. Fat molecules consist of hydrocarbon chains linked by glycerol, with most fats being triglycerides. Though usually in the form of butter or oil, lard is also often used in the pie business.

Milk also contains a fair amount of fats. The main role of fats is to ensure the sugar and flour are properly mixed during the processing, as well as preventing too much water absorption by the flour. Fats achieve this due to its hydrophobicity, meaning it doesn’t mix well with water2.

They are also involved in shortening, processes that keep the final product crumbly, such as in muffins. Fats help to keep the dough ‘short’ by slowing the water-induced aggregation of gluten and stops the dough from being too elastic. In addition, fats also lend pastries a smooth texture and a certain tenderness.

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Fats are the chemical equivalent to the tender loving care essential in all baking processes

Sugar, More than Just Sweet

You may be inclined to think that the only role sugar plays in baking is to excite those taste buds of ours, but you would be doing this crucial ingredient a major disservice. Sugar, also known by its posh name sucrose, is a disaccharide. It consists of two monosaccharides (hence the name), glucose and fructose.

Sucrose is involved in the chemistry of baking too, by increasing the rate of fermentation (if you’re using yeast as your leavener). It also locks in moisture in the finished product and increases its shelf life. That’s why artificial sweeteners like aspartame are so difficult to incorporate into pastries, because sugar is so much more than just sweet!

chemical structure of sucrose
Chemical structure of sucrose, consisting of glucose (left, 6-membered ring) and fructose (right, 5-membered ring) linked by an oxygen atom


Kickstarting the Chemical Reactions

Now that everything is put together, we need to provide energy for the chemical reactions to take place. That’s what the oven is for! Once the pastry is placed in the oven, energy in the form of heat speeds up all the chemical processes, releasing CO2 and expanding the dough.

Water vapor starts to form at 60°C which further expands and aerates the dough as the moisture escapes. The coagulation of the egg proteins occurs at 80°C, which also corresponds to gluten losing its stretchiness, causing the pastry to set.

Finally, at around 160°C, browning reactions take place within the now mostly dried out surface in the form of Maillard reactions (also found in coffee roasting). Remember the sugar from before? The nucleophilic amine on the amino acids in the dough attacks the carbonyl on sugar, producing a range of aromas/flavors that only baked goods can conjure.

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The actual process of baking a pastry in an oven is split into 3 stages: expansion, setting and browning.

These are not well characterized – probably because they end up in the stomachs of hungry chemists instead of ever making it to the lab.

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Did it for the memes.

Reference

  1. Wieser, H. (2007). Chemistry of gluten proteins. Food microbiology, 24(2), 115-119.
  2. Rogers, D. (2004). Functions of fats and oils in bakery products. INFORM-CHAMPAIGN-, 15, 572-574.

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