The Chemistry of Baking: Baking Soda

Have you ever had milk and cookies? Almost everyone has! But have you stopped and thought about why cookies taste so good? Most people probably haven’t. Read on and we’ll explain to you the science behind baking soda, an often overlooked ingredient that is incredibly important in making delicious cookies, brownies, and many of the baked desserts that we enjoy every day.

Some baked goods

These yummy baked goods probably wouldn’t taste very good at all without baking soda.

When you take a bite into a cookie, take a look inside! It’s not just a solid chunk – it’s full of little holes that make the cookie soft and give it texture. How do these little holes end up in the cookies? The answer is baking soda. Baking soda is pure sodium bicarbonate, an organic base that can react with acids to produce gas. In particular, baking soda reactions create sodium, water, and carbon dioxide. The holes that you see in your cookies are created by the carbon dioxide bubbles that are produced by the reaction. This reaction is also the reason why dough and batter rise while being baked, since the gas produced makes the food expand while in the oven. That’s why a flat hunk of batter will get bigger and puffier, becoming a delicious cookie. Often, the acid that is used to start the reaction is vinegar, but many other ingredients can create the same reaction.

Reaction with vinegar: NaHCO3(s) + CH3COOH(l) → CO2(g) + H2O(l) + Na+(aq) + CH3COO-(aq)

In baking terms, sodium bicarbonate is called a “leavening agent”. Yeast is another example of a leavening agent. What do they have in common? They both are added to doughs and batters in order to produce bubbles. As we mentioned above, these bubbles are very important because they are responsible for the texture of the baked good.

Baking soda has had many other uses throughout history. Besides cooking, it often used as a cleaning agent. Even the ancient Egyptians used baking soda, both as a cleaning agent as well as in making mummies. Thousands of years later, a British chemist named Alfred Bird created baking powder (a combination of baking soda and a powdered acid) to help his wife who had a yeast allergy. She was not able to consume most of the baked goods of the time since yeast was the traditional leavening agent . Later, Arm and Hammer became one of the most famous baking soda brands in the world because not only did they sell baking soda for cooking, but they also revolutionized the usage of baking soda in the kitchen as a cleaning agent. National magazines advertised the use of baking soda as a safe chemical cleaner. In addition to being a yeast substitute, baking soda quickly earned a reputation as an excellent cleaning product for use all around the house, as well as an odor reducer. A more complete history of leavening can be found here.

The disadvantage of baking soda was that one always had to add an acid in order for the reaction to occur. This is not an issue when one uses baking powder, as baking powder is made of powdered sodium bicarbonate (baking soda), a powdered acid salt (often, cream of tartar), and starch. Since the base, sodium bicarbonate, and the acid are in solid, powdered form, they are not able to react with one another (this is an important concept to note – many chemical reactions take place only when the reactions are dissolved in some solute (in this case, water), because the molecules can then move around much more freely and interact with each other). The starch is necessary because it ensures that the acid and the base powder stay dry until you decide to use it for cooking/baking. When that time comes, just add liquid, which will dissolve the acid and base and allow the reaction to occur, creating the gas bubbles necessary for the baking of your dessert.

Baking soda + water – no bubbles form

Baking powder + water – and here we have bubbles!

However, because baking powder is not pure baking soda, more baking powder must be used in order to have the same reaction occur as in a pure baking soda reaction. For example, instead of one teaspoon of baking soda, one would have to use about 3 teaspoons of baking powder.

An interesting point to note is that, as with many chemical reactions involving gases, the baking soda-and-acid reaction is affected by atmospheric pressure. Here on Earth, the air pressure decreases at higher altitudes. When baking with leavening agents, this results in the formation of more bubbles. This illustrates Le Chatelier’s Principle, which states that if the initial conditions of a reaction are changed, the equilibrium will shift in response. It shows that when pressure is increased, the reaction will shift toward the side of the reaction with the least amount of gas, and if pressure is decreased, the reaction will shift towards the creation of more gases. In this case, when the initial pressure is lower, the reaction will produce more carbon dioxide molecules as a result. Thus, you may sometimes notice that recipes will include instructions for anyone baking at high altitudes (for example, in the recipe we used). If you live in the mountains and wishes to bake cookies, or if you are baking cookies on a plane (this isn’t as crazy as it sounds – plenty of airplane food is prepared while in the air), you can add more flour and use the same amount of baking powder or soda. The additional flour makes the dough heavier and counteracts the excess CO2 production, ensuring that your cookies are just as delicious up high as they are at sea level.

To demonstrate exactly why baking soda is important, we baked some chocolate chip cookies and created this video showing our experiment and explaining our results

For the baking soda reaction we added vanilla extract, an acid, to start the gas-producing reaction.  For the baking powder reaction, we simply added water which dissolved the powdered base and acid and started the reaction. The baking powder reaction (which followed the original recipe) turned out the best, with the cookies staying soft and chewy for several days. The difference between these two batches of cookies may be caused by the amount of baking soda and baking powder. Because only a portion of baking powder is composed of sodium bicarbonate, in order to have exactly the same reaction, more baking powder must be used in comparison to baking soda for the same amount of gas to be produced and for the cookies to come out with the same texture.

As for the cookies made without baking powder or baking soda, there weren’t any holes, and the cookies came out solid and mushy, but later cooled into a very hard cookie chunk. This is because with no gas within the cookie, it takes longer for the cookie to bake and harden, leaving the center mushy if left in the oven for the same amount of time as the other cookies. Additionally, the solidity of the cookies (not having any bubbles) was due to the lack of any reaction happening that would create CO2. The cookies do not look particularly flat because they were placed on the pans in round ball shapes and they did not fully flatten during baking. This also has to do with the lack of baking soda/powder because with no gas spreading apart the dough, the structure of the dough is less likely to flatten.

So, in conclusion….

Now you know the chemistry behind those delicious cookies you love! The texture of cookies is all thanks to baking soda, i.e. sodium bicarbonate. Through a simple acid and base reaction, carbon dioxide bubbles form and your cookies end up perfectly soft and munchable.

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One thought on “The Chemistry of Baking: Baking Soda

  1. Pingback: 364/365: National Bicarbonate of Soda Day | Eat My Words

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