A debate broke out among students about red velvet cake during a recent chemistry class. We were studying natural acid-base indicators, and a student claimed that cocoa powder was an acid-base indicator. They even went as far as to claim that Red Velvet cake was simply chocolate cake with added acid that changes the dark brown chocolate color to the deep red color of a Red Velvet Cake. Most other students countered that red food coloring caused the red color.
Spoiler alert -- the brilliant red color of Red Velvet cake results from red food coloring; however, I thought it would be a worthwhile opportunity to deviate from the course schedule and test the claim that the first student made.
Will chocolate cake turn red at a low pH?
Procedure
Before baking a cake, we performed a quick experiment to see if the color of cocoa powder was dependent on pH.
First, we mixed raw, natural cocoa powder with vinegar to lower its pH to 2. Another sample was mixed with sodium bicarbonate to raise the pH to 9. After a few minutes, there was a subtle difference in color between the two samples. 24-hrs later, the color difference was more pronounced. The cocoa powder at a higher pH had turned a darker brown color while the lower pH sample was slightly orange (Figure 1)
Figure 1: Cocoa powder at pH 9 and pH 2
Confident that pH affected the color of cocoa powder, I was ready to try and change the color of a chocolate cake.
I used a slightly modified version of Mark Bittman's classic chocolate cake recipe1 to make the control cake. I used the same recipe for the "acidic chocolate cake" but added buttermilk instead of milk because it has a lower pH than milk and one tablespoon of vinegar.
Classic Chocolate Cake Recipe (Mark Bittman)
1 stick of butter
2 cups all-purpose flour
3 tablespoons of cocoa powder (original recipe called for 3 oz. of unsweetened chocolate)
2 teaspoons baking powder
½ teaspoon baking soda
½ teaspoon salt
¾ cup of sugar
2 eggs, separated
1 teaspoon vanilla extract
1 ¼ cup milk
Both cake batters were mixed for 10 minutes. The batter of the classic chocolate cake had a pH of 8, while the batter of the "acidic chocolate cake" had a pH of 6.
Both cakes were baked at 350℉ for 30 minutes.
Results
Although not nearly the shade of a traditional Red Velvet cake, I was happy to see that the color of the acidic cake was indeed redder in color than the control chocolate cake (Figure 2). Even better, both cakes were edible. I could not detect any unpleasant flavors from the vinegar or buttermilk in the "acidic chocolate cake."
Figure 2: Classic chocolate cake on the left and "acidic chocolate cake" on the right
Discussion
The color of chocolate cake is dependent on pH; however, it seems that a higher rather than a lower pH has a more significant effect on the color of cocoa powder because the "acidic chocolate cake" seemed to stay the same color as the original cocoa powder.
Incidentally, cocoa powder is often treated with a base to give a more rich and dark brown color, improve the flavor, and enhance its solubility in a process known as "Dutching." Natural cocoa powder is very light brown, and it gets darker brown as the pH increases (Figure 3). Under certain conditions, strongly alkalized cocoa powder can even turn black.2
Figure 3: Cocoa powder at pH 9 and pH 2 compared to original cocoa powder color
Chocolate contains polyphenols, and it is well established that polyphenols change color with changing pH.3 Although reactions involving polyphenols are likely involved with cocoa's color change, the chemical processes involved during alkalization are incredibly complex. Sugar degradation (caramelization) and the Maillard reaction (reaction between proteins and carbohydrates) are also enhanced during alkalization, contributing to color change. The control chocolate cake turned darker brown because of the higher pH from the addition of baking soda and baking powder.4
A more pronounced red color may even be possible at a pH slightly above rather than below 7. Three molecules in cocoa powder generate red chromophores when alkalized 5,6-Xanthenocatechin, 6’-Hydroxycatechinic acid radical, and 7,8-Xanthenocatechin.5 The most pronounced red color of these molecules seems to form at medium alkalization.4
Further Research
Tom Kuntzleman recently wrote a ChemEd X post that described his experiments with changing the color of the pigments in blueberries. He speculated that metal ions could also change the color of polyphenols (anthocyanins). It may be possible to enhance the color change of cocoa with the addition of edible sources of various metal ions.
References
- Bittman, M. (2016). How to bake everything: Simple recipes for the best baking. Houghton Mifflin Harcourt.
- Valverde Garcia, D., Perez Esteve, E., & Barat Baviera, J. M. (2020). Changes in cocoa properties induced by the alkalization process: A review. Comprehensive Reviews in Food Science and Food Safety, 19(4), 2200-2221.
- Sabnis, R. W. (2007). Handbook of acid-base indicators. CRC Press.
- Valverde Garcia, D., Perez Esteve, E., & Barat Baviera, J. M. (2020). Changes in cocoa properties induced by the alkalization process: A review. Comprehensive Reviews in Food Science and Food Safety, 19(4), 2200-2221.
- Germann, D., Stark, T. D., & Hofmann, T. (2019). Formation and characterization of polyphenol-derived red chromophores. Enhancing the color of processed cocoa powders: Part 1. Journal of agricultural and food chemistry, 67(16), 4632-4642.
NGSS
Asking questions and defining problems in grades 9–12 builds from grades K–8 experiences and progresses to formulating, refining, and evaluating empirically testable questions and design problems using models and simulations.
Asking questions and defining problems in grades 9–12 builds from grades K–8 experiences and progresses to formulating, refining, and evaluating empirically testable questions and design problems using models and simulations.
questions that challenge the premise(s) of an argument, the interpretation of a data set, or the suitability of a design.
Scientific questions arise in a variety of ways. They can be driven by curiosity about the world (e.g., Why is the sky blue?). They can be inspired by a model’s or theory’s predictions or by attempts to extend or refine a model or theory (e.g., How does the particle model of matter explain the incompressibility of liquids?). Or they can result from the need to provide better solutions to a problem. For example, the question of why it is impossible to siphon water above a height of 32 feet led Evangelista Torricelli (17th-century inventor of the barometer) to his discoveries about the atmosphere and the identification of a vacuum.
Questions are also important in engineering. Engineers must be able to ask probing questions in order to define an engineering problem. For example, they may ask: What is the need or desire that underlies the problem? What are the criteria (specifications) for a successful solution? What are the constraints? Other questions arise when generating possible solutions: Will this solution meet the design criteria? Can two or more ideas be combined to produce a better solution?
Planning and carrying out investigations in 9-12 builds on K-8 experiences and progresses to include investigations that provide evidence for and test conceptual, mathematical, physical, and empirical models.
Planning and carrying out investigations in 9-12 builds on K-8 experiences and progresses to include investigations that provide evidence for and test conceptual, mathematical, physical, and empirical models. Plan and conduct an investigation individually and collaboratively to produce data to serve as the basis for evidence, and in the design: decide on types, how much, and accuracy of data needed to produce reliable measurements and consider limitations on the precision of the data (e.g., number of trials, cost, risk, time), and refine the design accordingly.