red cabbage experiment three glasses

How Red Cabbage Changes Color – The Science of Blue, Pink and pH

Want to give your red cabbage salad a nice vibrant pinkish color? Just sprinkle in a little lime juice and your previously purple cabbage will turn pink! Or, to lighten up your cooked cabbage, add a few pieces of apple while cooking and it will turn into a lighter color.

Wondering why this happens? It’s a great combination of complex molecules, pH, and our eyes. And it’s not even unique to red cabbage. Plenty of products in nature change color, depending on their environment!

We recently had a LIVE event in which we discussed and demonstrated this concept! You get free access to the recording if you’re a Food Chemistry Basics student. (Read more here).

What gives red cabbage its color?

Colors are all around us. They’re so common that you might think they’re simple as well. However, that’s not the case. Color and how we perceive color is a fascinating (but complex) topic. Colors can change depending on how and where you look at them (e.g. under what type of light). It depends on how light is reflected from the color’s surface and gets into our eyes. For a color to be visible, certain wavelengths of light have to be reflected and absorbed to reach our eyes.

This reflection and absorption are often done by complex molecules with quite specific structures. In the case of red cabbage, the color stems from the presence of a group of molecules called anthocyanins. This is just one group of molecules that can give foods a red-ish color, there are many more.

slide of red cabbage
Red cabbage cross section

Anthocyanins

Anthocyanins are a large group of molecules that all have the same basic structure, containing three rings of atoms (see below). Each anthocyanin contains the same core structure which is crucial for absorbing and reflecting light to make a color. Just exactly how this works is too detailed for this article (we’d recommend reading one of the references below). In short, this special structure allows the molecule to spread a charge (positive or negative) over several atoms.

Each anthocyanin type will have a different configuration of the R-groups in the structure below. Some might be very simple and only contain hydrogen atoms in those locations. Others may contain larger complex structures. These different configurations will change how the molecule interacts with light and thus what color it is exactly. That said, all anthocyanins will have a red/black/purple/blue-ish color.

anthocyanin structure, source Wikipedia
The basic anthocyanin structure under acidic conditions. All anthocyanins share the core structure, but differ in the R-groups. Source: Wikipedia.

A mix of anthocyanins

Red cabbage doesn’t just contain one type of anthocyanin molecule. Instead, it contains over 15 different ones!

Anthocyanins are quite common in fruits and vegetables. Blueberries, blackberries, and certain red grapes, all contain anthocyanins. None contain just one type of anthocyanin molecule, it is always a mixture. It’s the mix of molecules that give each product its unique color and behavior.

Anthocyanins are responsible for most reds/purples in plants. There are a few notable exceptions: tomatoes (lycopene) & red beets (betanins)!

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Why does red cabbage change color?

So the chemical structure and coordination of anthocyanins determines their color. This structure happens to be very sensitive to the acidity of the environment surrounding it. It influences how it organizes the electrons within the molecule.

In an acidic environment there are a lot of protons (H+) present. This results in the anthocyanin structure we shared above (with the O+). However, under less acidic conditions, this O+-group will lose its positive charge, resulting in other structural changes. These structural changes cause the molecule to interact differently with light and that results in a different color. Some anthocyanins can even completely lose color due to these changes.

The measure for acidity is the pH-value. A value between 0-7 is acidic, from 7-14 it’s alkaline. We’ve discussed acid base reactions in more detail here.

sliced red cabbage with some lime juice making it pink instead of purple
The red cabbage salad we discussed at the start, see that pinkish line? That’s where lime juice has been sprinkled.

Red cabbage color changes

The type of anthocyanins present determines just how they will respond to a change in the pH-value of the environment. In the case of red cabbage the color roughly change as follows (from: Stanford):

  • pH ≈ 2: red/bright pink, lime juice (which we sprinkled on top of our salad) will cause the pH to sink down this low and make the cabbage red
  • pH ≈ 4: light purple
  • pH ≈ 6: violet (aka dark purple), this is approximately the color of your ‘native’ red cabbage, how you buy it
  • pH ≈ 8: blue

At even higher pH-value the red cabbage can even turn green/yellowish, however, you won’t generally encounter this when prepping red cabbage.

blueberry panna cotta with and without acid
Blueberries also contain pH-sensitive anthocyanins. These are two blueberry pannacottas, the only difference being the addition of some lemon juice to the pannacotta on the right. Notice what a different those few drops make!

Controlling the color of red cabbage

In order to keep red cabbage in your preferred color, you have to control the acidity of the environment. In most instances you’d probably be looking for a red/purple color as opposed to a blue one.

Salads

As discussed at the top of this article, add a squirt of lime (or lemon) juice or vinegar to your red cabbage salad to give it those bright and vibrant colors. These are all acidic ingredients that will keep the pH-value low.

Cooking red cabbage

If boiling red cabbage in water, you can add a little bit of acidity to the boiling liquid to keep it nice and red. Often, apples may be added. They add some sourness to keep the pH-value low but also add a nice hint of sweetness to the final dish!

Note that, unlike a lot of other colors in fruits and vegetables (e.g. chlorophyll), anthocyanins aren’t that sensitive to heat. You can safely cook or boil your red cabbage without losing all of your pigments. Do try to keep the volume of boiling liquid low. A lot of anthocyanins will leach out of the red cabbage, in extreme cases you can even end up with pale red cabbage!

red cabbage experiment three glasses

Changing the Colour of Red Cabbage

Since red cabbage can be changed in color so easily, it's a great vegetable to use in a real scientific experiment on pH!  You can force the cabbage to change color by adding something acidic or something alkaline (with a pH>7). You can also use the test to determine whether something's more acidic than another liquid you have. Let's see how that goes.

Materials

  • Red cabbage leaves (4 large leaves are plenty)
  • Water
  • Acids you'd like to test, for instance: lemon juice, lime juice, vinegar, orange juice, fruit juices, sodas! - take care to use regular food ingredients and no other chemicals, these might react a lot more strongly and vigorously.
  • Alkaline ingredients you'd like to test, for example: baking soda, soaps

Tools

  • A pot + stove
  • Three clear glasses (take more if you want to test more liquids)
  • Stirrer/spoons

Instructions

    1. Take the red cabbage leaves and break them in slightly smaller pieces. Place them in a pot and cover with water, you can add plenty of water (1-2 liters works well).
    2. Bring the cabbage to the boil and leave to boil for a few minutes, the water should have clearly changed color.
    3. Leave the cabbage + water to cool down until it's cool enough to pour in the glasses.
    4. Pour the 'red cabbage juice' in your glasses.
    5. Now the fun starts! Add one of your test liquids to a glass of red cabbage water. Start with just a few drops, stir where necessary and continue to add until you're happy with the color.
    6. !Be sure to be very careful when making acids and alkaline ingredients! These tend to react very intensely! For instance, baking soda and acid react and will bubble up quite intensely.

Notes

This is what happens when you add a bit of baking soda to the red cabbage juice, notice the blue color?!

Red cabbage juice + baking soda, pH science

Adding a squirt of lemon juice turns it pink!

Adding lemon juice to red cabbage - effect of pH on colour

Sources

Arapitsas, Panagiotis & Sjöberg, Per & Turner, Charlotta. (2008). Characterisation of anthocyanins in red cabbage using high resolution liquid chromatography coupled with photodiode array detection and electrospray ionization-linear ion trap mass spectrometry. Food Chemistry. 109. 219-226. 10.1016/j.foodchem.2007.12.030.

Ummi Kalthum Ibrahim, Ida Idayu Muhammad and Ruzitah Mohd Salleh, 2011. The Effect of pH on Color Behavior of Brassica oleracea Anthocyanin. Journal of Applied Sciences, 11: 2406-2410. link

Pericles Markakis, Anthocyanins as food colors, Elsevier, 2012, chapter 1, link ; chapter 1 contains an extensive deep dive into the chemistry and the dependency on pH of anthocyanins

Pourjavaher, Simin; Almasi, Hadi; Meshkini, Saeed; Pirsa, Sajad; Parandi, Ehsan, Development of a colorimetric pH indicator based on bacterial cellulose nanofibers and red cabbage ( Brassica oleraceae ) extract, Carbohydrate Polymers 156 (2017) 193–201, https://doi.org/10.1016/J.CARBPOL.2016.09.027

Science Buddies, Cabbage Chemistry–Finding Acids and Bases, Scientific American, Jan-26, 2012, link

Stanford, Red cabbage color indicator chart, here.

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