red cabbage experiment three glasses

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

A while back I prepared a red cabbage salad for a meal later that day. I added all the ingredients and ended with a sprinkle of lime juice over the top. I must not have mix the salad well, because when it was time to eat a few hours later the salad had a strip of pink across it, right where I had added some lime juice! The rest of the salad, without the lime juice, was still the original purple-red cabbage color.

So why does the color of red cabbage change when you sprinkle lime juice over it? It’s all to do with the science of colors and pH, a measure of acidity. As a matter of fact, you can’t just turn red cabbage pink, you can also turn it blue!

What gives red cabbage its color?

Anthocyanins are what give red cabbage its red/purple/blue color. Anthocyanins are a large group of molecules that all have the same basic structure (see below). They have the same ‘core’ and differ based on the side groups (shown as R in the image below) that are attached to this central structure.

anthocyanin structure, source Wikipedia
The basic anthocyanin structure. The central part is the same every time, but the R’s differ per anthocyanin. Source: Wikipedia.

Anthocyanins are quite common in fruits and vegetables. Blueberries, blackberries, and certain red grapes for instance all contain anthocyanins that color them blue/black/red. None contain just one type of anthocyanin molecule, it is always a mixture. Red cabbage for instance contains more than 15 different anthocyanins!

Anthocyanins all have different colors. The R-groups will influence just how the energy of light is absorbed and reflected from the molecule, causing the color to change. Nevertheless, most anthocyanins have a purple/red/blue hue to them.

Fruits and vegetables contain several more groups of molecules that contribute to color, such as as the carotenoids for orange colors. We give an overview of all those colors here.

Why does red cabbage change color?

The chemical structure of anthocyanins determines the color of the anthocyanin. If something in the environment influences this structure and coordination, the color of the molecule may change. An important environmental factor that impacts the color of anthocyanins is the acidity (expressed in pH-value). Some anthocyanins may lose their color completely if the pH-value changes drastically, others simply change in color. The last is what happens for red cabbage.

Remember that lime juice that I sprinkled on top of my red cabbage salad? Lime juice is known to be very acidic, it has a very low pH-value (below 3). Anthocyanins are known to change color at a low pH-value, starting at around 3. Under acidic conditions the anthocyanins turn a bright pink. At a higher pH, around 5, the color turns into a darker purple. It is the color of ‘regular’ red cabbage. At a neutral pH (approx. 7) or even higher the color turns blue or even a dark green.

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.

How to control the color of red cabbage

In order to keep the red cabbage pink, purple, or blue, you have to control the acidity of the environment of the red cabbage. Unlike other colors, anthocyanins don’t degrade greatly when heated (although they’re not stable indefinitely at high temperatures). As such, you should add some acidity, or some alkalinity to your red cabbage to have it turn color.

If you’re boiling red cabbage, chances are that your red cabbage will turn a darker purple/blue colour. When the cabbage cooks and breaks apart, the resulting liquid is quite neutral in acidity. A way to counteract this darker colour change is to add something acidic to the red cabbage. It’s one of the reasons people cook red cabbage with pieces of apple. Apples are slightly sour and they reduce the pH-value of the cooking liquid, changing the colour of the cabbage.

The same goes up for that lime juice in the salad. If you want a bright pink red cabbage salad, add some lime or lemon juice to your salad, an orange will also work if it’s sufficiently sour.

If you want to go more in-depth, do the experiment below to help see just exactly how you can change the color of red cabbage extract.

red cabbage experiment three glasses

Changing the Colour of Red Cabbage

Since red cabbage can be changed in colour so easily, it's a great vegetable to use in a real scientific experiment on pH!  You can force the cabbage to change colour 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 (from the tap is fine)
  • 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.
  • 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

What about other fruits & vegetables?

Red cabbage isn’t the only produce type that changes color based on the acidity. It is how, quite unique in the fact that it has a very wide range of pH-values for which it has a bright and ever changing color.

The photo below shows two panna cottas made with blended blueberries. They’re identical except for the fact that the right version contains just a few drops of lemon juice. Notice what a different those few drops make! Again, it’s the anthocyanins that change color here.

blueberry panna cotta with and without acid

Sources

If you’re interesting in digging a little deeper into the science of these colours in red cabbage have a look at these two research articles, focussed on analyzing the anthcyanins in red cabbage: 2006 & 2008.

Scientific American wrote a very nice article on using red cabbage for science experiments on acids & bases.

You can find a simple color indicator chart of red cabbage here.

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

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

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