Colour science & pH – On the changing colour of red cabbage

Recently I made a red cabbage salad, on which I sprinkled some lime juice. The salad wasn’t meant to be eaten the same day. When I looked back a day later I could see a line of light pink cabbage, exactly where I squeezed that lime juice, whereas the rest was still a dark purple…

A few days later I took the other half of the red cabbage and boiled it. This red cabbage had more or less turned blue/purplish, definitely not pink! Quite remarkable, the exact same vegetable, but two completely different colours!

Also seen this in your kitchen or in your production processes? It reminded me of a high school science lesson in pH-value. So I decided it was time to revisit the concept of acids and bases and see how we can explain this concept using red cabbage! A teacher or student? At the end of this post you will find a great experiment you can do with red cabbage and its colours.

What gives red cabbage its colour?

As we’ve seen before for oranges and tomatoes, the colour of a food tends to be determined by a few molecules present in that food. That is also the case for red cabbage. In red cabbage anthocyanins cause the red/purple colour.

Anthocyanins are a large group of molecules with all the same basic structure (see below). However, each type will have its own side groups attached to this basic structure (the R-groups in the drawing). In red cabbage there are more than 15 different anthocyanins!

Red cabbage isn’t the only food that contains these anthocyanins, blackcurrants, blueberries and grapes, to name just a few, also contain anthocyanins which give them a purplish colour.

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

Why does red cabbage change colour?

All those different anthocyanins will have slightly different colours. Also, not all of them are stable at all pH values (a measure for acidity, read more here). Some anthocyanins lose their colour quite easily with an increase or decrease of the pH-value. Others maintain a colour, but the colour changes, which is also the case in red cabbage.

In an acidic food there is an excess of protons (H+) whereas in an alkaline food there is an excess of OH. These small ions float around and will interact with other molecules present in the food. In the case of red cabbage they will interact with the anthocyanins. As a result of these interactions the anthocyanins will change colour.

At a low pH value (approx. 3) the colour of the anthocyanins will turn into a bright pink. At a higher pH, around 5 the colour will turn into a darker purple. At a neutral pH (approx. 7) or higher the colour will turn 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.

The colours red cabbage can get

As we discussed at the start of this post, my red cabbage salad on the top of the post has some sections which are a bright pink (see photo above). This colour change occurred a few hours after the lime juice had been sprinkled over. Lime juice is quite acidic (it has a pH < 7). Indeed, it so happens that an acidic environment turns the anthocyanins in cabbage a lighter pink! Since the other parts of the salad had not been in touch with this lime juice, they were not affected and maintained their colour. This purple colour represents a more neutral pH-value (closer to 7).

On the other hand, when boiling the red cabbage, the cabbage turned blue/purplish. This is caused by again an interaction of the anthocyanins, but this time, this causes the colours to turn darker, more towards blue than pink.

How to keep (cooked) red cabbage red?

A very common addition to red cabbage upon boiling the cabbage are some pieces of apple. It doesn’t only contribute sweetness though, it also influence the colour of the red cabbage! Apples tend to be slightly sour and thus they make the red cabbage a bit lighter and more pinkish in colour.

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

Red cabbage colour experiment

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

For this experiment you need:

  • Red cabbage leaves (4 large leaves are plenty)
  • A pot + stove
  • Water (from the tap is fine)
  • At least three transparent glasses, but take as many as you’d like to test liquids
  • Acids & bases you’d like to test, some suggestions:
    • Acids: lemon juice, lime juice, vinegar, orange juice, fruit juices, sodas!
    • Bases: baking soda, soaps

Method

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). Bring the cabbage to the boil and leave to boil for a few minutes, the water should have clearly changed colour. Leave the cabbage + water to cool down until it’s cool enough to pour in the glasses.

Pour the ‘red cabbage water’ in your glasses. Now the fun starts! Start adding your liquids to the red cabbage water. If necessary, give it a slight stir, but in most cases you will see it changing all by itself. Let’s show two examples:

Adding lemon juice to red cabbage - effect of pH on colour
Three glasses of red cabbage water, to the left some lemon juice is added, it changes colour immediately!
Red cabbage juice + baking soda, pH science
Now we continue that experiment but instead add some baking soda to the right glass. It turns a blueish colour!

If you’d like to do more experiments with acids, have a look at the experiment on FoodScienceSecrets.com, a great simple experiment to start learning more about acids, especially nice to do with kids.

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 colour indicator chart of red cabbage here.

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