neutral beet juice with added acids

How to Keep Beetroots Red

Have you had cooked red beetroots turn an unappetizing brown color? Luckily, there’s a simple way to keep cooked beetroots red. All you need is some acid!

We regularly do science experiments in our kitchen. Most of them are done on purpose, with the intention of figuring something out. However, this time, we did one by accident. In doing so, demonstrated how you can influence the color of your beetroot!

The Accidental beetroot color experiment

What did we do?

Within a week’s time, we cooked beetroot twice. Once, the recipe we used called for adding some acids, we added vinegar. The other time, we just cooked the beetroots in some water.

In both cases we had some leftover beetroots, which we stored in the fridge, to be used for another dish. Both beetroots had a nice red color.

A week later, we took our leftover beetroots, ready to use them. However, we noticed that the beetroots cooked in plain water had lost all of their vibrant red color! Instead, they had turned brown. The acidic beetroots on the other hand still their bright color.

We tried adding acids to the brown beetroots. But no matter what we did. The red color did not return. The brown beetroots remained brown.

neutral beet juice with added acids
Left: water used to boil beets, stored in the fridge for a week (no acids added) ; Middle: added a little vinegar, the color was somewhat diluted by the extra moisture, but still brownish ; Right: added some citric acid, still no vibrant red color. Strips next to liquids are pH strips, from left to right values are: 6-7, 3-4, 2-3, see also: how to measure pH value

The color of beetroots: betalains

What had happened to our beetroots? To understand, we first have to look at the color of beetroots in some more detail.

In fruits and vegetables, bright colors are very common. These colors originate from a select group of molecules. These molecules absorb and reflect light in such a way that we perceive them as a specific color.

The science of colors in food is more complex than you might expect! Take a deeper dive into color in food and how it can be measured.

The color of beetroots is caused by a group of molecules called betalains. These again can be divided into two classes:

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  1. Red-violet betacyanins
  2. Yellow betaxanthins (more prevalent in yellow/orange beetroots)

Betanin – The lead in reds

In red beets the betacyanins are what give the beetroots their red color. The betacyanins can again be split into several groups of molecules. The most important one, making up 75-95% of red colorants in beetroots, are betanins. One kilogram of red beetroots contains about 300-600mg of betanin. This is a small amount, showing how powerful a colorant it is.

Betanin structure
Betanin, an example of a betalain. Source: Wikipedia CC

Betanin can change color

Like so many other natural colorants, the color & stability of betanins depends on their surroundings. Betanins themselves can change color when the conditions are favorable. However, they can also break down quite easily, resulting in a complete loss of color!

The influence of pH on color

A very important factor for the color of betanin is the pH-value of the surrounding liquid. Remember that pH is a measure for the acidity of a liquid. A pH-value <7 indicates a liquid is acidic, above 7 is alkaline. You lower the pH of a solution by adding an acid, such as lemon juice or vinegar. Baking soda does the opposite, it’s alkaline so it will increase the pH value.

The pH value can influence the configuration of some molecules. In the case of betanin, this results in a change of its color!

When betanin is held at a pH value of 3,5-7, its color is a bright red. At lower values, it is still red but it might lose some of its intensity. At higher pH values, its color can turn purple/blue.

Beetroots aren’t the only vegetables of which the color depends on the surrounding pH value! The color of red cabbage also depends on the pH value. Adding a little acid to your cabbage will make it turn a bright(er) red!

Stability of betanins

Betanins are quite unstable molecules. They can break down due to hydrolysis reactions. Once they’re broken down, the red beetroots will lose their red/purple colors. Instead, they will turn brown.

Heating betanins for extended periods of time, and exposing them to oxygen, will cause betanins to break down. Since the air is full of oxygen, cooking red beetroots for extended periods of time can cause them to lose color.

You can reduce the rate of breakdown though. Again, you’d use the pH value of the surrounding liquid. Betanin is most stable at a pH value of roughly 4-6. When it’s kept under these conditions, the molecule won’t break down as quickly.

However, at higher pH values, larger than 7, betanin becomes more unstable. As a result, beetroots can lose their color over time if stored under these conditions. This can even happen at lower temperatures, for example in the fridge.

How to keep cooked beetroots red

Knowing the theory, we can now explain what happened to the beetroots that we cooked and stored in the fridge! Remember, we cooked two batches of beetroots. One was cooked with some acid, the other with just plain water. The acidic beetroot remained bright red after a couple of days of storage, whereas the other beetroot turned brown!

We now know this was due to the pH value of the liquid and the stability of the betanin molecules. In our acidic beetroots the lower pH value – which was around 4 – ensured a bright red color, as well as a stable molecule over time. As such, the betanin was still intact after storage. By adding some baking soda, an alkaline material, we could even still have the betanins turn purple!

acidic beet juice (left) with baking soda (right)
Left: water in which beets were boiled, with some added vinegar (stored in the fridge for a few days), pH is <4. Right: that same liquid but with added baking soda, the pH is approx. 11.

Our other non-acidic beetroot, on the other hand, didn’t fare as well. The color molecule wasn’t stabilized and over time, the betanins broke down. As a result, the beetroots turned brown. Adding some acid didn’t cause a color change any more. The betanins had broken down and couldn’t be brought back.

Add some acid, e.g. lemon juice or vinegar, to keep your beetroots bright red!

Using beetroot as a colorant – Stability is tricky!

Because of their bright red color, betanins have been studied extensively as a colorant of other foods. They are for instance used to color plant-based beef burgers. In the raw burger, which is kept slightly acidic, the red color is very stable. However, upon cooking, the pH value is not low enough to keep the molecule stable. As a result, it breaks down and the burger turns brown!

For a burger, this is actually a very desirable process to occur! However, in many other foods, such as candy, it’s not a good thing at all. Like so many other natural colorants, betanins need to be controlled very carefully to ensure they don’t lose their colors. It’s why so many foods still use artificial colors, which tend to be very stable under a very wide range of conditions.

References

Chandran J, Nisha P, Singhal RS, Pandit AB. Degradation of colour in beetroot (Beta vulgaris L.): a kinetics study. J Food Sci Technol. 2014;51(10):2678-2684. doi:10.1007/s13197-012-0741-9, link

M. A. Elbandy and M. G. Abdelfadeil, Stability of Betalain Pigments from Red Beetroot (Beta vulgaris), Egypt. J. Food Sci. 36, 2008, link

Anna Gliszczyńska-Świglo, Henryk Szymusiak, Paulina Malinowska. Betanin, the main pigment of
red beet – molecular origin of its exceptionally high free radical scavenging activity. Food Additives
and Contaminants, 2006, 23 (11), pp.1079-1087. 10.1080/02652030600986032. hal-00577387, link

Erum Akbar Hussain, Zubi Sadiq, Muhammad Zia-Ul-Haq, Betalains: Biomolecular Aspects, Springer, 2018, Chapter, 3.5, link

John M. deMan, John W. Finley, W. Jeffrey Hurst, Chang Yong Lee, Principles of Food Chemistry, Springer, 2018, p. 282, link

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