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Ever stored some cooked beetroots in your fridge, only to come back a few days later to find dull brown beetroots instead of bright red ones?
Or have you tried baking a red cake using beetroot as a colorant? Only to have it turn out brown instead of that beautiful bright red color?
If so, you’ve come across some fascinating color chemistry. Many natural colors, including that of beetroot, aren’t fixed. Instead, they change color, depending on their surroundings. To keep beetroots red, you need to make sure the environment remains acidic. Otherwise, chemistry will do its thing, and the beets will turn brown.
- The accidental beetroot experiment
- The color of beetroots: betalains
- Acids cause betanin to change color
- Can't make brown beetroots red again
- How to keep cooked beetroots red
The accidental beetroot experiment
We do experiments in our kitchen all the time. Change a little bit of this, add a little bit of that. Sometimes they’re well thought out, but other times, they just happen by chance. And that’s very much what happened when we ‘discovered’ how to keep our cooked beetroots red.
So, what did we do?
Within a week’s time, we cooked beets twice for two different meals. In one instance, the recipe called for adding some vinegar to the boiling water. The other beetroots were cooked in plain water. Both beetroots turned out tasting pretty much the same, and had a nice bright red color.
We didn’t eat all beets in one go, so stored the leftover beetroots in the fridge. A couple of days later, we decided to use our beetroot leftovers. And, to our surprise, one of the beetroots was still bright red, whereas the other had turned an unappealing brown color!
So which ones were brown and which were red?
It turns out, adding vinegar to the beetroots protected them from turning brown in the fridge. They were still bright red!
The color of beetroots: betalains
What had happened to our beetroots? To understand, let’s see what makes beetroots red in the first place.
A group of molecules called betalains are responsible for making beetroots red. These are all molecules with a similar build and structure causing them to reflect light in such a way that our eyes register a specific color. Betalains can be divided into two classes:
- Red-violet betacyanins
- Yellow betaxanthins
As you might expect, red beets mostly contain betacyanins, whereas the betaxanthins give yellow and orange beets their color.
Betanin – Makes beetroots red
Betacyanins can again be split into several groups of molecules, but for red beets we’re most interested in just one specific molecule: betanin. This one makes up 75-95% of red colorants in beetroots. A mere 300-600mg of betanin per kg of beetroot is enough for the stark red color. If you’ve prepared beetroots, you know how strong and staining the color can be.
Acids cause betanin to change color
Like so many other natural colorants, the color of betanin is not fixed. It can change color. Acids and bases (their counterparts) have a noticeable influence.
When the surrounding liquid is acidic, betanin is bright red. However, when it gets too acidic, the red color might become slightly less intense. It’s most stable red within a pH-value of 3.5-7. When the pH rises above 7, that is, it becomes alkaline instead of acidic, the betanin will turn purple or even blue.
This explains why our beetroot + vinegar mix was a bright red. Vinegar is an acid, with a pH-value well below 7, so ensures the betanin remains red.
Recall that the pH-value is a measure for the acidity or alkalinity of a liquid. Beetroot color isn’t the only vegetable color that’s impacted by this value. The color of red cabbage is as well, it can turn from red to purple to blue.
Color change can take some time
For the color to change the betanins do need to get in touch with this liquid. This can take some time. In raw beetroots, the color molecules are protected in their cells. Only when you start grinding or cooking the beetroots, does the color become easily accessible to outside liquids, and can it change color easily.
Can’t make brown beetroots red again
Keep in mind though that our non-acidic beetroots weren’t purple. They were red when we cooked them, and only after storing them for some time they turned brown.
They seemed to have lost their color altogether, betanin doesn’t turn brown.
So, we tried reverting the process, adding back some acid to the brown beetroots, seeing if they’d turn red again. Which, they didn’t.
Betanins have broken down in brown beetroots
It turns out, the betanins hadn’t just changed color in the brown beetroots, they’d broken down completely. The original color molecules were gone.
Betanins are actually quite unstable molecules. Once they’re broken down, the red beetroots will lose their red/purple colors, instead turning brown.
Heating betanins for extended periods of time, and exposing them to oxygen (aka air), causes betanins to break down. But, again you can use acids to ‘protect’ the color of the beetroot. 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.
At higher pH values, betanin is 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. It’s what happened to our beetroots cooked in water, which were likely held at a pH of about 7, or a little higher.
How to keep cooked beetroots red
So next time you want to keep your cooked beetroots red, even during storage in the fridge for a couple of days, add a little acid to help them out. It’s what we did when making a red beetroot red velvet cake.
If you look at jars or cans of cooked beetroot, you’ll notice that manufacturers use the same trick. In many cases, vinegar, or another type of acid has been added to the beetroot.
If you’d prefer a purple beetroot, add a little bit of baking soda or other alkaline ingredient. But, keep in mind that this color isn’t very stable over time!
Add some acid such as lemon juice or vinegar, to keep your beetroots bright red.
Keep the oxygen out
This method is not very practical for most home cooks, but, there’s another way to help beetroots maintain their color, and that’s to take out the oxygen. Betanin breaks down by reacting with oxygen. If oxygen isn’t present, this reaction can’t take place. It’s why you’ll commonly find cooked beetroots packaged in a vacuum pack, the air has been pulled out. Or, you might find a note ‘packaged in modified atmosphere’, which indicates that the composition of the air within is different from that outside. That is, it won’t contain the oxygen required for the red to turn brown!
Using beetroot as food coloring can be tricky
Because of their bright red color, you might expect that betanins would be great as red food coloring. And whereas they’re definitely used in a wide range of foods, they’re not suitable for every application. The fact that they’re not very stable, and can change color, definitely makes them harder to use than say an artificial colorant which will keep each color no matter what!
Betanins work great in plant-based burgers
Sometimes, the instability of betanins isn’t a disadvantage though. Beetroot color is commonly used to give color to plant-based beef burgers. In the raw burger, which is slightly acidic, the red color is very stable. However, it’s not stabilized enough to survive the extreme heat of cooking the burger. As a result, it breaks down, and the final burger is brown. Exactly what you’d want it to do!
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