Food Chemistry

The chemistry of aubergine (eggplant) colour, bitterness and browning

chemistry of aubergines v2
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The aubergine (or eggplant) is a fascinating fruit. And yes, you read that correctly – the aubergine is technically a fruit, not a vegetable. If you’ve ever wondered why they soak up oil like a sponge, and rapidly brown when cut, read on!

Aubergines come in many different varieties, and not all of them are the characteristic colour. However, many of them are a shade of violet or purple, and this is caused by the presence of anthocyanins. Anthocyanins are pigments which often give red, blue or purple colouration in plants. We’ve come across them previously in red cabbage, for instance.
Aubergines contain many different anthocyanins, and the exact balance of these compounds present depends on the variety. The most common anthocyanin, and that most often linked to the purple colour of the aubergine, is nasunin. Nasunin’s name comes from the Japanese name for aubergines (‘nasubi’).
Nasunin is present in two different isomeric forms in aubergine, cis and trans-nasunin. The trans isomer is the more stable and more common of the two. Studies have shown that these isomers interconvert under visible light.
Anthocyanins are a type of phenolic compound. Aubergine contains these compounds in abundance, and they are also responsible for its bitter taste. Not only that, but they are also the reason why aubergines brown rapidly when cut.
When you chop up an aubergine, its cells are damaged. This damage results in the release of the enzyme, polyphenol oxidase. Polyphenol oxidase converts the phenolic compounds in the aubergine, many of which are colourless, into quinones in the presence of oxygen in the air. These quinones can then polymerise to form complex brown polymers.
Several techniques are suggested to prevent browning. These include immediately putting the chopped aubergine into salted water, or water with lemon juice. Others suggest simply chopping with a sharp knife to minimise cell damage and release of polyphenol oxidase.
Even once you’ve stopped your aubergine from browning, there are other considerations before you throw it in the pan. Those who’ve tried to fry aubergines will know that it acts like a sponge for oil. This is due to the aubergine’s spongy texture. They have many tiny air pockets between cells, which shrink when cooked but also soak up oil.
There are steps that can be taken to prevent this. Pre-cooking in a microwave can help collapse the spongy structure. Even more simply, salting the chopped aubergine can also help. This draws out water from the auberinge cells, a process known as osmosis. This fills the air pockets and collapses the structure. The aubergines need to be left for around half an hour after salting to reap the full benefits of this method. Hopefully, this new-found science knowledge helps you perfect your aubergine preparation!
Finally, while here in the UK we refer to them as aubergines, I’m aware a large portion of the world refers to them as eggplants. To keep everyone happy, there’s also a version of the graphic below which has the word ‘eggplant’ substituted in for ‘aubergine’!
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