Why Do Apples Turn Brown ?

You open your lunch, only to find that your freshly sliced apple has turned into an ugly brown mess. This happens due to a chain of reactions known as ‘enzymatic browning’. Enzymatic browning is not unique to apples - pears, bananas and peaches are also prone to browning when cut.

When you slice into an apple, the plant tissue is exposed to oxygen. Due to this, an enzyme called polyphenol oxidase (PPO) begins to oxidise polyphenols in the plant’s tissue.

• Oxidising is a process where oxygen is added to a compound (or hydrogen is removed)

• Polyphenols (Organic Chemistry Concept) - Polyphenols are a large family of naturally occurring organic compounds characterized by multiples of phenol units. They are abundant in plants and structurally diverse. Polyphenols include flavonoids, tannic acid, and ellagitannin, some of which have been used historically as dyes and for tanning garments.

• Phenol - Phenol is an organic compound with the chemical formula C₆H₅OH. The molecule is composed of a phenyl group (−C6H5) bonded to a hydroxyl group (-OH).

This reaction creates a compound called 1,2 Benzoquinone (also called ortho-quinone or o-quinone). The o-quinones then react with amino acids (proteins) to produce melanin, a brown-coloured pigment. This is what gives the apple its brown colour.

• Melanin is the same pigment which is responsible for the colour of your skin.

How to prevent apples from browning?

PPO reacts best at room temperature. So, by refrigerating a sliced apple, you can prevent it from browning as quickly.

The pH of the apple plays a role in the reaction of PPO. PPO reacts best at a pH of 7. As a result, if you apply lemon juice on the cut apple, it will reduce its pH and slow the reaction down.

If you like, you can also coat the slices with caramel or sugar syrup. This blocks the oxygen from entering the cells, and will keep your apple from browning. However, Enzymatic browning is not always an undesirable process, it is responsible for the desirable brown colour of coffee, tea and cocoa. So there you go! Next time you get irritated by your apple turning brown, at least you’ll know why it happens :(

Sources

1. 30, July. “Why Do Apple Slices Turn Brown after Being Cut?” Scientific American, Scientific American, 30 July 2007, https://www.scientificamerican.com/article/experts-why-cut-apples-turn-brown/#:~:text=When%20an%20apple%20is%20cut,to%20brown%2Dcolored%20secondary%20products.

2. Fox, Rebecca, and August 21. “Why Do Apples Turn Brown after You Cut Them?” Let's Talk Science, 21 Aug. 2019, https://letstalkscience.ca/educational-resources/stem-in-context/why-do-apples-turn-brown-after-you-cut-them.

Photo by John Finkelstein: https://www.pexels.com/photo/red-apple-1630588/