# Why do bananas bruise?

Aug 2, 2015

Bananas bruise because an enzyme called polyphenol oxidase catalyzes oxidation of its polyphenols to brown pigments called melanins.

#### Explanation:

Bananas contain an enzyme called polyphenol oxidase (PPO).

While the cell structure remains intact, the enzyme and phenolic compounds are separated from each other.

When the cell structure breaks down, the enzyme and the phenols can come together and react with atmospheric oxygen.

The amino acid tyrosine

(from commons.wikimedia.org)

(from en.wikipedia.org)

which is found at exceptionally high levels in banana peel.

The dopamine is converted sequentially to dopamine quinone

2,3-dihydroindole-5,6-quinone (red)

(from OPSIN)

5,6-dihydroxyindole

(from OPSIN)

indole-5,6-quinone (purple)

(from OPSIN)

finally ending up as melanin, a brown or black cross-linked polymer with a partial structure in which the side chains are a mixture of $\text{COOH}$ groups and $\text{H}$ atoms.

(from en.wikipedia.org)