I know that often oxidation processes are mentioned when referring to the color change from yellow to brown in bananas (specifically: those that you get everywhere in supermarkets, with no seeds).

However, I am really interested if there is a specific oxidative pathway that is generally followed in this process, and whether it has been examined.

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    $\begingroup$ Note: When answering this question, please back up your claims with references to the scientific literature. (This is not a homework question [although phrased like one] but arose purely from personal interest and lack of time to pursue it properly.) $\endgroup$ – tschoppi Apr 13 '14 at 16:48
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    $\begingroup$ As a sidenote: Ethylene is an hormone involved in fruit ripening. $\endgroup$ – Remi.b Apr 13 '14 at 17:15
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    $\begingroup$ Irrelevant to why bananas go brown, but the process is faster in cold conditions than in warmer ones. So it's not worth putting bananas in a fridge. $\endgroup$ – Ultimate Gobblement Apr 15 '14 at 16:17

The reason for this is the oxidation of phenol residues in the banana (for example in the yellow color) which get oxidized by the enzyme Polyphenol oxidase (PPO) to melanins. The scheme (the image is from this website on food browning) looks like this (you can of course also use more complicated substrates):

enter image description here

For further information, see these references:

Polyphenol oxidases are widely found in fruits and cause most of the oxidative browning. See these articles:

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  • $\begingroup$ Could you explain how tyrosine gets in there? What is it that's being oxidized? I assume you mean free tyrosine molecules and not those incorporated into protein right? If I'm missing something obvious, please point it out, biochemistry was never my strong point and I haven't worked in a wet lab since university. $\endgroup$ – terdon Apr 13 '14 at 19:04
  • $\begingroup$ @terdon Tyrosine is a simple molecule used for demonstration. PPO's can oxidize quite a range of molecules, which have to contain phenyl groups. See this paper for some details. $\endgroup$ – Chris Apr 13 '14 at 19:45
  • $\begingroup$ I remember reading somewhere that PPO has a role to play in the change in colour(to brown) of cut apples when exposed to air. $\endgroup$ – biogirl Apr 14 '14 at 4:24
  • $\begingroup$ Ah OK, thanks. I thought you were suggesting that the oxidation of tyrosine molecules was responsible for the change of color. $\endgroup$ – terdon Apr 14 '14 at 4:41
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    $\begingroup$ @biogirl PPO's are present in almost any fruit. Appples, Bananas, they are responsible for the brown color of tea and cocoa, mangos and so on. $\endgroup$ – Chris Apr 14 '14 at 5:56

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