Why do cut avocados go brown in salads?

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Offline chris

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Why do cut avocados go brown in salads?
« on: 20/03/2015 08:13:01 »
Why do cut avocados go brown in salads? Is this similar to the chemistry that turns a cut mushroom a black colour?
I never forget a face, but in your case I'll make an exception - Groucho Marx

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Offline evan_au

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Re: Why do cut avocados go brown in salads?
« Reply #1 on: 20/03/2015 10:31:36 »
Cut avocados react with oxygen in the air.
Some people recommend rubbing with lemon juice to reduce the browning effect.

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Offline chris

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Re: Why do cut avocados go brown in salads?
« Reply #2 on: 22/03/2015 22:28:05 »
Do you know the chemistry of the avocado browing reaction, Evan?
I never forget a face, but in your case I'll make an exception - Groucho Marx

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Offline chiralSPO

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Re: Why do cut avocados go brown in salads?
« Reply #3 on: 22/03/2015 22:52:47 »
I'm fairly certain it has to do with the oxidation of the amino acid tyrosine, which is a phenol, so it is easily oxidized. Before the avocado is opened not enough oxygen can get in to complete the reaction, but once it is exposed to the air the reaction is fairly swift. Acid can slow the reaction down (I have my theories as to why, but I am not certain, and I don't want to present misinformation...)

It's the same thing with apples, and similar with mushrooms. Some mushrooms contain hydroquinones (very similar to phenols, but more reactive) which react to turn dark blue when the mushroom is damaged (https://www.youtube.com/watch?v=PoyHRmIw28k).

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Offline evan_au

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Re: Why do cut avocados go brown in salads?
« Reply #4 on: 24/03/2015 09:39:41 »
Browning is caused by enzymes in the fruit, when it is cut (polyphenol oxidase or PPO is one of them).
See: http://en.wikipedia.org/wiki/Browning_%28food_process%29

Or "Activity, Electrophoretic Characteristics and Heat Inactivation of Polyphenoloxidases from Apples, Avocados, Grapes, Pears and Plums"
http://www.sciencedirect.com/science/article/pii/S0023643897903029

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Offline chris

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Re: Why do cut avocados go brown in salads?
« Reply #5 on: 26/03/2015 22:12:56 »
Thank you; lovely answer.
I never forget a face, but in your case I'll make an exception - Groucho Marx