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So what exactly happens to the flavor of avocados when it oxidizes? I made some guacamole and as many are familiar, it forms that yucky garbage-green color.

Now, it tastes different. I'm not sure if it's necessarily bad though. Maybe it's just an acquired taste. The essence is still kinda there and I feel bad scraping that first 1/8 inch off and just chucking it away. I can't tell if it tastes spoiled because it looks that way, or if it's just undergone a really crappy reconstitution of it's flavors and there's been a quantifiable decrease in yumminess.

Side note: a friend taught me to cover the top layer of guac with sour cream. This works awesome. I just didn't have any on hand for my last batch.

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If you cover with plastic wrap touching the surface and clinging to it (not just over it), this will prevent oxidation. –  justkt Nov 22 '10 at 13:35
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The sour cream trick works because it's a fat, and fat is an almost airtight seal against oxygen. It's the same reason why you can keep stock or drippings in the fridge for a week or two with the layer of congealed fat on top, but once you skim the fat, it only lasts a few days. –  Aaronut Nov 22 '10 at 15:52
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2 Answers 2

up vote 16 down vote accepted

Whenever you cut into the avocado, you activate an enzyme, polyphenol oxidase, which causes the monophenols in the avocado to hydroxylate to polyphenols. This results in the browning you see.

You can stop this reaction dead in it's tracks by introducing an acid. Which acid depends on what you're making, but I usually use citric acid, in the form of some lime juice. You can however use whichever acid you'd like.

Covering your guacamole with sour cream certainly work as well, the lactic acid within might help (although seeing as sour cream isn't really all that acidic, it's probably a pretty minor role), but this effect comes mostly from the fact that you're covering it, so that it isn't exposed to air.

You can do the same thing with plastic wrap, just make sure that you don't leave any air pockets between the plastic wrap and the guacamole.

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Brilliant explanation Magnus! –  BaffledCook Nov 22 '10 at 14:47
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You can also disolve vitamin C tablets in a little water and brush with that. You get all the Citric acid goodness without much of the lemon flavor (if you are trying to avoid the addition of extra ingredients). –  FoodTasted Nov 22 '10 at 17:09
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Actually, vitamin c tablets are ascorbic acid. Depending on where you live (e.g. Sweden), you may be able to buy ascorbic acid in the spice section of your local grocery store, and it's usually cheaper than in the supplement section. –  Magnus Nordlander Nov 22 '10 at 22:51
    
I guess this explains why store-bought guac goes from a nice, bright green to brown almost the instant you open the package. –  TokenMacGuy Nov 23 '10 at 0:04
    
@TokenMacGuy Funny you should mention store bought guacamole. The one we have in Finland contains 1.5% (!) avocado. santamaria.fi/servlet/… –  johnny Nov 23 '10 at 8:11
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I found sources that claim that you can prevent darkening by squezing lemon juice on top of it. Recipes I have tried also uses lemon. I've never tried without so I can't say whether it helps or not.

http://www.wikihow.com/Keep-an-Avocado-from-Darkening-when-Mashed

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The same method works with apples and pears –  johnny Nov 22 '10 at 8:12
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