I too am beginning to think browning avocado is a myth. I attended a brief demo class presented by John Ashe and he said that merely running tap water over the cut surface will keep it from turning. That works too. Maybe anything works?

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I dunno, I have had avocados turn brown literally minutes after removing them. Possibly some difference in the temperature.

Lemon Juice DOES help. I have had guacamole stay green for several days in the fridge with lemon juice, while guacamole without lemon juice becomes brown within a couple hours in the fridge.

Ya know what... Maybe it's the damn fridge.

THERE's a test. Sounds weird... But I bet an avocado left outside the fridge (though drier, maybe) would look the same or better than one left in the refrigerator. Commence testage.

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How quickly an avocado turns from a delicious green to an unappealing brown will vary with room conditions. The main culprit in turning color is air, and the more air that touches the avocado, the faster it will turn. This happens in breezy environments (outdoors, room or ceiling fans, etc) and warm environments.

If you'd like to verify this, instead of just cutting the avocado in half, try cutting it into several chunks, and put one chunk in front of a room fan. Actually, put a little lemon or lime juice over some of the chunks, and put those, along with some of the un-juiced chunks, in front of the fan and see what happens.

Also, you didn't mention how you were measuring color? Did you have some fabric or something that resembled the color when you first cut into the avocado? If not, your avocado might have changed color just subtly enough for you not to notice, or mind, all that much.

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Conclusion: avocados turn brown after about 6 hours and 15 minutes ...at that temperature, humidity, and air pressure. On that particular avocado (ripeness, pesticides, etc.).

The browning reaction and preventative measures are addressed in Hervé This' Molecular Gastronomy.

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the avocado browns at different speeds depending on how ripe it is when you cut into it

if your avocado is firm it will take longer to brown than a soft avocado

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Air has nothing to do with it. I've seen tons of guac turn brown from exposure to light.

Air is not a variable. Put a bit of guac under a flourescent light and you'll see something like bark in a matter of hours.

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The browning is real, and it is done by an enzyme polyphenal oxidase. This enzyme is a catalyst to the browning process and requires oxygen to complete the reaction. Lemon or lime juice works in guac because of the citric acid denatures the enzyme, making it inoperable. Another thing that will help is heat, I forget the exact numbers, but if you heat the avocados up for a bit, that will denature the enzyme just as well. This method works better for recipes that don't call for citric fruits.

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Bill, air has everything to do with it, or more precisely, oxygen. Bob, méwyn and Nicolai all have very good comments on this "experiment", if you can even call it that. so, i guess from Sarah's conclusion, people who have avocados that turn brown in minutes are liars? or live in some weird alternate reality?

my girlfriend and i were just discussing ways to actually test this and quantify the reaction. unfortunately, we'd have to do a lot of testing, and use a lot of expensive laboratory equipment. between the two of us, we could probably do it, but i think our supervisors and/or professors wouldn't appreciate halting research to test avocados.

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Also, how much 'damage' you inflict to the fruit will have some significant effect on how fast it browns. Guacamole will brown faster than an avocado that was simply sliced in half.

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Here's a bit of correction for the debate I see going on above:

Air browns guacamole, but so does light and heat. It's not just oxygen that's the culprit, it's *oxidation*, which does not necessarily require oxygen. (Chlorine is a very potent oxidizer, for example; oxidation is named after oxigen, but the same effect can happen through radiation or through other elements with high electronegativity.)

Guacamole (and avacados in general) are rich in Omega-3 fatty acids, which have sensitive, easily broken double-bonds. Light, especially higher frequency light like UV, and oxigen, easily breaks these bonds, which is the primary cause of oil rancidity. That's why linseed oil (a.k.a. flax seed oil), fish oil, guacamole, and nut oils tend to go rancid so quick unless they are sealed air tight in light-proof tins and kept refrigerated.

Poly-unsaturated oils are "unsaturated" because there are carbon bonds that are not occupied; these oils (which are generally good for you) tend to go rancid very easily. Hydrogenation of oils breaks these bonds and saturates all of the now free bonds with hydrogen atoms. With all the easily broken bonds now split and occupied with hydrogen, the hydrogenated oils are less likely to go rancid (in other words, without the double bonds available to be attacked by light or oxygen, the oil keeps better.)

Once a little bit of oil in a large batch gets rancid, the whole batch tends to go really quick, because when those double bonds are broken by an oxidizer, the part of the bond that isn't occupied by the oxidizer seeks out things it can bond to. This is what is called a "free radical". This bond usually seeks out and destroys the double-bonds of neighboring molecules, and leads to a chain reaction. In some oils, this causes polymerization and solidification, until all the oil molecules are bonded into massive chains. For example, walnut oil will solidify entirely if left to oxidize. This particular trait, namely the solidification of Omega-3 fatty acids that are left to oxidze, is the primary reason linseed (a.k.a. flax seed) oil and walnut oil are used as wood finishes. If you try to finish wood with crisco, you just end up with greasy wood, but walnut oil and linseed oil will give you a hard shiny finish.

That's the detailed answer for why avocados and nut oils/fish/flax oils go bad so quickly.

As for how to prevent it: use plastic wrap, and press it down on the guacamole so all the air is pressed out, and refrigerate it. And when you mash the guacamole, don't mash in a bunch of air; that defeats the purpose. The pit does nothing: the reason the guacamole under the pit remains fresh is that the pit keeps the guacamole that its in contact with from contacting the air. The same effect can be found if you substitute a light blub for the pit. Plastic wrap does even better than light blubs and avocado pits.

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FYI, flourescent lights brown avocados faster because they give off more light in the higher frequencies. Higher frequency light (especially UV light) will break the double-bonds in omega-3 fatty acids at a higher rate. Try this experiment: put an avocado under a black light, and you'll see how much more quickly it turns brown. Black lights give off UV light in huge quantities, which is very potent at breaking double bonds.

It is precisely this bond-breaking action that bleaches color out of bright colors, and turns other things yellow or brown. Sunlight fades colors because it has so much UV, and the colors you see in carbon-based dyes depend on the double bonds and other sensitive bonds in the dyes. Chlorine bleach does the same thing to colors and dyes (it snaps the bonds responsible for color absorbtion), but chemically, rather than through high frequency light.

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Is this the same/similar reaction that causes banana skins to turn brown in the fridge??

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Actually, if you cut the avacado pit in half, and set the halves cut-side up on top of the guac, then wrap it with plastic wrap before refridgerating, it keeps a lovely green shade. not sure if it's something chemical to do with the inside of the pit, or just luck or avacado juju, but there you go.

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You can still eat it brown. Just saying.

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Um, did you think that maybe you should, you know, take it out of the skin? The browning happens when you take it out of the skin, mash it, and mix it with other vegetables/fruits. In this case, yes, it browns.

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