Sometimes I don't need a whole avocado, especially when I'm making food for just myself, so I'm often left with a spare avocado half. As we all know, the cut surface of an avocado quickly turns brown and slimy, making it hard to store it in the refrigerator for very long. Read on for my best tip ever for keeping a cut avocado green, fresh and tasty. (And no, it does not involve the pit!)
Why do avocados turn brown when cut open?
An enzyme in avocados causes the flesh to oxidize when exposed to air, turning it an unappealing brown color. This is why an avocado is often green underneath the pit but brown on the exposed surfaces surrounding it — the surface underneath the pit is protected from oxygen. But the protection is not about the pit itself, only the way the pit acts as a barrier.
The usual solution
The usual approach is to limit the exposure of the cut surface to oxygen. Many people simply press a piece of plastic wrap over the cut surface and stick it in the refrigerator. I've tried this and I've found it lacking. First, I don't like plastic wrap and second, it seldom creates a tight enough seal to keep the air out.
Another solution is to squirt it with lemon or lime juice but I've found that over time this just makes for a slimy avocado. Ugh.
I've found the best way to keep a cut avocado green is to place it in a clean cottage cheese or similarly sized airtight container with a piece of cut up onion. Cover it with a lid and refrigerate. I've had a cut avocado stay fresh for several days this way. I don't know why this is, but I suspect that it has to do with the sulfur compounds that the onion releases. This is the same sulfur that makes you cry when you chop onions, but it is also used as a preservative. The onion's smell and taste don't seem to transfer to the avocado, as far as I can tell. But I'm not overly sensitive to onions, so you may want to check this out first if you are.
It works for guacamole too!
You can also adapt this technique to keep guacamole from turning brown. I've found that scattering some of the chopped onion called for in the recipe over the top of the guacamole, then covering the bowl works really well. Just stir in the onion before serving!
More Avocado Tips on The Kitchn
Have you ever tried this method? What's your favorite way to keep a cut avocado fresh?
This post has been updated. Originally published 9/15/10.