What you see: Guacamole that’s brown and yucky on top
What it is: Guac that turned brown because it was exposed to air
Eat or toss? Toss the brown layer, but excavate under it until you get to green. As long as your guacamole is relatively fresh, any green areas should still be fine to eat.
This guacamole may look like a mucky swamp, but buried beneath that layer of yuck, there’s hope for your tortilla chips.
The guac turned brown simply because it was exposed to air. An enzyme in the avocado reacted with oxygen and that sad brown color was the result. The same thing happens to apples and other fruits and vegetables when they’re exposed to air. So, nothing to fear here, but the oxidized avocado may not taste so great.
But, if you scrape off the brown part, you’ll get to a subterranean layer of perfectly good guacamole. That’s because the top layer of guac guarded the layer below from the oxygen in the air.
How to avoid brown guac
So, brown guacamole is still salvageable, but there are also loads of ways to prevent this problem in the first place. The key is just to block air from coming into contact with the avocado-y goodness. You can do this by stirring in or coating the top layer with lemon or lime juice, which will make conditions too acidic for the browning reaction to take place.
Or, you can try other methods to prevent contact with air. Such tricks range from applying plastic wrap directly to the surface of the guac to simply pooling water on top before putting it in the fridge, and then dumping the water off before serving. I tend to use the lemon juice method. The Today Show evaluated a number of tactics and they liked the water trick the best.
Thanks to Joe G. for submitting these photos. Joe reports that he successfully scraped off the brown layer. For science, he sampled the brown area and found it to be “yucky.”
“Not overpowering, but it had sort of a stale taste,” he wrote. “Wasn’t pleasant.”
But, he kept digging and reported: "The green beneath the brown tasted perfect."
Frequently Asked Avocado Questions - AvocadoCentral.com
The best way to keep guacamole green, revealed - Today Show
Apple slices turned brown: Are they really bad apple or just misunderstood? - EatOrToss.com
The Chemistry of An Avocado - Compound Interest
What's up, Guac?