How to tell if an avocado is bad

Dark green doesn’t necessarily mean rotten.
By Sarah Jampel | Food52 |


FOOD52 -- A few years ago, an Australian company called Naturo Technologies invented a machine—the Natavo Zero, aka the Avocado Time Machine. This ATM supposedly miraculously slows the avocado ripening process, keeping it from turning brown for up to 10 days without the use of chemicals—or olive oil, or lemon juice, or red onion. (Naturo Technologies is currently working on a new process of pasteurization that leaves more of the natural vitamins and enzymes in the milk.)

The implications of this Avocado Time Machine? Let’s say you did all the sleuthing at the store for the perfectly-ripe avocado: You sought out fruits (yes, they’re a fruit) with a darker green, almost black skin color. You squeezed only a handful (ha) of avocados at the store (applying gentle pressure only, obviously). You found the one that was perfectly ripe and ready, and nicked off the stem to check the color of the round (perfectly light green). But then, you got home, and forgot about the avocado—not remembering until days later, and your once-perfectly ripe avocado is now a brown and mushy avocado.

When the ATM was first getting publicity, it seemed like everyone I knew emailed me one article or another (from MashableEaterFoodbeastFood & WineGrub StreetTasting Table...). Even my dad told me about it!

But here's the thing: Rather than being marketed or intended for home use, the machine is meant for large-scale food service. A press release from the company states that the "list of possible beneficiaries is long and includes the food service, quick service restaurants, catering companies, canteens, sushi chains, supermarkets and all kinds of retailers. It also permits the export of locally grown avocado fruit to countries with difficult import restrictions or hard to access markets." Moreover, the ATM, says Naturo, eliminates potential pathogens at a level that exceeds the strictest food safety codes—its benefit is in increased food safety, not just in aesthetics.

As Clint Rainey of Grub Street put it, "The idea is cool and all, even if it’s the work of somebody whose plan seems to be tackling the world’s problems in reverse order of importance, and is of absolutely no benefit to home cooks." So why all the attention? What's with our never-ending obsession with keeping avocados as green as possible as long as possible? What's really going on when an avocado turns brown? Is it just an unsightly, unappetizing phenomenon—or can you really taste the difference?

The FAQ page of Avocado Central, a website run by the Hass Avocado Board, does not offer much information beyond, "Oxidation (exposure to air) can cause the fruit of an avocado and/or guacamole to turn brown." Kids' stuff.

Compound Interest, a site (and accompanying book) that dives into the chemistry of everyday food and drinks, offers a bit more explanation: In the presence of oxygen, the enzyme polyphenol oxidase converts the phenolic compounds in the avocado into quinones (more on how quinones affect flavor later). These quinones polymerize—they join smaller molecules into long chains—to create polyphenols, and it is this polymerization that appears as the brown coloration. It's not only oxygen exposure that results in the browning—it's also the damaging of the structure of the plant cells, which allows the phenolic compounds and the polyphenol oxidase enzyme to interact.

The ATM works to combat this by using steam to create pressure fluctuations that "switch off" polyphenol oxidase, thereby slowing the browning process.

If all that went over your head and you just want to know whether avocados that have browned from oxidation actually taste worse, here’s Monte Nesbitt, an Extension Program Specialist at the Texas A&M AgriLife Extension, to explain.


How To Tell If An Avocado Is Bad

1. Check The Color

Not oxidation (your light green avocado turning dark green, even brown), but quinones "are responsible for bitter flavor,” Nesbitt says.”There can be some loss of eating quality as the process continues, although perceptions of these bitter flavors vary among people.”


So the impact of oxidation on avocado flavor is a bit up in the air. Eric Focht, a staff research associate at the University of California Riverside, said he wasn't aware of any blind taste tests of browned versus freshly-cut fruit. But, he'd "bet that if the subject couldn't see what they were eating, they'd notice little difference; I know I don't." Most of the aversion, he hypothesizes, is psychological: "We don't like the way it looks, so it's off-putting. A big part of food is its presentation, and we associate browning or discolored fruit with being bad or overripe or even rotten."


"On the other hand," Focht reminded me, "there are types of browning and discoloration in avocado fruit that DO affect flavor"—like chilling injury, which is browning or blackening of the skin and browning or graying of internal flesh due to low-temperature storage—"and it's important to distinguish between [these] and phenolic response."

So, a few years ago, we did a mini, totally unscientific, mostly anecdotal blind taste test at the Food52 offices. One of our avocados was truly funky—brown-gray from the moment we opened it up—and it tasted discernibly different, though not bad, per se: more like there was something obscuring the true avocado flavor; then-staffer Amanda Sims put it well when she said it tasted like an unripe avocado (slightly blah-ish) but with a soft texture.

The avocado that was pristine when we cut into it but left out to brown for a few hours, on the other hand, was harder to differentiate from the freshly-cut one: Maybe it tasted slightly bitter... maybe. It was hard to say! (Of course, the oxidation that occurs after a few hours is surely less impactful than oxidation that happens over a number of days.)

In all scenarios we realized that avocados—at least the ones we're getting in New York City (that is, shipped in from Mexico and California)—aren't so palatable on their own. But when you add the necessary salt and pepper and lime juice, any deterioration in taste due to oxidizing probably wouldn't be noticeable at all.


2. Taste It (Yes, Really!)

As for safety concerns? Nesbitt said that there's "no safety problem with consuming dark avocados unless they have also been exposed to room temperature and bacteria, thus recommendations to refrigerate them are valid." So if you're tasting something funky or rancid or "off" in your brown avocado (or the sandwich you're saving for the next day), it's important to think about why it's brown: It could be due to something other (or maybe more nefarious) than oxidation.

If presented with an unsightly, but otherwise very fine (largely light-green-fleshed, some dark bruisings OK, buttery but not liquified, absolutely no foul smells); avocado, we’re on Team Just Eat It. And it sounds like you are too: When we asked you your feelings about oxidizing avocados, most of you commented that you simply cut around discolored areas or use the less-than-perfect fruit in smoothies, on toasts, or in guacamole.

But! If you can't get behind browning avocados (and you're upset that the Natavo Zero is meant for commercial use, not home kitchens) and they do taste bitter or funky to you, you can take precautionary measures to stymie their browning (counterintuitively, the best choice may be to do nothing at all!).

Or, you can seek out special varieties less prone to browning (many of which aren't available in all parts of the country). Focht told me that "some avocados suffer from [browning] a lot more than others and Hass is one of the varieties that does have particular problems with browning after the flesh is cut. Many other avocados, however, show a significantly less pronounced response" and "some of them really do not seem to brown at all (Sir Prize, a UCR release [a cross that originated at UC Riverside] from the late 90s comes to mind)."



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