Can placing an avocado seed in guacamole prevent discolouration?

I like guacamole. But I like it to be green. This mix of mashed avocado, peppers and tomatoes has been a South American staple for hundreds of years. And for just as long, avocado lovers have waged a battle against the “browning reaction.” Everyone knows that the beautiful enticing green color of fresh guacamole can quickly turn to a dismal unappetizing brown. Perhaps the reason that guacamole is traditionally made with peppers and tomatoes stems from the observation that these components retard the browning reaction.

In terms of chemistry, the reaction is fascinating. When the tissues of the avocado are damaged, enzymes known as polyphenol oxidases are released. These catalyze the reaction of naturally occurring polyphenols in the avocado with oxygen of the air to form dark colored aggregates. One theory is that these oxidized polyphenols have an antifungal effect and serve to protect the fruit from invading organisms. They may protect the fruit, but they take away our appetite. But there are solutions. Adding lemon juice works because the enzymes are inactivated at high acidity. Adding vitamin C, or ascorbic acid, also works because it acts as an “antioxidant,” reversing the reaction of the polyphenols with oxygen. But you need a great deal of vitamin C. Tomatoes and peppers are acidic and contain vitamin C, so South American cooks may have noticed the benefits of these ingredients long before modern chemists offered an explanation.

But there is another anti-browning tradition that is on much shakier grounds. Supposedly, placing an avocado seed in the guacamole prevents the discoloration. Actually it doesn’t, except directly underneath the seed where it eliminates contact between the guacamole and oxygen from the air and thus prevents browning. The fact that the seed possesses no magical quality can be clearly demonstrated by using a light bulb instead. The guacamole under the bulb stays green. So what is the best method to reduce the browning? Cover the guacamole with plastic wrap that is impervious to oxygen. This used to be simple when Saran Wrap was still made of polyvinylidene chloride. That was an excellent barrier, but because of environmental concerns about chlorinated compounds, Saran Wrap is now made of polyethylene which is a poor oxygen barrier. So if you want to keep your guacamole green, you’ll have to get your hands on some ethylvinylalcohol copolymer, a superb barrier film.

A final point. The name of the avocado derives from an Aztec word meaning testicles. There is somewhat of a resemblance. Perhaps this explains the fruit’s reputation as an aphrodisiac. Indeed, the first European explorers spoke about the effect of avocados on their sexual appetites. Nobody, as far as I know, has investigated this matter scientifically. Maybe it was because they lost their appetite due to the brown color. But perhaps with the right oxygen barrier wrap this theory can be explored.

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