Whether you're a guacamole lover or not, everyone knows that the beautiful enticing green colour 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 from the air to form dark coloured aggregates. One theory is that these oxidized polyphenols have an anti-fungal effect that serves to protect the fruit from invading organisms. Unfortunately, while they may protect the fruit it also takes away our appetite. Luckily, there are solutions. Adding lemon juice works because the enzymes are inactivated at high acidity. Adding vitamin C, (or ascorbic acid), also works since it acts as an "antioxidant," reversing the reaction of the polyphenols with oxygen. The only trouble here is that you need a large amount of vitamin C, which interestingly enough may explain why tomatoes and peppers are used in guacamole. South American cooks may have noticed the acidic benefits of these ingredients long before modern chemists offered an explanation.
But there is another anti-browning tradition that is on much shakier grounds. Placing an avocado seed in the guacamole is said to prevent discoloration yet this is not entirely true. The only place where it will prevent browning is directly underneath the seed, as the seed works to eliminate contact between the guacamole and oxygen from the air. Don't believe me? Try putting a lightbulb in your guacamole instead of the avocado seed itself. The guacamole under the bulb will stay green.
So what is the best method to reduce the browning? Cover the guacamole with plastic wrap, which will make it impervious to oxygen. Back in the day Saran Wrap was the best option, as it was made from polyvinylidene chloride. But with today's environmental concerns about chlorinated compounds, Saran Wrap is made of polyethylene, which is a poor barrier to oxygen. So if you want to really be sure that your guacamole stays green, you will have to get your hands on some ethyl vinyl alcohol copolymer, a plastic resin commonly used as an oxygen barrier in food packaging.