Dear Straight Dope:
Some guacamole recipes say to leave the avocado pit in the bowl with the guacamole to prevent the guacamole from turning brown. Does this work, and if so, how?
Does it work? Yes. Does it work for some really cool and exotic reason? No.
Most fruits and vegetables change color when their flesh is exposed to the air due to oxidation–that is, reaction with oxygen in the air. Some fruits and vegetables, such as the avocado, are more susceptible than others because they contain an enzyme called polyphenol oxidase. This enzyme works on phenolic compounds in the flesh of the avocado, changing their chemical structure and thus their color.
So there are two culprits in this browning process–the enzyme in the avocado, and the oxygen in the air. Logic suggests that if the avocado pits prevents browning, one of two things must be happening: Either the avocado pit chemically changes the guacamole, or it prevents oxygen from getting to the guacamole in the first place. I originally proposed to Ed that I do some hard-core experimentation with multiple guacamole preparations, but I was talked out of it, and referred instead to the book The Curious Cook by Harold McGee. McGee did experiments with guacamole and avocado pits and discovered that the secret was simply that the avocado pit physically blocked air from oxidizing the guacamole. In fact, the best way to prevent oxygen intrusion is to take plastic wrap and seal it over the guacamole, pressing it down into the surface of the food so no air is trapped above the surface.
You can attack the other culprit, the enzyme, in a couple of ways. Refrigeration will slow down the action of the enzyme, but the whole fruit should not be refrigerated at very low temperatures, as the avocado is very susceptible to chilling injury. Boiling can also be used to slow down or stop the enzyme action, but I don’t advise boiling your guacamole–for one thing, the tannins in it are reportedly bitter when cooked. Since the enzyme doesn’t like acidic conditions, adding something acidic to the guacamole such as lime or lemon juice will slow the reaction of the enzyme with oxygen. Both lime and lemon juice have pH values that are fairly acidic, being between pH 2.0 and 3.0, so choose whichever better suits your recipe. Ascorbic acid (vitamin C) will also help to slow down oxidation (note 1), and since lemon and lime juice both contain large amounts of it, they seem ideally suited for guacamole, which might explain why they appear in many guacamole recipes.
Other chemicals (such as sulfites) and other methods for preparation (such as freeze-drying and high hydrostatic pressure) are sometimes used in industrial applications, but for the standard kitchen, your best bet is to add some lemon or lime juice, and judiciously apply plastic wrap.
Note 1. Avocados already contain some ascorbic acid, but adding a moderate amount to the guacamole can help to slow browning.
"Keeping Freshness in Fresh-Cut Produce," USDA Agricultural Research Magazine, February, 1998.
Lidia Dorantes, PhD., Lidia Parada, MSc., Alicia Ortiz, PhD. Post Harvest Compendium, Chapter 30 Avocado: Post-Harvest Operation, report for the Information Network on Post-Harvest Operations, Food and Agriculture Organization of the United Nations.
On Food and Cooking: The Science and Lore of the Kitchen by Harold McGee, 1984.
The Curious Cook: More Kitchen Science and Lore by Harold McGee, 1992.
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