Avocados contain an enzyme called polyphenol oxidase (it’s also in other fruit, like apples). When it consorts with oxygen, brown pigments form in the green flesh. That’s why when you leave out freshly made guacamole or a cut-up avocado, it turns brown quickly. But packaged guacamole always seems to be bright green upon purchase, despite having sat on the shelf for countless days.
Manufacturers have to make sure there’s no oxygen present in their packaging if they want the product to stay green, says Ardy Haerizadeh, the CEO of guacamole manufacturer Yucatan Foods. They can do this by vacuuming the oxygen from packages, having plastic film in contact with the top of the guacamole, or filling the headspace of tubs with nitrogen gas.
The enzyme is also sensitive to temperature and acid, so manufacturers can maintain the green color of their guacamole by refrigerating the product and by adding citric acid and/or ascorbic acid, says Diane M. Barrett, director of the Center for Fruit & Vegetable Quality at the University of California–Davis. Acids lower the pH “to the point that the enzyme is not that active,” explains Barrett, who adds that enzymes are also “less active at cold temperatures.”
Refrigeration, adding lemon or lime juice, and limiting exposure to oxygen can also help keep homemade guacamole from turning brown. Chowhounds offer a helpful guacamole timetable for a seven-layer dip.
On a related note, if you do buy commercially prepared guacamole, read the ingredients carefully. Haerizadeh says the FDA hasn’t created a standard of identity for the dish, so manufacturers can label pretty much anything they want as guacamole, whether it’s 95 percent avocado, or a “guacamole flavor” dip that contains less than 2 percent avocado.