Why Avocados Turn Brown
Avocados contain an enzyme called polyphenol oxidase. When the flesh of the avocado is exposed to air, the enzyme oxidizes. This enzyme creates browning in many different fruits and vegetables. The enzyme transforms chemical chains formed with a benzene ring with one hydroxyl substituent, called monophenols, into benzene rings with two hydroxyl substituent, called o-diphenols. When oxygen is added, they produce another chemical chain called o-quinones; in this process, black, brown or red pigments can be created. It is these pigments that create the browning.
Prevent Avocados From Turning Brown
There are a number of methods to prevent browning from occurring in avocados. One commonly purported method is to leave the pit in the avocado, but in relation to the chemistry involved, it just doesn't make sense. However, it is true that putting an avocado in the refrigerator down the enzymes. You can also tightly wrap the avocado to prevent the oxygen from getting to the enzyme. Also, the enzyme doesn't like acidic conditions, so adding an acid will slow the reactions; try lemon or lime juice, as these flavors go well with many avocado dishes.
How Long It Really Takes
Sometimes an avocado will turn brown in minutes, while at other times, it can take hours; it really depends on the avocado. The change in color won't affect the taste of the dish, but the vibrant green color of an avocado is an important part of many dishes.
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