Why does guacamole turn brown? -The enzyme polyphenol oxidase reacts with oxygen and discolors the avocado. 

Is it OK to eat guacamole when it turns brown? -Yes. It's safe to eat! Well, it's safe as long as the guac has been stored in the fridge for not longer than two or three days.

This Is How to Keep Guacamole from Turning Brown............

The guacamole must be stored in an airtight container—like these glass storage containers 

With a metal spoon, flatten the surface, taking care to remove all air pockets. 

Pour water over the guacamole

Refrigerate the container of guacamole, covered tightly, for up to two days. 

A fresh bowl of guac but like a brown avocado, brown guacamole can be safely eaten.