Blackened avocado: why?
You noticed that the cut avocado turned black in 15-20 minutes. Why is that, why does avocado turn black?

Causes of blackening
The secret is that avocado has enzymes that oxidize and change the color of the pulp. Blackening is due to the exposed flesh, dark guacamole, and all due to contact with oxygen.
How to keep half an avocado from turning black?
Wrap the avocado in a Uf.Bee wax wrapper and store the cut avocado in the refrigerator only in the wrapper.
Wax wrapper
