When an apple or banana is cut, a chemical reaction with air causes the flesh to turn brown. placing the fruit in a cold place, such as a refrigerator, can keep it from getting brown as quickly. explain this effect!

Apples turn brown when exposed to air because it undergoes aerial oxidation.  Due to this, when the inside of the apple is exposed to the air containing oxygen and water, it turns brown. When apple is uncut, skin of the apple protects it from this process. 

If the peeled apple in kept in the refrigerator, the oxidation reaction is greatly slowed down. This is because, rate of chemical reaction decreases with temperature. Hence, in refrigerator it would take several days for it to turn brown.