What makes bruised apple flesh brown? (BioE80 Spr2015)

I had an apple in my backpack that got badly bruised, with a dark, mushy spot on its side. Bruises happen when you accidentally dent the fruit, but I found myself wondering about biochemical explanations for the browning. What’s happening inside the apple to darken the flesh? I used my Foldscope to investigate. I first…