What makes bruised apple flesh brown? (BioE80 Spr2015)

May 27, 2015 • 6:45 AM UTC

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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 had to take a sample of unbruised flesh to see how apples are normally structured. I cut out a slice of unbruised flesh from the apple and made my Foldscope slide:

The underlying structure was astonishing. I expected some sort of porous matrix, but it actually was composed of relatively long fibers. The surrounding solution was clear, with occasional brown specks.

It was then time to check out the bruised portion. I cut out a slice of bruised flesh and made a slide:

The bruised flesh had fibers like the normal flesh, but the surrounding solution was murky and brown!

It looks like whatever causes the brown flesh is hiding in the solution itself, too small for the Foldscope’s gaze. Maybe it’s bacteria, or fine particles; maybe even single molecules. In fact, scientists know that an enzyme called polyphenol oxidase triggers tiny phenol molecules to link together into huge pigment chains, which contributes to browning. Bioengineers have designed apples without this enzyme, and they don’t turn as brown. However, we don’t know yet what other factors are involved, or why this even happens when you bruise the apple. All the more reason to keep exploring!
Ian Hull

What makes bruised apple flesh brown? (BioE80 Spr2015)

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