Debunking the apple myth

So why do apples darken?

Safe­ty pre­cau­tions

Warn­ing! Only un­der adult su­per­vi­sion.

Equip­ment

  • fresh ap­ples;
  • lemon juice;
  • bowl of wa­ter;
  • re­seal­able bag;
  • potas­si­um thio­cyanate so­lu­tion.

Step-by-step in­struc­tions

Cut a few ap­ples – af­ter a while, the slices dark­en. Is this due to a high iron con­tent? If you blend an ap­ple with wa­ter, the re­sult­ing mix­ture won’t turn red if potas­si­um thio­cyanate is added. To keep ap­ple slices fresh longer, put them in a bowl of wa­ter and add some lemon juice. Then put the slices in a re­seal­able bag.

Process de­scrip­tion

Ap­ple cells con­tain polyphe­nols. When an ap­ple is cut, its cells are de­stroyed, and un­der the ac­tion of en­zymes and oxy­gen, the polyphe­nols are ox­i­dized. This tints the ap­ple slices brown. The iron con­tent in ap­ples is about 0.0001%, which is in­suf­fi­cient not only to dark­en the slices, but even to cause ap­ple juice to red­den when potas­si­um thio­cyanate is added. For com­par­i­son, an iron(III) chlo­ride so­lu­tion in­stant­ly turns red with the ad­di­tion of potas­si­um thio­cyanate due to the for­ma­tion of a new iron com­pound. Lemon juice con­tains an­tiox­i­dants that in­hib­it the ox­i­da­tion of the polyphe­nols. There­fore, if you moist­en ap­ple slices even with di­lut­ed lemon juice, they will stay fresh longer!

More sci­en­tif­ic se­crets await you in the MEL Sci­ence sub­scrip­tion!

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