Why
Apple Slices Turn Brown and How to Stop Sliced Apples from Browning

You get a beautiful apple, slice it, put it in a plastic bag in your child's lunch.  That afternoon your child comes home from school and there in the lunch pail is the uneaten sliced apple, all brown and mushy. When asked when he or she didn't eat it, you child says "It looked brown and gross!"

Why did the apple turn brown?

When you slice an apple the cut cells of the apple are exposed to oxygen in the air. Enzymes, called polyphenol oxidase (PPO), in the apple's chloroplast cells react to this apparent injury by converting some naturally occurring  phenolic compounds into another type of compound called  o-quinones.  Then these colorless o-quinones react with other naturally occurring compounds in the apple to create the brown-colored secondary compounds.

Some apples seem to brown faster than others

While most plant tissues contain PPO, the level of  PPO and the phenolic compounds, varies between varieties of fruits.  This is why some varieties like Granny Smith brown less and les quickly than others, like Red Delicious. Also PPO levels vary due to weather, growing conditions and the maturity of the fruit.

What can you do to stop the browning?

There are several ways:

1. Dip the apple slices in a bowl of water (say 2 quarts) with 1/2 cup of lemon juice, or

2. Dip freshly cut apples in sugar or sugar syrup, which coats the cut apples preventing oxygen from coming into contact with the cut cells, or

3. Blanching the apples; immersing them in boiling water for 60 seconds, then using a slotted spoon to scoop them out and cool them in ice water for a couple of minutes.  This slows the enzymes.

References:

  1. Preservative Treatments for Fresh-Cut Fruits and Vegetables, Elisabeth Garcia and Diane M. Barrett, Dept. of Food Science and Technology, University of California, Davis
  2. Scientific American, July 30, 2007
  3. Wiki-How
  4. Wafler Nursery