Why cut apples turn brown in color?

I was cutting the apples into pieces when kids entered the science class. Noticing that, they all assumed I was going to give it to them to eat :-) which I didn't. So, one of them asked me " Aunty, you had left the cut apple pieces out. Will they not turn brown in color?". I was glad that they had initiated the discussion for the day. Further, I asked them to think about things that turn brown when left outside. That triggered them to wonder

1. Why cut apple pieces  turn brown in colour?

2. Why copper vessels change color from pink to brown when kept out?

3. Why does Iron rust when exposed to moisture?

Let us first check out why do cut apples turn brown in colour?

Have you ever bitten into an apple, left it for sometime to find that it had turned brown in colour. When apple is cut or bruised, oxygen is introduced into the cells which then reacts with the enzyme (polyphenol oxidase) in the chloroplast to turn into brown colour. The fruit starts to oxidize, by transferring electrons to the air and the food turns brown. It is like edible rust on your food. 

Oxidation can be prevented or slowed down by not allowing the oxygen to enter the apples. So , we tried the following experiment to find out what can prevent the cut apples becoming brown in color.

Materials

Fresh cut apple pieces - 4 

Bowls - 4 

lemon juice - 2 tsp

salt - 1 tsp

water - 1/4 cup

Procedure

1. Cut apple into 4 slices.

2. Rub lemon juice on one slice, salt on the other slice and the third one inside the water. The fourth one is plain apple piece.

3. Leave the apple pieces outside. 

4. Observe the changes on the apples after 30 minutes.

Results and Observation

What we found was that the apple pieces rubbed with lemon juice stayed the same compared to other pieces. The piece left inside the water had turned more brown than others. Salt rubbed apple piece also had become brown.

So, if you want your apple pieces to stay fresh you should rub it with lemon juice. The citric acid in the lemon prevents it from reacting with oxygen in the air .

 Variations

You can try the same experiment with pears, bananas, egg plants cut too .