Techniques for preserving food
- Cut an apple into three pieces.
- Place one piece in the refrigerator.
- Leave the other two pieces uncovered on a plate, and baste one of them with vinegar.
- Wait for two hours.
Which of the three pieces has turned brown? Why?
Only the piece of apple left in the open air and without vinegar has turned brown. Once fruit is cut open, the skin no longer protects the inner flesh. When it comes into direct contact with air, it turns brown.
The cold temperature in the refrigerator slows down the reaction of the flesh to air. This is why the chilled piece of fruit does not turn brown. Vinegar is acidic and acts on the flesh, preventing it from mixing with air.
Cold and vinegar are ways of preserving cut up fruit. Lemon juice is also acidic, so can be used as an alternative to vinegar.