Enzymes. Oxidation. Browning. And how to keep cut apples from turning brown.
1-Minute NomNom
You put fresh apple slices in your lunchbox in the morning, and by the time you get to eat them, they’ve turned a mottled brown. What gives?
Cutting the apple damages the fruit’s tissues and allows oxygen into the cells. The chloroplasts of plant cells (see diagram below — don’t worry about the other terms) contain an enzyme called polyphenol oxidase (PPO). In the presence of oxygen, PPO oxidizes organic compounds in the fruit’s tissues to produce new organic compounds called ortho-quinones.
These ortho-quinones play an important role of protecting the apple from bacteria and fungi. They also react with amino acids, proteins and oxygen to form the pigment melanin. This is the mottled brown you see.
The whole process is called enzymatic browning. Apple slices that are a little brown are safe to eat; however, the fruit will lose flavor and nutritional value over time.
We can slow the chemical process of enzymatic browning in five ways:
1) Keep cut apples cold — the lower temperature lowers enzyme activity.
2) Put the cut apples in a sugar or salt solution, or at least rub salt or sugar onto them (which will form a solution with the moisture in the apple) — this slows down oxygen diffusion into the apple.
3) Wrap the slices in plastic or store in an airtight container — this limits further oxygen exposure.
4) Spritz lemon juice — lemon juice is not only an anti-oxidant, but it also helps by lowering the pH level and denaturing the PPO, thus reducing enzyme activity.
5) Cook the apple (e.g. for apple pie) — heat denatures the PPO and cuts down enzyme activity substantially; but that also means the apple tastes different (just compare fresh apples with the apple slices in a pie).
Second Helpings
Incidentally, melanin is also the same brown compound we see on bananas after they have ripened for a while; melanin is also what causes your skin to tan with sun exposure. Does that make us all apples and bananas (to some degree)?
Feed Me!
What other fruits display the food-nomenon of enzymatic browning? Share your experiences in the comments below!
Like them apples? Don’t let me turn brown — like me to discover more. All you need is a minute a day to explore the world’s marvels through the phenomenon of food!
