If you smashed all the bottles of chemicals in a chemistry lab with a baseball bat, you wouldn’t be surprised at any unusual chemical reactions that occurred as their spilled contents ran together on the floor, would you?
Well, plants are made up of remarkably packaged, exquisitely organized little “bottles of chemicals” called cells. When physical damage is done to a fruit, the cells are broken open and the chemicals that were previously isolated from one another in different parts of the cells spill out and mix.
When you bruise or cut into an apple, pear, or avocado, for example, the damaged flesh soon turns brown from the action of oxidizing enzymes called polyphenol oxidases, which are released from their captivity as soon as the cell walls are broken.
These enzymes act upon the fruit’s phenols, a large group of antioxidant compounds responsible for flavor, color, and many other characteristics of our edible plants, sending them along a chemical path leading to a variety of large molecules (polymers), many of which are brown in color.
This so-called enzymatic browning, to distinguish it from both caramelization and Maillard browning, can be minimized by deactivating the enzymes with heat (in other words, cook those apples promptly) or with an acid. Lemon and lime juices are the most acidic substances in our kitchens, more acidic than vinegar.
Instead of destroying the oxidation-encouraging enzyme, we can cut off the oxygen to the cells, for example by covering the cut surface of the fruit with plastic wrap. Or we could treat it with any of a variety of chemical compounds that inhibit oxidation, such as sulfur dioxide, ascorbic acid (vitamin C), or citric acid in the form of (again) lemon juice.
In some fruits, the enzyme-driven browning reactions do produce sweet sugars. But in others, including apples, sour acids or bitter flavors are produced.
So don’t physically abuse your fruits in an effort to make them sweeter. Uninjured fruits always look and taste best.