Brown apples and brown tea, in this Moment of Science^®.

When you bite or cut into an apple, you crush some of the cells the apple is made of.  Substances that normally occupy separate compartments within cells now have the chance to combine.  Chemical reactions happen that would not happen in an uninjured apple, and one of those reactions leads to browning.

In particular, a family of substances called phenols from one compartment of each apple cell comes into contact with oxygen from the air.  When certain enzymes, the so-called phenolases, reach that mixture, a whole chain of chemical reactions takes place, and one of the end products is a brown-colored pigment.  That pigment doesn't change the apple's taste or nutritional value, but it does hurt the apple's visual appeal.

To prevent cut apples from browning, you must either inactivate the browning enzymes with heat or with acid such as lemon juice, or you must cut off the oxygen supply by immersing the fruit in water or by coating it with salad dressing.  Vitamin C also prevents browning by starting a chemical reaction that uses up oxygen.

Browning also happens with bananas, pears, avocados, peaches, and raw potatoes, among other fruits and vegetables.  The process apparently benefits the fruit, because it produces not only the brown pigment but a substance that attacks fungi that might otherwise become established in the injured fruit tissue.

This "enzymic browning" has a practical application.  In the processing of tea, the green leaves are crushed and exposed to the air for a few hours.  Crushing of cells, mixing of substances, reaction with oxygen, and production of brown pigment all happen basically as they do with an apple.  We don't like brown fruit, but we do like brown tea.

This moment of science comes from Indiana University. 

I'm Yaël Ksander.