Discoloration: Enzymatic Browning

Appears in
On Food and Cooking

By Harold McGee

Published 2004

  • About
Many fruits and vegetables—for example apples, bananas, mushrooms, potatoes— quickly develop a brown, red, or gray discoloration when cut or bruised. This discoloration is caused by three chemical ingredients: 1- and 2-ring phenolic compounds, certain plant enzymes, and oxygen. In the intact fruit or vegetable, the phenolic compounds are kept in the storage vacuole, the enzymes in the surrounding cytoplasm. When the cell structure is damaged and phenolics are mixed with enzymes and oxygen, the enzymes oxidize the phenolics, forming molecules that eventually react with each other and bond together into light-absorbing clusters. This system is one of the plant’s chemical defenses: when insects or microbes damage its cells, the plant releases reactive phenolics that attack the invaders’ own enzymes and membranes. The brown pigments that we see are essentially masses of spent weapons. (A similar kind of enzyme acting on a similar compound is responsible for the “browning” of humans in the sun; here the pigment itself is the protective agent.)