Browning is the process of becoming brown, especially referring to food. Browning foods may be desirable, as in caramelization, or undesirable, as in an apple turning brown after being cut. Foods, including beverages, can turn brown through either enzymatic or non-enzymatic processes.
Enzymatic browning is a chemical process, involving polyphenol oxidase, catechol oxidase and other enzymes that create melanins and benzoquinone, resulting in a brown color. Enzymatic browning generally requires exposure to oxygen, thus the browning that occurs when an apple, for example, is cut.
Enzymatic browning can be beneficial for:
Developing flavor in tea
Developing color and flavor in dried fruit such as figs and raisins.
Enzymatic browning is often detrimental to:
Fresh fruit and vegetables, including apples, potatoes and bananas
Seafood such as shrimp
A variety of techniques exist for preventing enzymatic browning, each exploiting a different aspect of the biochemical process.
Lemon juice and other acids lower the pH and remove the copper cofactor necessary for the responsible enzymes to function
Blanching to denature enzymes and destroy responsible reactants
Cold temperatures can also prevent enzymatic browning by reducing rate of reaction.
Inert gas, like nitrogen, prevent necessary oxygen from reacting
Chemicals such as sodium bisulfite and citrates