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Browning (chemical process)

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 turn brown through an enzymatic or a non-enzymatic process.

Enzymic browning

Enzymic browning is a chemical process involving polyphenol oxidase or other enzymes that create melanins, resulting in a brown color. Enzymic browning is an important color reaction in fruit, vegetables, and seafood. Enzymatic browning of fruits and vegetables creates heavy economic losses for growers.

Enzymic browning is beneficial for:

Developing flavor in tea (here the reaction is incorrectly called fermentation)
Developing color and flavor in dried fruit such as figs and raisins.

Enzymatic browning is detrimental to:

Fresh fruit and vegetables, in particular apples and potatoes
Seafood such as shrimp

Enzymatic browning is usually controlled with chemicals (such as sodium bisulfite), or by destroying the responsible chemicals with heat. Blanching to destroy the enzymes is commonly used to preserve color in vegetables. Lemon juice and other acids are used to preserve color in fruit, particularly apples, by lowering the pH and removing the copper cofactor necessary for the enzyme to function.

Non-enzymatic browning

Non-enzymatic, or oxidative, browning is a chemical process that produces a brown color in foods without the activity of enzymes. Melanins and other chemicals are responsible for the brown color. The two main forms of non-enzymatic browning are caramelization and the Maillard reaction. Both vary in reaction rate as a function of water activity.

Caramelization is the oxidation of sugar. It is used extensively in cooking for the resulting nutty flavor and brown color. As the process occurs, volatile chemicals are released producing the characteristic caramel flavor.

The Maillard reaction is a chemical reaction between an amino acid and a reducing sugar, usually requiring the addition of heat. The sugar interacts with the amino acid, producing a variety of odors and flavors. The Maillard reaction is the basis of the flavoring industry, since the type of amino acid involved determines the resulting flavor.