Browning (food process)

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Caramelization in sucrose: the brown color in the photograph is entirely a result of caramelization, as sucrose is normally colorless

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.

Browning has an important economic cost, causing deterioration of the value of products in the market of food.

Enzymatic browning[edit]

An example of enzymatic browning in the skin of a banana

Enzymatic browning is a chemical process, involving polyphenol oxidase, catechol oxidase, and other enzymes that create melanins and benzoquinone from natural phenols, resulting in a brown colour. In general, enzymatic browning requires exposure to oxygen, for example the browning that occurs when an apple is cut.

Enzymatic browning can be beneficial for:

Enzymatic browning is often detrimental to:

A variety of techniques for preventing enzymatic browning exist, each exploiting a different aspect of the biochemical process.

Arctic Apples are a suite of trademarked apples that contain a nonbrowning trait. Specifically, gene silencing is used to turn down the expression of polyphenol oxidase (PPO), thus preventing the fruit from browning.

Non-enzymatic browning[edit]

The crust of brioche bread, which is golden-brown due to the Maillard reaction

Contrary to enzymatic or oxidative browning, non-enzymatic browning is a chemical process that produces a brown color in foods without the activity of enzymes. 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 pyrolysis 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; it also produces toast.

Aged white wine with brown color.

The wine color is altered with wine aging by reactions between different active molecules present in the wine, these reactions, in general, giving rise to a browning of the wine, leading from red to a more tawny color.

Melanoidins are brown, high molecular weight heterogeneous polymers that are formed when sugars and amino acids combine (through the Maillard reaction) at high temperatures and low water activity. Melanoidins are commonly present in foods that have undergone some form of non-enzymatic browning, such as barley malts (Vienna and Munich), bread crust, bakery products and coffee. They are also present in the wastewater of sugar refineries, necessitating treatment in order to avoid contamination around the outflow of these refineries.

See also[edit]