right|thumb|253px|Enzymatic browning (oxidize): [[Fuji apple - 32 minutes in 16 seconds (video).]]

Browning is the process of food turning brown due to the chemical reactions that take place within. The process of browning is one of the chemical reactions that take place in food chemistry and represents an interesting research topic regarding health, nutrition, and food technology. Though there are many different ways food chemically changes over time, browning in particular falls into two main categories: enzymatic versus non-enzymatic browning processes.

Browning has many important implications on the food industry relating to nutrition, technology, and economic cost. Researchers are especially interested in studying the control (inhibition) of browning and the different methods that can be employed to maximize this inhibition and ultimately prolong the shelf life of food.

Enzymatic browning

right|thumb|446px|Example of a general reaction of polyphenols by polyphenol oxidase (PPO) that catalyzes enzymatic browning. The production of quinones undergoes more reactions which eventually form brown pigments on the surface of the food.

Enzymatic browning is one of the most important reactions that takes place in most fruits and vegetables as well as in seafood. These processes affect the taste, color, and value of such foods. whose strong electrophilic state causes high susceptibility to a nucleophilic attack from other proteins. The rate of enzymatic browning is reflected by the amount of active polyphenol oxidases present in the food.

  • Developing color and flavor in dried fruit such as figs and raisins.

Examples of non-beneficial enzymatic browning:

  • Fresh fruit and vegetables, including apples, potatoes, bananas and avocados.
  • Oxidation of polyphenols is the major cause of melanosis in crustaceans such as shrimp.

Control of enzymatic browning

thumb|250px|Irradiated [[guava]]

The control of enzymatic browning has always been a challenge for the food industry. A variety of approaches are used to prevent or slow down enzymatic browning of foods, each method aimed at targeting specific steps of the chemical reaction. The different types of enzymatic browning control can be classified into two large groups: physical and chemical. Usually, multiple methods are used. The use of sulfites (powerful anti-browning chemicals) have been reconsidered due to the potential hazards that it causes along with its activity. Much research has been conducted regarding the exact types of control mechanisms that take place when confronted with the enzymatic process. Besides prevention, control over browning also includes measures intended to recover the food color after its browning. For instance, ion exchange filtration or ultrafiltration can be used in winemaking to remove the brown color sediments in the solution.

Physical methods

  • Heat treatment − Treating food with heat, such as blanching or roasting, de-naturates enzymes and destroys the reactants responsible for browning. Blanching is used, for example, in winemaking, tea processing, storing nuts and bacon, and preparing vegetables for freezing preservation. Meat is often partially browned under high heat before being incorporated into a larger preparation to be cooked at a lower temperature which produces less browning.
  • Cold treatment − Refrigeration and freezing are the most common ways of storing food, preventing decay. The activity of browning enzymes, i.e., rate of reaction, drops in low temperatures. Thus, refrigeration helps to keep the initial look, color, and flavour of fresh vegetables and fruits. Refrigeration is also used during distribution and retailing of fruits and vegetables.
  • Oxygen elimination − Presence of oxygen is crucial for enzymatic browning, therefore eliminating oxygen from the environment helps to slow down the browning reaction. Withdrawing air or replacing it with other gases (e.g., N<sub>2</sub> or CO<sub>2</sub>) during preservation, such as in vacuum-packaging or modified atmosphere packaging, using impermeable films or edible coatings, dipping into salt or sugar solutions, keeps the food away from direct contact with oxygen. Impermeable films made of plastic or other materials prevent food being exposed to oxygen in the air and avoid moisture loss. There is an increasing activity in developing packaging materials impregnated with antioxidants, antimicrobial, and antifungal substances, such as butylated hydroxytoluene (BHT) and butylated hydroxyanisole (BHA), tocopherols, hinokitiol, lysozyme, nisin, natamycin, chitosan, and ε-polylysine. Edible coatings can be made of polysaccharides, proteins, lipids, vegetable skins, plants or other natural products.
  • Irradiation − Food irradiation using UV-C, gamma rays, x-rays, and electron beams is another method to extend the food shelf life. Ionizing radiation inhibits the vitality of microorganisms responsible for food spoilage and delays the maturation and sprouting of preserving vegetables and fruits.

Chemical methods

  • Acidification − Browning enzymes, as other enzymes, are active at a specific range of pH. For example, PPO shows optimal activity at pH 5-7 and is inhibited below pH 3.

Non-enzymatic browning

thumb|left|The crust of [[brioche bread, which is golden-brown due to the Maillard reaction|225x225px]]

The second type of browning, non-enzymatic browning, is a process that also produces the brown pigmentation in foods but without the activity of enzymes. The two main forms of non-enzymatic browning are caramelization and the Maillard reaction. Both vary in the reaction rate as a function of water activity (in food chemistry, the standard state of water activity is most often defined as the partial vapor pressure of pure water at the same temperature).

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

thumb|308x308px|Example caramelization of table sugar (sucrose) caramelizing to a brown nutty flavor substance (furan and maltol)thumb|308x308px|Overview of the mechanism of non-enzymatic Maillard reaction in foods. The Schiff base loses a CO<sub>2</sub> molecule and adds to water. Notice the interaction between the amine group of the amino acid (asparagine here) and the carbonyl carbon of the sugar (glucose). The end product is [[acrylamide. For more information, visit Maillard reaction. ]]

The other non-enzymatic reaction is the Maillard reaction. This reaction is responsible for the production of the flavor when foods are cooked. Examples of foods that undergo Maillard reaction include breads, steaks, and potatoes. It is a chemical reaction that takes place between the amine group of a free amino acid and the carbonyl group of a reducing sugar, since the type of amino acid involved determines the resulting flavor.

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.

Browning of grapes during winemaking

Like most fruit, grapes vary in the number of phenolic compounds they have. This characteristic is used as a parameter in judging the quality of the wine.