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 2 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 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. Generally, it is a chemical reaction involving polyphenol oxidase, catechol oxidase, and other enzymes that create melanins and benzoquinone from natural phenols. Enzymatic browning (also called oxidation of foods) requires exposure to oxygen. It begins with the oxidation of phenols by polyphenol oxidase into quinones, whose strong electrophilic state causes high susceptibility to a nucleophilic attack from other proteins. These quinones are then polymerized in a series of reactions, eventually resulting in the formation of brown pigments (melanosis) on the surface of the food. The rate of enzymatic browning is reflected by the amount of active polyphenol oxidases present in the food. Hence most research investigating methods to inhibit enzymatic browning has focused on hindering polyphenol oxidase activity. However, not all browning of food produces negative effects.
Examples of beneficial enzymatic browning:
- Developing color and flavor in Coffee, Cocoa beans, and tea.
- 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.
- Polyphenols oxidases is the major reaction in the formation of Melanosis in crustaceans such as shrimp.
Control of enzymatic browning
A variety of methods are used to prevent or slow down enzymatic browning of foods, each method aimed at targeting specific steps of the chemical reaction. The control of enzymatic browning has always been a challenge for the food industry. In addition, the use of chemicals to inhibit browning, such as sulfite (a powerful antibrowning chemical) have been reconsidered due to the potential hazards[clarification needed] it causes along with its activity. Much research has been conducted regarding the exact types of control mechanisms that take place when confronted with these enzymatic process. The different types of enzymatic browning control can be classified into different groups.
- Lemon juice and other acids lower the pH and remove the copper cofactor necessary for the responsible enzymes to function
- Blanching or roasting of foods, to denature the enzymes, and to destroy the responsible reactants, as used in the "kill green" phase of tea processing.
- Low temperatures can also prevent enzymatic browning by reducing rate of reaction.
- Use of ascorbic acid in certain pH's to control browning of apples under certain conditions by changing their phenolase activity. Different pH values affect phenolase activity of apples differently.
- Proteins can exert an inhibitory effect on PPO activity by Chelating the essential copper at the active site of PPO through competitive inhibition, inhibiting its activity 
- During wine synthesis, the use of Ion-exchange filtration is used to remove the brown color sediments in the solution.
- Arctic Apples have been genetically modified to silence the expression of polyphenol oxidase, thereby delaying a browning effect, and improving apple eating quality.
The second type of browning, nonenzymatic 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.
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, usually with the addition of heat. The sugar interacts with the amino acid, producing a variety of odors and flavors. The Maillard reaction is the basis for producing artificial flavors for processed foods in the flavoring industry, 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. The general process of winemaking is initiated by the enzymatic oxidation of phenolic compounds by polyphenol oxidases. Contact between the phenolic compounds in the Vacuole of the grape cell and the Polyphenol oxidase Enzyme (located in the cytoplasm) triggers the oxidation of the grape. Thus, the initial browning of grapes occurs as a result of "compartmentalization modification" in the cells of the grape.
Implications in food industry and technology
Enzymatic browning affects the color, flavor, and nutritional value of foods, causing huge economic loss when not sold to consumers on time. It is estimated that more than 50% of produce is lost as a result of enzymatic browning. The increase in human population and consequential depletion in our natural resources has prompted many biochemists and Food engineers alike to find new and improved techniques to preserve food longer, by using methods to inhibit the browning reaction, and effectively increase the shelf life of foods. A better understanding of the enzymatic browning mechanisms, specifically, understanding the properties of the enzymes and substrates that are involved in the reaction, may help food technologists to control certain stages in the mechanism and inhibit browning.
Apples are fruits commonly studied by researchers due to their high phenolic content, which make them highly susceptible to enzymatic browning. In accordance with other findings regarding apples and browning activity, a correlation has been found between high phenolic amount and enzymatic activity of apples. This provides a hope for food industries in an effort to genetically modify foods to decrease polyphenol oxidase activity and thus decrease browning. An example of such accomplishments in food engineering is in the production of Arctic Apples. These apples, engineered by Okanagan Specialty Fruits Inc, are a result of gene splicing, a technique that has allowed for the reduction in polyphenol oxidase.
Another type of issue that is closely studied is the browning of seafood. Seafood, in particular shrimp, is a delicacy consumed by people all over the world. The browning of shrimp which is actually referred to as Melanosis, creates a great concern for food handlers and consumers. Melanosis mainly occurs during postmortem handling and refrigerated storage. Recent studies have found a plant extract that acts as an anti melatonin polyphenol oxidase inhibitor and serves the same function as sulfites but without the health risks.
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