Browning is the process of food turning
brown
Brown is a color. It can be considered a composite color, but it is mainly a darker shade of orange. In the CMYK color model used in printing or painting, brown is usually made by combining the colors orange and black. In the RGB color model used ...
due to the
chemical reactions
A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the positions of electrons in the forming and breaking ...
that take place within. The process of browning is one of the chemical reactions that take place in
food chemistry
Food chemistry is the study of chemical processes and interactions of all biological and non-biological components of foods. The biological substances include such items as meat, poultry, lettuce, beer, milk as examples. It is similar to biochemist ...
and represents an interesting research topic regarding health,
nutrition
Nutrition is the biochemical and physiological process by which an organism uses food to support its life. It provides organisms with nutrients, which can be metabolized to create energy and chemical structures. Failure to obtain sufficient n ...
, and
food technology
Food technology is a branch of food science that deals with the production, preservation, quality control and research and development of the food products.
Early scientific research into food technology concentrated on food preservation. Nic ...
. Though there are many different ways food chemically changes over time, browning in particular falls into two main categories:
enzymatic
Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products. A ...
versus non-enzymatic browning processes.
Browning has many important implications on the
food industry
The food industry is a complex, global network of diverse businesses that supplies most of the food consumed by the world's population. The food industry today has become highly diversified, with manufacturing ranging from small, traditiona ...
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
Shelf life is the length of time that a commodity may be stored without becoming unfit for use, consumption, or sale. In other words, it might refer to whether a commodity should no longer be on a pantry shelf (unfit for use), or no longer on a ...
of food.
Enzymatic browning
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
Polyphenol oxidase (PPO; also polyphenol oxidase i, chloroplastic), an enzyme involved in fruit browning, is a tetramer that contains four atoms of copper per molecule.
PPO may accept monophenols and/or ''o''-diphenols as substrates. The ...
(PPO),
catechol oxidase
Catechol oxidase is a copper oxidase that contains a type 3 di-copper cofactor and catalyzes the oxidation of ortho-diphenols into ortho-quinones coupled with the reducing agent, reduction of molecular oxygen to water. It is present in a variety ...
, and other
enzyme
Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products. A ...
s that create
melanin
Melanin (; from el, μέλας, melas, black, dark) is a broad term for a group of natural pigments found in most organisms. Eumelanin is produced through a multistage chemical process known as melanogenesis, where the oxidation of the amino ...
s and
benzoquinone Benzoquinone (C6H4O2) is a quinone with a single benzene ring. There are 2 (out of 3 hypothetical) benzoquinones:
* 1,4-Benzoquinone, most commonly, right image (also ''para''-benzoquinone, ''p''-benzoquinone, ''para''-quinone, or just quinone)
* 1 ...
from
natural phenol
In biochemistry, naturally occurring phenols are natural products containing at least one phenol functional group. Phenolic compounds are produced by plants and microorganisms. Organisms sometimes synthesize phenolic compounds in response to eco ...
s. Enzymatic browning (also called oxidation of foods) requires exposure to
oxygen. It begins with the oxidation of
phenols by
polyphenol oxidase
Polyphenol oxidase (PPO; also polyphenol oxidase i, chloroplastic), an enzyme involved in fruit browning, is a tetramer that contains four atoms of copper per molecule.
PPO may accept monophenols and/or ''o''-diphenols as substrates. The ...
into
quinone
The quinones are a class of organic compounds that are formally "derived from aromatic compounds uch as benzene or naphthalene
Uch ( pa, ;
ur, ), frequently referred to as Uch Sharīf ( pa, ;
ur, ; ''"Noble Uch"''), is a historic city in the southern part of Pakistan's Punjab province. Uch may have been founded as Alexandria on the Indus, a town founded by Alexand ...
by conversion of an even number of –CH= groups into –C(=O)– groups with any necessary rearrangement of double ...
s,
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
Melanosis is a form of hyperpigmentation associated with increased melanin.
It can also refer to:
* Melanism
* Ocular melanosis
* Smoker's melanosis
* Oral melanosis
* Riehl melanosis
See also
* List of cutaneous conditions
Many skin cond ...
) 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 into methods of preventing enzymatic browning has been directed towards inhibiting 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
fig
The fig is the edible fruit of ''Ficus carica'', a species of small tree in the flowering plant family Moraceae. Native to the Mediterranean and western Asia, it has been cultivated since ancient times and is now widely grown throughout the world ...
s and
raisins.
Examples of non-beneficial enzymatic browning:
* Fresh fruit and vegetables, including
apples,
potatoes,
banana
A banana is an elongated, edible fruit – botanically a berry – produced by several kinds of large herbaceous flowering plants in the genus ''Musa''. In some countries, bananas used for cooking may be called "plantains", distinguis ...
s and
avocados.
* Oxidation of polyphenols is the major cause of melanosis in crustaceans such as shrimp.
Control of enzymatic browning
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
Sulfites or sulphites are compounds that contain the sulfite ion (or the sulfate(IV) ion, from its correct systematic name), . The sulfite ion is the conjugate base of bisulfite. Although its acid (sulfurous acid) is elusive, its salts are widel ...
(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
Roasting is a cooking method that uses dry heat where hot air covers the food, cooking it evenly on all sides with temperatures of at least from an open flame, oven, or other heat source. Roasting can enhance the flavor through caramelization ...
, de-naturates enzymes and destroys the reactants responsible for browning. Blanching is used, for example, in
winemaking,
tea processing, storing
nuts
Nut often refers to:
* Nut (fruit), fruit composed of a hard shell and a seed, or a collective noun for dry and edible fruits or seeds
* Nut (hardware), fastener used with a bolt
Nut or Nuts may also refer to:
Arts, entertainment, and media Com ...
and
bacon
Bacon is a type of salt-cured pork made from various cuts, typically the belly or less fatty parts of the back. It is eaten as a side dish (particularly in breakfasts), used as a central ingredient (e.g., the bacon, lettuce, and tomato sand ...
, 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.,
N2 or
CO2) during preservation, such as in vacuum-packaging or
modified atmosphere packaging
Modified atmosphere packaging (MAP) is the practice of modifying the composition of the internal atmosphere of a package (commonly food packages, drugs, etc.) in order to improve the shelf life. The need for this technology for food arises from t ...
,
wine or juice bottling, 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
antioxidant
Antioxidants are compounds that inhibit oxidation, a chemical reaction that can produce free radicals. This can lead to polymerization and other chain reactions. They are frequently added to industrial products, such as fuels and lubricant ...
s,
antimicrobial, and
antifungal
An antifungal medication, also known as an antimycotic medication, is a pharmaceutical fungicide or fungistatic used to treat and prevent mycosis such as athlete's foot, ringworm, candidiasis (thrush), serious systemic infections such as crypto ...
substances, such as
butylated hydroxytoluene
Butylated hydroxytoluene (BHT), also known as dibutylhydroxytoluene, is a lipophilic organic compound, chemically a derivative of phenol, that is useful for its antioxidant properties. BHT is widely used to prevent free radical-mediated oxidatio ...
(BHT) and
butylated hydroxyanisole
Butylated hydroxyanisole (BHA) is a synthetic, waxy, solid petrochemical. Its antioxidant properties have caused it to be widely used as a preservative in food, food packaging, animal feed, cosmetics, pharmaceuticals, rubber, and petroleum produc ...
(BHA),
tocopherols,
hinokitiol,
lysozyme,
nisin,
natamycin,
chitosan, and
ε-polylysine.
Edible coatings can be made of
polysaccharide
Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrates found in food. They are long chain polymeric carbohydrates composed of monosaccharide units bound together by glycosidic linkages. This carbohydrate can react with wa ...
s,
proteins,
lipids, vegetable
skins, plants or other natural products.
*Irradiation −
Food irradiation
Food irradiation is the process of exposing food and food packaging to ionizing radiation, such as from gamma rays, x-rays, or electron beams. Food irradiation improves food safety and extends product shelf life (preservation) by effectively ...
using
UV-C,
gamma rays,
x-rays, and
electron beam
Cathode rays or electron beam (e-beam) are streams of electrons observed in discharge tubes. If an evacuated glass tube is equipped with two electrodes and a voltage is applied, glass behind the positive electrode is observed to glow, due to ele ...
s is another method to extend the food
shelf life
Shelf life is the length of time that a commodity may be stored without becoming unfit for use, consumption, or sale. In other words, it might refer to whether a commodity should no longer be on a pantry shelf (unfit for use), or no longer on a ...
.
Ionizing radiation
Ionizing radiation (or ionising radiation), including nuclear radiation, consists of subatomic particles or electromagnetic waves that have sufficient energy to ionize atoms or molecules by detaching electrons from them. Some particles can travel ...
inhibits the vitality of microorganisms responsible for
food spoilage and delays the maturation and
sprouting
Sprouting is the natural process by which seeds or spores germinate and put out shoots, and already established plants produce new leaves or buds, or other structures experience further growth.
In the field of nutrition, the term signifies ...
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.
Acidifying agents and
acidity regulator
Acidity regulators, or pH control agents, are food additives used to change or maintain pH (acidity or basicity). They can be organic or mineral acids, bases, neutralizing agents, or buffering agents. Typical agents include the following acids ...
s are widely used as
food additive
Food additives are substances added to food to preserve flavor or enhance taste, appearance, or other sensory qualities. Some additives have been used for centuries as part of an effort to preserve food, for example vinegar (pickling), salt (salt ...
s to maintain a desired pH in food products.
Acidulant Acidulants are chemical compounds that give a tart, sour, or acidic flavor to foods or enhance the perceived sweetness of foods. Acidulants can also function as leavening agents and emulsifiers in some kinds of processed foods.Berry, S.K.. (2001). R ...
s, such as
citric acid,
ascorbic acid
Vitamin C (also known as ascorbic acid and ascorbate) is a water-soluble vitamin found in citrus and other fruits and vegetables, also sold as a dietary supplement and as a topical 'serum' ingredient to treat melasma (dark pigment spots) an ...
, and
glutathione, are used as anti-browning agents. Many of these agents also show other anti-browning effects, such as chelating and antioxidant activities.
*Antioxidants − Many
antioxidant
Antioxidants are compounds that inhibit oxidation, a chemical reaction that can produce free radicals. This can lead to polymerization and other chain reactions. They are frequently added to industrial products, such as fuels and lubricant ...
s are used in food industry as food additives. These compounds react with oxygen and suppress the initiation of the browning process.
Also, they interfere with intermediate products of the following reactions and inhibit
melanin
Melanin (; from el, μέλας, melas, black, dark) is a broad term for a group of natural pigments found in most organisms. Eumelanin is produced through a multistage chemical process known as melanogenesis, where the oxidation of the amino ...
formation.
Ascorbic acid
Vitamin C (also known as ascorbic acid and ascorbate) is a water-soluble vitamin found in citrus and other fruits and vegetables, also sold as a dietary supplement and as a topical 'serum' ingredient to treat melasma (dark pigment spots) an ...
,
N-acetylcysteine,
L-cysteine,
4-hexylresorcinol,
erythorbic acid
Erythorbic acid (isoascorbic acid, D-araboascorbic acid) is a stereoisomer of ascorbic acid (vitamin C). It is synthesized by a reaction between methyl 2-keto-D-gluconate and sodium methoxide. It can also be synthesized from sucrose or by strains ...
,
cysteine hydrochloride
Cysteine (symbol Cys or C; ) is a semiessential proteinogenic amino acid with the formula . The thiol side chain in cysteine often participates in enzymatic reactions as a nucleophile.
When present as a deprotonated catalytic residue, sometime ...
,
glutathione are examples of antioxidants that have been studied for their anti-browning properties.
*Chelating agents − Polyphenol oxidase requires
copper as a cofactor for its functionality, thus copper-
chelating agent
Chelation is a type of bonding of ions and molecules to metal ions. It involves the formation or presence of two or more separate coordinate bonds between a polydentate (multiple bonded) ligand and a single central metal atom. These ligands are ...
s inhibit the activity of this enzyme. Many agents possessing chelating activity have been studied and used in different fields of food industry, such as
citric acid,
sorbic acid,
polyphosphates,
hinokitiol,
kojic acid,
EDTA,
porphyrins, polycarboxylic acids, different proteins.
Some of these compounds also have other anti-browning effects, such as acidifying or antioxidant. Hinokitiol is used in coating materials for
food packaging.
Other methods
*Natural agents − Different natural products and their extracts, such as
onion,
pineapple,
lemon, and
white wine, are known to inhibit or slow the browning of some products.
Onion and its extract exhibit potent anti-browning properties by inhibiting the PPO activity. Pineapple juice have shown to possess anti-browning effect on apples and bananas. Lemon juice is used in making doughs to make the
pastry products look brighter. This effect is possibly explained by the anti-browning properties of
citric and
ascorbic acid
Vitamin C (also known as ascorbic acid and ascorbate) is a water-soluble vitamin found in citrus and other fruits and vegetables, also sold as a dietary supplement and as a topical 'serum' ingredient to treat melasma (dark pigment spots) an ...
s in the lemon juice.
*Genetic modification −
Arctic apples
Arctic apple is the trademark for a group of patented apples that contain a nonbrowning trait (when the apples are subjected to mechanical damage, such as slicing or bruising, the apple flesh remains as its original color) introduced through biote ...
have been genetically modified to silence the
expression of
PPO, thereby delaying the browning effect, and improving apple eating quality.
Non-enzymatic browning
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
Sugar is the generic name for sweet-tasting, soluble carbohydrates, many of which are used in food. Simple sugars, also called monosaccharides, include glucose, fructose, and galactose. Compound sugars, also called disaccharides or double ...
. 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
Caramel ( or ) is an orange-brown confectionery product made by heating a range of sugars. It can be used as a flavoring in puddings and desserts, as a filling in bonbons, or as a topping for ice cream and custard.
The process of caramelizatio ...
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
Polyphenol oxidase (PPO; also polyphenol oxidase i, chloroplastic), an enzyme involved in fruit browning, is a tetramer that contains four atoms of copper per molecule.
PPO may accept monophenols and/or ''o''-diphenols as substrates. The ...
enzyme
Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products. A ...
(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 natural resources has prompted many biochemists and
food engineers alike to find new or improved techniques to preserve food and for longer by using methods to inhibit the browning reaction. This effectively increases the shelf life of foods, solving this part of the waste problem. 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 ultimately apply that knowledge to 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 higher phenolic quantities and increased enzymatic activity in apples.
This provides a potential target and thus hope for food industries wishing 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
Arctic apple is the trademark for a group of patented apples that contain a nonbrowning trait (when the apples are subjected to mechanical damage, such as slicing or bruising, the apple flesh remains as its original color) introduced through biote ...
. These apples, engineered by ''Okanagan Specialty Fruits Inc,'' are a result of applying
gene splicing
Recombinant DNA (rDNA) molecules are DNA molecules formed by laboratory methods of genetic recombination (such as molecular cloning) that bring together genetic material from multiple sources, creating sequences that would not otherwise be foun ...
, a laboratory technique that has allowed for the reduction in polyphenol oxidase.
Another type of issue that is closely studied is the browning of seafood.
Seafood
Seafood is any form of sea life regarded as food by humans, prominently including fish and shellfish. Shellfish include various species of molluscs (e.g. bivalve molluscs such as clams, oysters and mussels, and cephalopods such as octopus an ...
, in particular shrimp, is a staple consumed by people all over the world. The browning of shrimp, which is actually referred to as
melanosis
Melanosis is a form of hyperpigmentation associated with increased melanin.
It can also refer to:
* Melanism
* Ocular melanosis
* Smoker's melanosis
* Oral melanosis
* Riehl melanosis
See also
* List of cutaneous conditions
Many skin cond ...
, 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 serves the same function as sulfites but without the health risks.
See also
*
Browning (partial cooking)
*
Decomposition
*
Gravy
*
Water activity
References
{{Food preservation
Food science
Biochemistry
pl:Ciemnienie nieenzymatyczne