What are Antioxidants?

All living organisms utilize oxygen to metabolize and use the dietary nutrients in order to produce energy for survival. Oxygen thus is a vital component for living. Oxygen meditates chemical reactions that metabolize fats, proteins, and carbohydrates to produce energy.

Dark side of oxygen

While oxygen is one of the most essential components for living, it is also a double edged sword. Oxygen is a highly reactive atom that is capable of becoming part of potentially damaging molecules commonly called “free radicals.”

Free radicals

These free radicals are capable of attacking the healthy cells of the body. This may lead to damage, disease and severe disorders. Cell damage caused by free radicals appears to be a major contributor to aging and diseases like:

  • cancer
  • heart disease
  • decline in brain function
  • decline in immune system etc.

Overall, free radicals have been implicated in the pathogenesis of at least 50 diseases.

Since free radicals contain an unpaired electron they are unstable and reach out and capture electrons from other substances in order to neutralize themselves. This initially stabilizes the free radical but generates another in the process. Soon a chain reaction begins and thousands of free radical reactions can occur within a few seconds on the primary reaction.

Reactive oxygen species (ROS)

ROS is a term which encompasses all highly reactive, oxygen-containing molecules, including free radicals. Types of ROS include the hydroxyl radical, hydrogen peroxide, the superoxide anion radical, nitric oxide radical, singlet oxygen, hypochlorite radical, and various lipid peroxides. These can react with membrane lipids, nucleic acids, proteins and enzymes, and other small molecules.

Oxidative stress

Oxidative stress means an unbalance between pro-oxidants and antioxidant mechanisms. This results in excessive oxidative metabolism. This stress can be due to several environmental factors such as exposure to pollutants, alcohol, medications, infections, poor diet, toxins, radiation etc. Oxidative damage to DNA, proteins, and other macromolecules may lead to a wide range of human diseases most notably heart disease and cancer.

Control of free radicals

Normally free radical formation is controlled naturally by various beneficial compounds known as antioxidants. When there is deficiency of these antioxidants damage due to free radicals can become cumulative and debilitating.

Antioxidants are capable of stabilizing, or deactivating, free radicals before they attack cells.

Antioxidants from food

There are several nutrients in food that contain antioxidants. Vitamin C, vitamin E, and beta carotene are among the most commonly studied dietary antioxidants.

Vitamin C is the most important water-soluble antioxidant in extracellular fluids. Vitamin C helps to neutralize ROS in the water or aqueous phase before it can attack the lipids.

Vitamin E is the most important lipid soluble antioxidant. It is important as the chain-breaking antioxidant within the cell membrane. It can protect the membrane fatty acids from lipid peroxidation. Vitamin C in addition is capable of regenerating vitamin E.

Beta carotene and other carotenoids also have antioxidant properties. Carotenoids work in synergy with vitamin E.

Antioxidant deficiencies

A diet low in fats may impair absorption of beta carotene and vitamin E and other fat-soluble nutrients. Fruits and vegetables are important sources of vitamin C and carotenoids. Whole grains and high quality vegetable oils are major sources of vitamin E.

Many plant-derived substances are known as “phytonutrients,” or “phytochemicals”. These also possess antioxidant properties. Phenolic compounds such as flavonoids are such chemicals. These are found in several fruits, vegetables, green tea extracts etc.

Antioxidants within the human body

Apart from diet, the body also has several antioxidant mechanisms that can protect itself from ROS mediated damage. The antioxidant enzymes – glutathione peroxidase, catalase, and superoxide dismutase (SOD) are such enzymes. They require micronutrient cofactors such as selenium, iron, copper, zinc, and manganese for their activity. It has been suggested that an inadequate dietary intake of these trace minerals may also lead to low antioxidant activity.

Antioxidant: The Oxidative Challenge In Biology

Oxygen is the vital requirement for living beings. This is required for the complex metabolic pathways. The paradox of oxygen requirement is the high reactivity of the oxygen molecule. These reactive oxygen molecules damage living organisms by producing reactive oxygen species.

Reactive oxygen species (ROS)

The reactive oxygen species produced in cells include:

  • Hydrogen peroxide (H2O2)
  • Hypochlorous acid (HOCl) & hypochlorite radical
  • Free radicals such as the hydroxyl radical (·OH)
  • Superoxide anion (O2−)
  • nitric oxide radical
  • singlet oxygen
  • lipid peroxides

These can react with membrane lipids, nucleic acids, proteins and enzymes, and other small molecules.

Why are free radicals unstable?

These free radicals contain an unpaired electron. This makes them unstable and they seek out and capture electrons from other substances in order to neutralize themselves. This initially stabilizes the free radical but generates another in the process. Soon a chain reaction begins and thousands of free radical reactions can occur within a few seconds on the primary reaction.

The hydroxyl radical

The hydroxyl radical is particularly unstable and will react rapidly and non-specifically with most biological molecules. This species is produced from hydrogen peroxide in metal-catalyzed redox reactions such as the Fenton reaction. These reactions lead to lipid peroxidation and oxidizing DNA or proteins to mediate cell damage.

The superoxide anion

The superoxide anion is produced as a by-product of several steps in the electron transport chain. There is the reduction of coenzyme Q in complex III. This forms a highly reactive free radical as an intermediate called the (Q·−). This intermediate leads to electron "leakage", when electrons jump directly to oxygen and form the superoxide anion. Peroxide is also produced from the oxidation of reduced flavoproteins.

Free radicals in plants

In plants, as well as algae, reactive oxygen species are also produced during photosynthesis, particularly under conditions of high light intensity. This is prevented by carotenoids in photoinhibition. These antioxidants react with over-reduced forms of the photosynthetic reaction centres to prevent the production of ROS.


Each of the living organisms thus have a complex network of antioxidant metabolites and enzymes that act together to prevent oxidative damage to cellular components such as DNA, proteins and lipids. These systems of antioxidants prevent these reactive species from being formed or remove them before they can damage vital components of the cell.

Oxidative stress, imbalance and disease

Despite an effective antioxidant network there are several diseases that can result from an imbalance between pro-oxidants and antioxidants. The term “oxidative stress” has been coined to represent a shift towards the pro-oxidants. This stress can be due to several environmental factors such as exposure to pollutants, alcohol, medications, infections, poor diet, toxins, radiation etc.

Oxidative damage to DNA, proteins, and other macromolecules may lead to a wide range of human diseases most notably heart disease, lung diseases like asthma, and cancer.

Antioxidant Metabolites

The antioxidant networks in the body are complex and are composed of several components. These may be endogenous factors such as Glutathione, thiols, haem proteins, Coenzymes Q, bilirubin and urates. These may also be endogenous enzymes like GSH reductase, GSH transferase, GSH peroxidises, Superoxide dismutase and Catalases.

Dietary factors

Some nutritional and dietary factors also function as antioxidant metabolites or parts of the antioxidant metabolic pathways. These include Ascorbic acid or vitamin C, Tocopherols or vitamin E, beta carotenes and retinoids, Selenium, Methionine etc.

Antioxidant metabolite Solubility Concentration in human serum (μM) Concentration in liver tissue (μmol/kg)
Ascorbic acid (vitamin C) Water 50 – 60 260 (human)
Glutathione Water 4 6,400 (human)
Uric acid Water 200 – 400 1,600 (human)retinol (vitamin A): 1 – 3
α-Tocopherol (vitamin E) Lipid 10 – 40 200 (human)

Classification of antioxidant metabolites

Antioxidant metabolites are further classified as soluble in water (hydrophilic) or in lipids (hydrophobic). Water-soluble antioxidants react with oxidants in the cell cytosol and the blood plasma. On the other hand lipid-soluble antioxidants protect cell membranes from lipid peroxidation.

The actions of each of these metabolites are dependent on each other as the metabolic pathways are complex. Selenium and zinc are commonly referred to as ''antioxidant nutrients''. These alone do not have antioxidant properties but are required for the activity of some antioxidant enzymes.

Vitamin C or Ascorbic acid

This is a monosaccharide antioxidant found in both animals and plants. This is one of the essential nutrients for living organisms like humans.  It must be obtained from the diet of humans and is a vitamin. Most other animals are able to produce this compound in their bodies and do not require it in their diets.

The vitamin is maintained in its reduced form by reaction with glutathione within the cell. It can be catalysed by protein disulfide isomerase and glutaredoxins.

Since it exists as a reduced agent, it can neutralize reactive oxygen species such as hydrogen peroxide. Ascorbic acid also is a substrate for the antioxidant enzyme ascorbate peroxidise. This is important for preventing oxidative stress particularly in plants.

Vitamin E or tocopherols

Vitamin E includes around eight related tocopherols and tocotrienols. These are fat-soluble vitamins with antioxidant properties.

Of these, alpha tocopherol is the most studied component as it has the highest bioavailability. The body absorbs this vitamin along with fats. It has been claimed that the α-tocopherol form is the most important lipid-soluble antioxidant. This vitamin protects membranes from oxidation by reacting with lipid radicals produced in the lipid peroxidation chain reaction. The reaction removes free radical intermediates and prevents the propagation reaction.

Once completed the oxidised α-tocopheroxyl radicals can be recycled back to the active reduced form through reduction by other antioxidants, such as ascorbate, retinol or ubiquinol. This α-tocopherol protects glutathione peroxidase 4 (GPX4)-deficient cells from cell death.


This is an endogenous antioxidant factor. Glutathione contains cysteine and is a peptide found in most forms of aerobic life. It is not required in the diet and is instead synthesized in cells from its constituent amino acids.

Glutathione contains a thiol group in its cysteine moiety which is a reducing agent and can be reversibly oxidized and reduced. In cells, glutathione is maintained in the reduced form by the enzyme glutathione reductase. This reduced glutathione reduces other metabolites and enzyme systems, such as ascorbate in the glutathione-ascorbate cycle, glutathione peroxidases and glutaredoxins.

Antioxidant: Pro-Oxidant Activities

Antioxidants that reduce the pro-oxidant or oxidative stress causing action of various reactive oxygen species and free radicals also have a dark side.

For example, vitamin C or ascorbic acid that acts as an antioxidant when it reduces oxidizing substances such as hydrogen peroxide also reduces metal ions that generate free radicals through the Fenton reaction.

The relative importance of the antioxidant and pro-oxidant activities of antioxidants are of importance. Vitamin C of course has more antioxidant activity than pro-oxidant activity.

The Fenton reaction

The Fenton reaction is as follows:

2 Fe3+ + Ascorbate → 2 Fe2+ + Dehydroascorbate

2 Fe2+ + 2 H2O2 → 2 Fe3+ + 2 OH· + 2 OH

When antioxidant enzymes deviate from physiological antioxidant activity they may have a dramatic effect on the resistance of cells to oxidant-induced damage to DNA and cell killing. The Fenton reaction is responsible for DNA damage produced under oxidative stress.

Beneficial and detrimental effects of antioxidants

The development of a beneficial or a detrimental cellular response by a nutrient depends on the nutrient's antioxidant or pro-oxidant characteristics. This depends on the product or the cellular oxygen environment.

Nutrients such as carotenoids, tocopherols or ascorbate derivatives will demonstrate an antioxidant or pro-oxidant characteristic depending on the redox potential of the individual molecule and the inorganic chemistry of the cell.

Most antioxidant nutrients that act as chemopreventives prevent excessive cell growth. When an inappropriate pro-oxidant activity develops in normal cells, the reactive oxygen metabolites generated could damage the DNA and cellular membranes. Thus the labile redox character of each nutrient must be considered in terms of the extracellular and intracellular microoxygen environment.

Several animal and human studies have shown the beneficial and detrimental effects of dietary antioxidants.

Antioxidant Enzyme Systems

There are several enzyme systems that catalyze reactions to neutralize free radicals and reactive oxygen species. These enzymes include:

  • superoxide dismutase
  • glutathione peroxidise
  • glutathione reductase
  • catalases

These form the body’s endogenous defence mechanisms to help protect against free radical-induced cell damage. The antioxidant enzymes – glutathioneperoxidase, catalase, and superoxide dismutase (SOD) – metabolize oxidative toxic intermediates.

These enzymes also require co-factors such as selenium, iron, copper, zinc, and manganese for optimum catalytic activity. It has been suggested that an inadequate dietary intake of these trace minerals may compromise the effectiveness of these antioxidant defense mechanisms. The consumption and absorption of these important trace minerals may decrease with aging.

Glutathione enzymes and system

Glutathione, an important water-soluble antioxidant, is synthesized from the amino acids glycine, glutamate, and cysteine. Glutathione can directly neutralize ROS such as lipid peroxides, and also plays a major role in xenobiotic metabolism.

Xenobiotics are toxins that the body is exposed to. Exposure of the liver to xenobiotic substances means the body prepares itself by increasing detoxification enzymes, i.e., cytochrome P-450 mixed-function oxidase.

When an individual is exposed to high levels of xenobiotics, more glutathione is utilized for conjugation. Conjugation with Glutathioone renders the toxin neutral and makes it less available to serve as an antioxidant. Research suggests that glutathione and vitamin C work interactively to neutralize free radicals. These two also have a sparing effect upon each other.

The glutathione system includes glutathione, glutathione reductase, glutathione peroxidases and glutathione ''S''-transferases. Of these glutathione peroxidase is an enzyme containing four selenium-cofactors that catalyzes the breakdown of hydrogen peroxide and organic hydroperoxides. Glutathione ''S''-transferases show high activity with lipid peroxides. These enzymes are at particularly high levels in the liver.

Lipoic acid

This is another important endogenous antioxidant. It is categorized as “thiol” or “biothiol”. These are sulfur-containing molecules that catalyze the oxidative decarboxylation of alpha-keto acids, such as pyruvate and alphaketoglutarate, in the Krebs cycle.

Lipoic acid and its reduced form, dihydrolipoic acid (DHLA), neutralize the free radicals in both lipid and aqueous domains and as such has been called a “universal antioxidant.”

Superoxide dismutase

Superoxide dismutases (SODs) are a class of enzymes that catalyse the breakdown of the superoxide anion into oxygen and hydrogen peroxide. These enzymes are present in almost all aerobic cells and in extracellular fluids.

SODs contain metal ion cofactors that, depending on the isozyme, can be copper, zinc, manganese or iron. For example, in humans copper/zinc SOD is present in the cytosol, while manganese SOD is present in the mitochondrion. The mitochondrial SOD is most biologically important of these three.

In plants, SOD isozymes are present in the cytosol and mitochondria. There is also an iron SOD found in chloroplasts.


Catalases are enzymes that catalyse the conversion of hydrogen peroxide to water and oxygen, using either an iron or manganese cofactor. This is found in peroxisomes in most eukaryotic cells. Its only substrate is hydrogen peroxide. It follows a ping-pong mechanism.

Here, its cofactor is oxidised by one molecule of hydrogen peroxide and then regenerated by transferring the bound oxygen to a second molecule of substrate.


There are peroxidases that catalyze the reduction of hydrogen peroxide, organic hydroperoxides, as well as peroxynitrite. These may be of three basic types - typical 2-cysteine peroxiredoxins; atypical 2-cysteine peroxiredoxins; and 1-cysteine peroxiredoxins. Peroxiredoxins seem to be important in antioxidant metabolism.

Oxidative Stress In Disease

Reactive oxygen species or ROS and free radicals can cause severe damage to the normal cells of the body. This damage can be to the DNA, proteins, and other macromolecules. This damage forms the basis of a wide variety of diseases, most notably heart disease and cancer.

There are numerous studies that prove that since these diseases are mediated by oxidative stress and disbalance between pro-oxidant and antioxidant factors, antioxidants may play a pivotal role in preventing or slowing the progression of these conditions.

Some of the notable diseases caused due to oxidative stress include:

Heart Disease

Heart disease is the leading cause of death worldwide. Heart disease risk is raised by several factors including high cholesterol levels, high blood pressure, cigarette smoking, and diabetes. These promote atherosclerosis. Atherosclerosis refers to formation of hardened walls of the arteries that impairs blood flow to the heart and other vital organs.

It is speculated that a critical step in development of atherosclerosis is oxidation of low-density lipoprotein (LDL) (a type of bad cholesterol in blood) within the arterial wall. Several studies show an association between low intakes of dietary antioxidants to an increased frequency of heart disease.

On the other hand, those with high blood levels of antioxidants have lower risk of heart disease. For examples, humans who took more vitamin E on a regular basis had a 41% lower incidence of heart disease than those who took less amounts as seen in a study on nurses. Dietary increases in antioxidant vitamins may reduce the risk of heart disease by 20-30%.(1)


Cancer kills millions worldwide. Diet may be the cause for cancer in as much as 35% of all human cancers. Low antioxidant intake in diet may also be responsible. Low dietary intake of fruits and vegetables doubles the risk of most types of cancers.

Pro-oxidants, or those who generate free radicals, stimulate cell division and these form the beginnings of mutagenesis and tumor formation. When a cell with a damaged DNA strand divides, it gives rise to disturbed and deformed clusters of cells that form the cancer.

Antioxidants exert their protective effect by:

  • decreasing oxidative damage to DNA and
  • decreasing abnormal increases in cell division

In addition, cigarette smoking and chronic inflammation lead to strong free radical generation that seems to be the reason for many cancers. Some research has indicated that people who smoke tend to have lower antioxidant levels than non-smokers and this makes smokers more at risk of cancers.

Lung diseases

The respiratory system is a well known target for free radical insult. This comes from endogenous factors as well as exposure to air pollutants and toxins, cigarette smoke etc.

Recent studies suggest that free radicals may be involved in the development of pulmonary disorders such as asthma. Antioxidants have been seen to reduce the development of asthmatic symptoms. Vitamin C, vitamin E, and beta carotene supplementation has been associated with improved lung function.

Neural disorders

Free radicals can also damage nerves and the brain. Neural tissue may be particularly susceptible to oxidative damage. This is because the brain receives a disproportionately large percentage of oxygen and has large amounts of polyunsaturated fatty acids which are highly prone to oxidation and oxidative damage.

Diseases implicated to oxidative stress include:

  • Alzheimer's disease
  • Parkinson's disease
  • dementia etc.


Formation of cataracts is believed to involve damage to lens protein by free radicals. This leads to opacity of the lens. Cataract formation may be slowed with the regular consumption of supplemental antioxidants like vitamin E, vitamin C, and the carotenoids.

Other diseases

Other diseases like Diabetes, Rheumatoid arthritis etc. are also associated with low antioxidant levels in blood.

Reviewed by April Cashin-Garbutt, BA Hons (Cantab)




Antioxidant Health Effects

Antioxidants, like oxidative injury causing pro-oxidants, have a profound role in health and diseases in humans. Some of the major beneficial roles include those in disease prevention and treatment.

Disease prevention by antioxidants

There are several antioxidant systems within the body that help cope with the oxidative stress that results from regular metabolic processes. Antioxidants in diet can also cancel out the cell-damaging effects of free radicals. These antioxidant supplements act in addition to the endogenous systems and their lack can cause several ill-consequences of oxidative stress.

There is evidence that some types of vegetables and fruits protect against a number of cancers and other diseases. Large studies have shown that people who took regular antioxidants in fruits and vegetables seemed to have lesser incidence of these diseases. In addition, those who took fewer amounts of antioxidants, or had excessive exposure to pro-oxidants like cigarette smoking etc., had a higher risk of these disorders.

For example, oxidation of low density lipoprotein (LDL) in the blood contributes to heart disease. Those taking Vitamin E supplements had a lower risk of developing heart disease.

The exact amounts of antioxidant supplement and their exact preventive role, however, could not be determined. This meant that some people did get cancers and other oxidative stress related disorders despite adequate fruits and vegetables and antioxidant consumption.

In prevention of heart disease, for example, seven large clinical trials were conducted to test the effects of antioxidant supplement with Vitamin E, in doses ranging from 50 to per day. None of these trials found a statistically significant effect of Vitamin E on overall number of deaths or on deaths due to heart disease.

This said, there are several essential vitamins, minerals and antioxidants that include resveratrol (from grape seeds or knotweed roots), beta carotene (provitamin A), vitamin C, vitamin E and Selenium, or herbs that contain antioxidants - such as green tea and jiaogulan.

Disease treatment with antioxidants

Several vital organs like the heart, lungs and the brain are vulnerable to oxidative injury. Brain in particular is vulnerable because of its high content of oxygen, high metabolic rate and elevated levels of polyunsaturated lipids - the target of lipid peroxidation.

Several antioxidant supplements are available to treat neural injury with oxidative stress. Brain injury may result in damage to parts of the brain after a stroke, Alzheimer’s disease, Parkinson’s disease and other neurodegenerative disorders. After a stroke for example the brain undergoes reperfusion injury that is mediated by oxidative stress.

Superoxide dismutase mimetics, sodium thiopental and propofol are used to treat reperfusion injury and traumatic brain injury, while the experimental drug NXY-059 and ebselen are being applied in the treatment of stroke. These compounds appear to prevent oxidative stress in neurons. They help in preventing neural cell death.

Antioxidants are also being investigated as possible treatments for neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis.

Detrimental effects of antioxidant supplements

Some of the antioxidants when taken in excess in diet may cause more harm than good. For example, when a person takes in excessive amounts of strong reducing agents as antioxidants, he or she may develop deficiency of several minerals like iron and zinc. The absorption of these minerals is prevented from the gastrointestinal tract.

Notable examples are oxalic acid, tannins and phytic acid, which are high in plant-based diets. In addition, there may be Calcium and iron deficiencies in persons who take too much phytic acid from beans, legumes, maize and unleavened whole grain bread. Similarly oxalic acid is present in cocoa, chocolate, spinach, turnip and rhubarb and tannins are present in cabbage, tea and beans. Excess of these in diet may prevent mineral absorption.

Eugenol, an antioxidant present in oil of cloves, also possesses toxic effects in high levels.

Toxicity associated with high doses of water-soluble antioxidants such as ascorbic acid are less of a concern since these can be excreted rapidly in urine. Very high doses of some lipid soluble antioxidants may have harmful long-term effects.



Antioxidants in Food

Antioxidants are present in large amounts in several foods. However, actual amount of antioxidants in several plant products may differ due to several factors. These include:

  • soil type and chemistry
  • available nitrogen and levels of other plant nutrients
  • moisture levels
  • temperature
  • pests

It has been seen that plants which are exposed to stress are driven to synthesize antioxidants and are richer in these polyphenols and flavonoids.

Phenolic antioxidants are present in plants at concentrations up to several grams per kilogram. In general, levels are higher in the rinds and skins of the fruits rather than within them. A number of chemical assays have been developed to measure different antioxidants. In vitro assays are designed to test antioxidant levels in foods, while other assays measure levels in blood, urine or blood cells.

Antioxidant food sources

Some food sources of antioxidants include:


Antioxidant Nutrients

Acorn squash, pumpkin, winter squash

Beta carotene



Apricots, cantaloupe, peaches

Beta carotene


Catechins, vitamin E



Bell peppers

Beta carotene, vitamin C


Anthocyanins, catechins, ellagic acid (in raspberries and strawberries), resveratrol (in blueberries),vitamin C

Broccoli, greens, spinach

Beta carotene, lutein, vitamin C

Brown rice



Beta carotene



Citrus fruits

Vitamin C




Lutein (in yolks); selenium, vitamin A



Garlic and onions


Grapefruit, pink

Lycopene, vitamin C

Grapes, red wine

Anthocyanins (in red and purple grapes), resveratrol

Mango and papaya

Beta carotene, vitamin C


Vitamin A

Nuts, nut butters, oils, seeds

Vitamin E







Salmon, tuna, seafood


Sweet potatoes

Beta carotene, vitamin C

Tea, black or green


Tomatoes (canned)

Lycopene, vitamin C


Lycopene, vitamin C

Wheat germ, whole grains

Selenium, vitamin E

In addition to the above:

  • Linseed is the major source of lignans in the diet. Lignans are also found in other cereals, legumes and vegetables
  • Stilbenes are found in only low quantities in the human diet. Resveratrol is the most widely studied stilbene and is found in grape seeds, peanuts or knotweed roots. These stilbenes have cancer protective effects.
  • Flavonoids are divided into six classes including flavonols, flavones, isoflavones, Flavonols (catechins and proanthocyanidins), Flavanones and Anthocyanidins. Of these Flavonols are the most common flavonoids in foods.


The richest sources of flavonols are onions, kale, leeks, broccoli, and blueberries. Flavonol concentrations are highest in or near the peel or rinds of fruits since their biosynthesis is stimulated by light.

In green leafy vegetables, the outer leaves often contain flavonol concentrations more than 10 times the concentrations found in inner leaves. Also smaller fruits of the same species, compared to larger fruits, tend to have higher concentrations of flavonols due to the relationship between surface area and fresh weight.

Flavonols exist in both the monomer form (catechins) and polymer form (proanthocyanidins). Catechins are present in green tea and chocolate and in apricots. Red wine is another major source.

Flavones, isoflavanones and flavanones

Parsley and celery are important sources of flavones.

Isoflavones are found almost exclusively in legumes and soybeans.

Flavanones are found mostly in citrus fruits, tomatoes, mint etc.


Anthocyanins are pigments that give fruits and vegetables their color. Levels increase as fruits ripen and are highest in the skins and peels of fruits.

Reviewed by April Cashin-Garbutt, BA Hons (Cantab)



Antioxidants in Technology

Antioxidants have a variety of uses in the industry. They are most commonly used as food preservatives and supplements. The industrial and other uses of antioxidants can be summarized as follows.

Antioxidants as food preservatives

Antioxidants are used to retard the oxidation of an organic substance. This increases the useful life or shelf life of that material.

For example, in fats and oils, antioxidants delay the onset of oxidation or slow the rate of oxidizing reactions. Fats and oils commonly spoil as oxidation of the lipids cause production of compounds that lead to different odors and taste and continue to affect other molecules in the food. These foods spoil due to exposure to oxygen and sunlight that lead to oxidation of food.

Foods can be preserved by keeping in the dark and sealing it in containers or even coating it in wax, as with cucumbers. However, if the plant products are stored without oxygen that is vital for their respiration, it may lead to unpleasant flavors and unappealing colors. Thus packaging of fresh fruits and vegetables contains around 8% oxygen atmosphere. This may cause oxidant mediated oxidative damage.

The main purpose of using an antioxidant as a food additive is to maintain the quality of that food and to extend its shelf life rather than improving the quality of the food. Antioxidants are an especially important class of preservatives. Unlike bacterial or fungal spoilage of food, oxidative damage can occur even in refrigerated and sealed food items. Antioxidants can prevent this type of food spoilage.

Natural antioxidants and synthetic antioxidants

These preservatives include natural antioxidants such as ascorbic acid (AA, E300) and tocopherols (E306), as well as synthetic antioxidants such as propyl gallate (PG, E310), tertiary butylhydroquinone (TBHQ), butylated hydroxyanisole (BHA, E320) and butylated hydroxytoluene (BHT, E321).

Some of the fats such as olive oil are partially protected from oxidation by their natural content of antioxidants, but remain sensitive to photooxidation or oxidative damage by light.

Antioxidant preservatives are also added to fat-based cosmetics such as lipstick and moisturizers to prevent rancidity. In addition, use of antioxidants also reduces the wastage of raw materials and widens the range of fats that can be used in specific products.

Antioxidant use in industry

Several industrial products contain antioxidants. Some of these include:

  • Antioxidants are added to fuels and lubricants to prevent oxidation, and in gasolines to prevent the polymerization - this polymerization of gasoline leads to residues that can damage the engines.
  • Antioxidants are added to polymers such as rubbers, plastics and adhesives to prevent their oxidative damage and loss of strength and flexibility. Polymers with double bonds are especially vulnerable and benefit with this addition.

The breakdown leads to ozonolysis or cracking. Ozone cracking is especially damaging to elastomers such as natural rubber, polybutadiene and other double-bonded rubbers. They can be protected by antiozonants. Others include polypropylene and polyethylene.

  • Additives such as AO-22 and 29 are added to turbine oils, transformer oils, hydraulic fluids, waxes, and greases in industrial use. AO-29 is added to gasolines as well.
  • Additives such as AO-24 are added to low-temperature oils
  • Additives such as AO-30, 31, 32 and 37 are added to jet fuels and gasolines, including aviation gasolines.
Fuel additive Components Applications
AO-22 N Turbine oils, transformer oils, hydraulic fluids, waxes, and greases
AO-24 N,N'-di-2-butyl-1,4-phenylenediamine Low-temperature oils
AO-29 2 Turbine oils, transformer oils, hydraulic fluids, waxes, greases, and gasolines
AO-30 2 Jet fuels and gasolines, including aviation gasolines
AO-31 2,4-dimethyl-6-tert-butylphenol Jet fuels and gasolines, including aviation gasolines
AO-32 2,4-dimethyl-6-tert-butylphenol and 2,6-di-tert-butyl-4-methylphenol Jet fuels and gasolines, including aviation gasolines
AO-37 2 Jet fuels and gasolines, widely approved for aviation fuels