Food Chemistry: Antioxidants and Oxidation Reactions in Foods

Antioxidants and Oxidation Reactions in Foods

An antioxidant is a substance that delays the onset or slows the rate of oxidation. It is also used to extent the self-life of a food. Antioxidants play an important role in food chemistry.

Antioxidants can be divided into two categories:

Naturally occurring (vitamin E, vitamin C, β-carotene) (Fig. I1)
Synthetic (2-BHA, 3-BHA, BHT) (Fig. I1)

The unsaturated bonds present in all fats and oils represent active centers that react with oxygen. This reaction leads to the formation of primary, secondary, and tertiary oxidation products that may make the fat or fat-containing foods unsuitable for consumption. This deterioration in flavor of fats and fatty foods is described as rancidity. Oxidation, in nearly all cases, leads to flavors that are not desirable, so we normally strive to reduce or avoid oxidation during storage and processing of food products. Most food components, are vulnerable to oxidation, and oxidation may change their flavor, color, and nutritive value.

 

What are the main factors that affect the rate of oxidation?

amount of oxygen present
degree of unsaturation of the compounds
presence of antioxidants
presence of catalysts for the oxidation (i.e. copper, heme-containing molecules)
light exposure
temperature of storage
nature of packaging material.

 

 

What is the mechanism of the oxidation reaction?

The oxidation reaction (autoxidation) can be divided into the following three steps:

Initiation: Hydrogen is abstracted from an olefinic compound to yield a free radical. The removal of hydrogen occurs at the carbon atom next to the double bond in the presence of a catalyst (metal) or light.

RH → R* + H*

Propagation: Once a free radical has been formed it will combine with oxygen to form a peroxy-free radical, which can in turn abstract hydrogen from another unsaturated molecule to yield a peroxide and a new free radical (propagation reaction).

R* + O₂ → RO₂*

RO₂* + RH → ROOH + R*

The above reaction may be repeated up to several thousand times and has the nature of a chain reaction.

Termination: Termination occurs if the free radicals react with themselves to yield nonactive products.

R* + R* → R-R

R* + RO₂* → RO₂R

nRO₂* → (RO₂)n

 

What are the oxidation products produced when food products are oxidized?

Oxidation is a chain reaction with free radicals as reactive intermediates. Products generated can be divided as:

Primary oxidation products (hydroperoxides)
Secondary oxidation products (carbonyls)
Tertiary oxidation products (fatty acids)

The peroxides (ROOH) formed in the propagation step of the free radical reaction are the primary oxidation products. These oxidation products are generally unstable and decompose into the secondary oxidation products, which include a variety of compounds, including carbonyls, which are the most important.

The peroxides do not affect the flavor of foods. Their oxidation products though –aldehydes and ketones amongst other – known as secondary oxidation products are mainly responsible for flavor deterioration. As the aldehydes are themselves oxidized, fatty acids are formed. These free fatty acids may be considered tertiary oxidation products.

 

How antioxidants prevent food oxidation? Which are the main naturally occurring antioxidants? Which are the main synthetic antioxidants?

Antioxidants are added to foods such as oils, fats and butter as they react with oxygen-containing free radicals and prevent oxidation. Amongst the most important naturally occurring antioxidants are:

Vitamin E (Fig. I.1), a fat soluble vitamin, is a very effective natural antioxidant. It is found in foods such as nuts, wheat germ, seeds, whole grain and in vegetable oils
Vitamin C (Fig. I.2), is present in all animal and plant cells, mostly in free form, and it is probably bound to protein as well. Vitamin C is particularly abundant in rose hips, black and red currants, strawberries, parsley, oranges, lemons (in peels more than in pulp), grapefruit, a variety of cabbages and potatoes
β-Carotene (provitamin A), occurs in large amounts in apricots, cherries, cantaloups, carrots and peaches.
The element selenium, is present in fish, shellfish, meat, eggs, grain and chicken.

 

Since for economic reasons, it is not always possible to use natural antioxidants several synthetic antioxidants have been prepared. Most of them are phenols with a hydroxyl group attached to the benzene ring.

Amongst them the most important synthetic antioxidants are:

BHA (Fig. I.2) Commercial BHA is a mixture of two isomers, 2- and 3-tert-butyl-4-hydroxyanisole
tert-Butylhydroquinone (TBHQ) (Fig. I.2) is a particularly powerful antioxidant used, for example, for stabilization of soya oil.
Propyl gallate, (PG) (Fig. I.2) is very active in fats and oils since it is enriched at the surface of fat and come in contact with air

 

In order to be used as an antioxidant, a synthetic compound has to meet the following requirements:

it should not be toxic
it has to be highly active at low concentrations (0.01–0.02%)
it has to concentrate on the surface of the fat or oil phase.

Utilization of antioxidants is often regulated by governments through controls on the use of food additives. In North America incorporation of antioxidants is permitted at a maximum level of 0.01% for any one antioxidant, and a maximum of 0.02% for any combination. The regulations related to permitted levels often vary from country to country.

 

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Relevant Posts

Food Chemistry: Chemical reactions in cooking

Caramelization in Cooking - Caramelization Reactions

Food Preservatives: Sulfites and SO2

 

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References

1. E.N. Frankel, “Lipid Oxidation”, The Oily Press: Dundee, U.K. 1998
2. H-D. Belitz et al., “Food Chemistry”, 4th Edition, Springer Verlag, 2009
3. P. Barham et al., Chem. Rev., 110, 2313 (2010)
4. L.H. Skibsted, “Lipid Oxidation Pathways”, AOCS Press: Urbana, Illinois: 2008

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Key Terms

antioxidants, rancidity, oxidation of foods, vitamin E, vitamin C, ,BHA, , PG, TBHQ

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