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Antioxidants are molecules that are able to slow or prevent other molecules from being oxidised. Oxidation is a chemical reaction where electrons are transferred from one particular substance to the oxidising agent, therefore the oxidising agent gains electrons and is reduced. Oxidation reaction reactions can produce free, radicals, which can lead to chain reactions, which can severely damage cells. Antioxidants play an important role in preventing this by terminating the chain reactions. They do this by removing the free radical intermediates so that they cannot participate in any further reactions. The Antioxidants are again oxidised in this process.
In the natural condition animal and plant tissue have their own natural antioxidants that protect their tissues from oxidation. However in processed foods the antioxidants are not as affective and the tissues are prone to oxidation.
Oxidation of food is a destructive process, causing loss of nutritional value and changes in chemical composition. Oxidation of fats and oils leads to rancidity and, in fruits such as apples; it can result in the formation of compounds which discolour the fruit.
Antioxidants are particularly important in the preservation of food as unlike bacterial and fungal growth oxidation can still occur whilst the food is refrigerated or even frozen.
Food rich in unsaturated fats are particularly susceptible to oxidation: vegetable oil, meat, fish, poultry, margarine and dairy products.
When meat is oxidised it can turn rancid. This leads to discolouration and a metallic/sulphurous taste that is very undesirable. Key chemicals that are responsible for the rancid taste are aldehydes, ketones and alcohols.
When the fat is broken down by oxygen, peroxides are produced, which cause the rancidity. Oxidative-rancidity (also called Autoxidation) is a free radical chain reaction consisting of three main phases: initiation propagation and termination.
Mechanism of oxidative rancidity
During this stage radicals are produced by the attack of oxygen on the fatty acid.
This can occur in a number of different ways depending on the fatty acid.
In Polyenoic (essential) fatty acids the CH2 between the two double bonds is the primary site for oxygen attack. (-CH=CH-CH2-CH=CH-)
The methyene group is converted into free radicals R-Hâ†’R*+H*
In Monoenoic fatty acids free radicals are formed by the cleavage of a hydrogen atom on either side of the double bond (-CH2-HC=CH-CH2-) â†’(-C*H-CH=CH-C*H-)
The unsaturated fatty acid formed in the initiation stage can absorb a molecule of oxygen, forming a peroxy radical. The peroxy radical is very reactive and forms hydroperoxide by accepting a hydrogen atom. The hydroperoxide molecule is easily cleaved to give a peroxy radical and a hydrogen radical or an alkoxy radical and a hydroxyl radical.
R*+O2 ïƒ R-O-O* Unsaturated fatty acid radical reacts with a molecule of oxygen
Peroxy radical accepts hydrogen to form hydroperoxide and an alkyl radical
R-O-O* +R-H ïƒ R-O-OH + R*
Hydroperoxide cleaved to give a peroxy radical and a hydrogen radical or an alkoxy radical and a hydroxyl radical. This part of the reaction can be repeated many times so is called a chain reaction
R-O-O-H ïƒ R-O-O* H* OR R-O* + H-O*
The chain reactions occurring during propagation can be interrupted by the recombination of the two free radicals. This terminates the reaction by creating stable products.
R* + R-O-O* ïƒ R-O-O-R
How are the bad smells created?
The hydroperoxide created during the propagation stage is what causes the rancid taste in meats.
Hydroperoxides are the most important reaction product and are produced during the propagation stage of the reaction. Hydroperoxides can easily decompose forming free radicals that can lead to more chain reactions.
Molecule cleaved either side of the alkoxy group. To create an aldehyde and a radical
Hydroperoxide cleaved to form an alkoxy radical and a hydroxyl radicalThe flow diagram shows how aldehydes are produced from the hydroperoxide.
The radicals formed can react with other radicals formed from previous reactions to form alcohols and other aldehydes as well as hydrocarbons.
It is the aldehydes, alcohols and hydrocarbons that are produced in the food that gives the rancid smell
How Do Metal ions speed up the oxidation process?
Metal ions catalyse the oxidation of the hydroperoxide. A metal ion reacts with the hydroperoxide creating two radicals. Without the metal ions the hydroperoxide would have been in a stable state and would not have reacted.
R-O-O-R + Me2+ ïƒ R-O-O* + Me2+ + H+
The free radicals produced can initiate further chain reactions.
Copper and Iron as well and manganese and copper (to a lesser extent) are particular promoters or oxidation
Fatty foods including dairy products as well as some meats should therefore not be stored in metal containers.
How does Light speed up the oxidation of Foods
In the presence of light a Triplet oxygen molecule can be converted into a Singlet molecule. A triplet oxygen molecule is the ground state is the state in which oxygen is usually found in.
The Triplet Oxygen has two unpaired electrons as can be seen in the MO diagram
In triplet oxygen both electrons have the same spin therefore each electron has to be
Placed in different Ï€* orbitals (Hund's Rule of Maximum Multiplicity)
Singlet oxygen also has two unpaired electrons however they occupy only one of the Ï€* orbitals and air spin paired. Singlet oxygen is far more reactive than triplet Oxygen and will cleave hydroperoxide far more readily increasing the rate of the chain reaction and ultimately the rate of the oxidation.
How do Antioxidants prevent fatty foods from turning rancid?
There are a number of different types of antioxidant that inhibit the oxidation of foods in different ways
First there are the normal antioxidants that work by reacting with peroxy radicals produced during the oxidation of fatty acids, forming a hydroperoxide molecule and a free radical of the antioxidant. The free radicals of the antioxidant are relatively stable and so do not initiate any chain oxidation reactions, unless present in excess.
The next class are the Hydroperoxide deactivators
These substance decompose Hydroperoxides in a non-radical way. They convert Hydroperoxides into less-reactive hydroxyl derivatives which are relatively stable and do not cause chain reactions. One compound which acts to deactivate Hydroperoxides is sulphur derivatives which are found in foods such as Onion and Garlic.
These are substances that have no antioxidant activity of their own but work synergistically in increasing the activity of antioxidants. Citric and tartaric as well as ascorbic and phosphoric acid are good examples of synergists. Synergistic activity can include the conversion of radicals into ions, metal Chelation and the regeneration of antioxidants. These three different activities increase the activity of the antioxidant.
Singlet Oxygen Quenchers
Singlet oxygen quenchers deactivate singlet oxygen and convert it back to the more stable and less reactive triplet oxygen there for reducing the rate of oxidation. Caretenoids found commonly in fruit and vegetables are examples of singlet oxygen quenchers
I explained earlier how metal ions present in the food can speed up oxidation. It is therefore important to find a way of removing these metal ions. Chelating agents do just this. Synergists, such as phosphoric acid, citric, tartaric, malic or ascorbic acids, all posses pronounced chelating activities. These substance bind to the metal ions forming complexes and making them unavailable to form radicals and preventing more chain reactions.
Sequestrants are a special class of additives that work in a similar way to Chelating agents. Sequestrants are compounds that "capture" metal ions such as those of copper, iron and nickel and remove them from contact with the food. In their free state the metal ions can potentially increase the rate of oxidation as I have already explained however by removing the metal ions they cannot have this affect.
EDTA, citric acid, Sorbitol and tartaric acid are all examples of Sequestrants
Ascorbic acid (vitamin C)
Beers, cut fruits, jams, dried potato.
Oils, meat pies. Obtained from soya beans and maize.
Butylated hydroxyanisole (BHA)
Oils, margarine, cheese, crisps.
Jam, tinned fruit, biscuits, alcoholic drinks, cheese, dried soup.
These are one of the most common synthetic antioxidants. They have limited activity in vivo which is why they are synthetic. They are, however, found naturally in many foods including lots of fruit and vegetables as well as Red wine, coffee, chocolate, green tea, olive oil, bee pollen (honey) and many grains.
Include info on Nitrites