1. (D) The Chemistry of Cooking and the Oxidation of FoodHC
CHEMISTRY HC
CHEMISTRY
NATURES’ CHEMISTRY
(D) The Chemistry of Cooking and the Oxidation of Food

2. (D) The chemistry of cooking and the oxidation of FoodHC
CHEMISTRY HC
CHEMISTRY
(D) The chemistry of cooking and the oxidation of Food
Anti-Oxidants
After completing this lesson you should be able to :
Antioxidants in foods.
Oxygen reacts with edible oils giving the food a rancid flavour. Antioxidants are molecules which will prevent these oxidation reactions taking place.
Learn about the chemistry of the antioxidant molecules which prevent oxidation reactions in foods from taking place.

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Hydrolytic

4. COPY Oxygen is killing you
An antioxidant is a molecule that inhibits the oxidation of other molecules. Oxidation is a chemical reaction involving the loss of electrons which can produce free radicals. In turn, these radicals can start chain reactions. When the chain reaction occurs in a cell, it can cause damage or death to the cell.
A free radical is an atom, molecule, or ion that has unpaired valency electrons. Free radicals are highly reactive and can cause damage to molecules in the cell.

5. Oxidation of food
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Oxidation reactions can occur when food is exposed to oxygen in the air.
Foods containing fats or oils are at the greatest risk of oxidation.

6. Foods rich in fats and oils

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Oxidation of Fats
Fats and oils, or foods containing them, are the most likely to have problems with oxidation. Fats react with oxygen and even if a food has a very low fat content it may still need the addition of an antioxidant. They are commonly used in:
vegetable oil
snacks (extruded)
animal fat
meat, fish, poultry
margarine
dairy products
mayonnaise / salad dressing
baked products
potato products (instant mashed potato)

8. Effects of oxidation on food
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Effects of oxidation on food
When fats react with oxygen they are broken down, causing:
deterioration of flavour (rancidity)
loss of colour
loss of nutritional value
a health risk from toxic oxidation products.

9. Oxidation of fat or oil COPY
As the fat/oil decomposes and reacts with oxygen, chemicals called peroxides are produced. These change into the substances characteristic of the smell and soapy flavour of a rancid fat
Antioxidants prevent the formation of peroxides and so slow the process of the food 'going off'. Some antioxidants react with oxygen itself and so prevent the formation of peroxides.
Air-tight packaging, using inert gases like nitrogen, vacuum packing and refrigeration can all be used to delay the oxidation process. However, these can still be inefficient and adding antioxidants can be an effective way of extending the shelf life of a product.

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Fat breaking down
Fat
Fat
Oxygen

11. Fat molecules COPY Radicals attack near the double bond
(NB ‘R’ represents the remainder of the fat molecule)

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Antioxidants
Antioxidants are chemicals that are added to food to prevent the food from ‘going off’.
An antioxidant is a substance that slows down or prevents the oxidation of another chemical.
Chemistry in every cup

13. Oxidative damage COPY Oxidation reactions can produce free radicals.
A free radical is a highly reactive species containing an unpaired electron.
Free radicals can damage food by removal of an electron.
Antioxidant molecules ‘mop up’ free radicals to protect the foodstuff.

14. COPY Radical now in a stable pair Damaging free radical
Neutralised free radical
Electron
transferred
Antioxidant
Antioxidant converted to a stable free radical

15. How does an antioxidant cancel out a free radical?
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How does an antioxidant cancel out a free radical?
The antioxidant molecule donates an electron to the potentially damaging free radical.
A stable electron pair is formed, stabilising the free radical.
The antioxidant itself becomes oxidised (loses an electron).

16. Antioxidants in action
The apple is protected when dipped in orange juice containing the antioxidant vitamin C
Oxidation occurs when the apple is left exposed to air

17. COPY The table shows some typical antioxidants: Antioxidant E-number
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The table shows some typical antioxidants:
Antioxidant
E-number
Typical foods
Ascorbic acid (vitamin C)
E300
Beers, cut fruits, jams, dried potato. Helps to prevent cut and pulped foods from going brown by preventing oxidation reactions that cause the discolouration. Can be added to foods, such as potato, to replace vitamin C lost in processing.
Tocopherols
E306
Oils, meat pies. Obtained from soya beans and maize. Reduces oxidation of fatty acids and some vitamins.
Butylated hydroxyanisole (BHA)
E320
Oils, margarine, cheese, crisps. Helps to prevent the reactions that break down fats and cause the food to go rancid .
Citric acid
E330
Jam, tinned fruit, biscuits, alcoholic drinks, cheese, dried soup. Naturally-occuring in citrus fruits like lemons. Helps to increase the anti-oxidant effects of other substances. Helps to reduce the reactions that can discolour fruits. May also be used to regulate pH in jams and jellies.

18. Antioxidants
Oxidation reactions happen when chemicals in the food are exposed to oxygen in the air.
In natural conditions, animal and plant tissues contain their own antioxidants but in foods, these natural systems break down and oxidation is bound to follow.
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 added to food to slow the rate of oxidation and, if used properly, they can extend the shelf life of the food in which they have been used.

19. Vitamin C (ascorbic acid)
The antioxidant vitamin C can act as a reducing agent (electron donor), preventing oxidation (electron loss) from the foodstuff.
C6H8O6   C6H6O6  +  2H+  +  2e-
Ascorbic acid
Dehydroascorbic acid

20. Antioxidants and health benefits
There may be health benefits from the use of antioxidants. Oxidation reactions in the body could be linked to the build-up of fatty deposits that cause blockages in arteries that can cause heart attacks. Antioxidants may be important in preventing this and there could also be a link with the prevention of certain cancers, arthritis and other conditions. The picture is not yet clear and a great deal of research needs to be undertaken.

21. Do antioxidants help us live longer?
Studies involving 230,000 men and women across the UK have shown that there is no convincing proof that antioxidants have any effect on how long people can live. However 40% of women and 30% of men are reportedly taking these supplements and spending over £333 million on them per year.

22. Impact of antioxidants on health
Free radicals in living cells
Free radicals are present in all living cell and are a part of the cell processes. However excessive free radicals in our cells can attack the cell membranes (the outer coat of the cell). This attack causes cell and tissue damage.
Radicals can also break strands of DNA (the genetic material in the cell). Some of the chemicals known to cause cancer, do so by forming free radicals.
The imbalance between free radicals and antioxidants can lead to disease and ill health. The 4 main non-enzymatic antioxidants metalonin, α-tocopherol (Vitamin E), ascorbic acid (Vitamin C) and β-carotene (precursor for Vitamin A) can be found in a range of foods in our diet. However medical opinions are divided as regards the impact these antioxidants have our on general health.

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Melatonin
This is a hormone which helps to regulate sleep in our bodies. This compound can be termed as a terminal or suicidal antioxidant as once it has removed the free radicals it has to be replaced.

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α-tocopherol
This is a form of vitamin E and can be found in vegetable oil, nuts and seeds. It has been suggested that it is good for the skin.

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Ascorbic Acid
This is also known as Vitamin C and is commonly found in fruits and vegetables. It is one of the essential vitamins and the human body is unable to synthesize it. It can be easily oxidised and acts as a hydroxyl or superoxide anion radical scavenger.

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β-carotene
This is a precursor to vitamin A. It is a highly red-orange pigment found in plants and fruits. In particular it gives carrots their orange colour. It helps human cells to absorb vitamin A.