Rancidity & Lipid Peroxidation

Preshantha GovenderRoll #25

Biochemistry Seminar

Shelf life The shelf life of a product refers to the

length of time a product may be stored without becoming unsuitable for use or consumption.

After the shelf life of a product has reached, the food undergoes processes which results in rancidity.

Examples of products and their shelf life stamps

What is Rancidity? Rancidity, is the natural process of

decomposition of fats or oils by either hydrolysis or oxidation, or both.

The process of degradation converts fatty acid esters of oils into free fatty acids.

This gives rise to an unpleasant odour and taste in food.

These lipids degrade to the point of becoming either unpalatable or unhealthy to ingest.

Examples of rancid food

Factors Causing Rancidity Catalysts: trace metal ions & inorganic

salts Temperature Amount of PUFA Time Light Water

Types of RancidityThere are 3 types/pathways of rancidity:1. Oxidative Rancidity Known as autoxidation It is due to the auto-oxidation of PUFA

present in triacylglycerols by the atmospheric O2 on free radicals.

The end product is the formation of aldehyde epoxide and peroxide.

Example of oxidative rancidity

2. Hydrolytic Rancidity

Known as hydrolysis/enzymatic oxidation.

It is due to the contamination of fat by lipase leading to the formation of diacyl & triacylglycerols with free fatty acids.

The end product is the formation of aldehyde epoxide and peroxide.

Structures of TAG, DAG & lipase enzyme

End products aldehyde epoxide &

peroxide

3. Microbial Rancidity

In which micro-organisms such as bacteria, moulds and yeast use their enzymes to break down chemical structures in the oil, producing unwanted odours and flavours.

Water needs to be present for microbial growth to occur.

Can be prevented by sterilization

Structures of yeast, mould spores &

bacteria

Lipid Peroxidation The main cause of rancidity of lipids is the

oxidative deterioration of lipids via a free-radical chain mechanism, which is also called lipid peroxidation.

This process occurs only within our bodies. It occurs in three stages or phases:

>Initiation>Propagation>Termination

Stage 1: Initiation

Molecular oxygen combines with unsaturated fatty acids, producing hydroperoxides and peroxyl free radicals, both of which are highly reactive and unstable.

Stage 2: Propagation

This is when these unstable by-products of the first stage react with other lipids, starting a continuing free radical lipid peroxidation chain reaction called autoxidation. This results in a continuing and cyclical oxidative degradation process, breaking down the lipid.

Stage 3: Termination

Is marked by the slowing or stopping of reactions, completion of making unreactive compounds or when an antioxidant is added or encountered. There are two basic types of oxidative by-products, primary and secondary.

Diagram representing lipid peroxidation

Measuring RancidityRancidity is most commonly detected by taste or smell, but it is also accompanied by a marked increase in the acid value of the fat, which is tested by using two basic laboratory tests:

Peroxide Value (PV) for primary oxidation products

Anisidine Value (AnV) for secondary oxidation products.

Harmful Effects of Rancid Food

Leads to deficiencies such as anaemia, hair loss & dermatitis.

Kidney & heart disease Neurodegeneration Cancer

Examples of anaemia, cancer cells & dermatitis

Harmful Effects of Lipid Peroxidation Damage to the cell membrane Cardiovascular diseases Joint diseases Ageing Cancer

Prevention of Rancidity

Prevention of Lipid PeroxidationAdding antioxidants (A, C & E) to one’s diet retards lipid peroxidation.

ReferencesWebsites www.wikipedia.org www.cyberlipid.org www.slideshare.net www.naturalproductsinsider.com

Textbooks Textbook of medical biochemistry (8th Edition) Principles of biochemistry (4th Edition)

Thank You