Lipid Oxidation

Lipid oxidation generally occurs after a long induction period. Once started it is generally a very rapid reaction. Lipid oxidation proceeds by a free radical mechanism.

A free radical is a compound with an odd number of unpaired electrons.

Two free radicals were formed. These radicals are very reactive and generally do not have long life times. They enter into three main types of reactions:

1. Abstraction:

2. Addition:

3. Combination:

Lipid oxidation follows three main steps:

Initiation
Propagation
Termination

1. Initiation involves the formation of free radicals.

Mechanism later:



In some cases may add oxygen directly to the double bond to form a biradical

Once the initial radicals have formed, the formation of other radicals proceeds rapidly. The new radicals will not be at the double bond. To remove a hydrogen from a double bond requires 80 Kcal/mole. To remove a hydrogen alpha to a double bond only requires 15 Kcal/mole. This lower energy requirement can be explained by the phenomenon known as resonance

There can also be resonance within a lipid free radical:

The radicals can abstract H atoms from other lipids or react with other radicals to form alcohols and ketones:

A.

B.

C.

Of great importance to the food industry is the splitting of C-C bonds and the formation of aldehydes:

There can also be cleavage on the other side:

It is possible for two of the radicals formed to combine:

The change from the alcohol to the aldehyde is called a keto - enol shift. If the two free radicals do not react with other molecules they may combine with each other.