Initiation of lipid oxidation:

Singlet versus Triplet

Molecules with two electrons in the outer shell may have electrons arranged in a number of different ways. If the electrons are in the same orbital, they must have opposite spins. There is only one magnetic moment derived from electrons in the same orbital and this is known as the singlet state. If the electrons are not in the same orbital, there are three possible arrangements:

Each arrangement has a different magnetic moment and all three arrangements can be detected. This is the triplet state. With enough energy, normal triplet oxygen can be converted to singlet oxygen. When the energy involved is considered, a rule states that compounds in one state can not react with compounds in another state. In this case, the term can not react, is really a statement of probability. Reactions between compounds in different states can occur, but only very slowly. Photosynthetic pigments are capable of converting ground state triplet oxygen into the excited singlet state ( DG = 23 Kcal/mole ).

¹S = Sensitizer (Chlorophyll ) in the singlet state
³S = Sensitizer in the triplet state
³O₂ = oxygen in the triplet state
¹O₂ = oxygen in the singlet state

RH and ROOH are in the singlet state. Their reaction with normal triplet oxygen is very slow. When oxygen is in the singlet state, the reaction is at least 1,450 times faster.

Factors contributing to lipid oxidation:

Energy is the form of heat and light
Metals
Number of double bonds
Enzymes
Oxygen content
Type of oxygen
Other components

.Singlet Oxygen Formation

Superoxide Anion

The reaction of xanthine with xanthine oxidase does not produce singlet oxygen directly. It produces a compound, O₂-, the superoxide anion, that can lead to the formation of Singlet oxygen.

Superoxide and singlet oxygen

Spontaneous Dismutation

O₂^- + O₂^- + 2H+Æ H₂O₂ + ¹O₂

Superoxide Dismutase

O₂^- + O₂^- + 2H+Æ H₂O₂ + ³O₂

Harber Weiss Reaction

H₂O₂ + O₂^- Æ ³O₂ + OH- + *OH

Reaction with iron

H₂O₂ + Fe++Æ Fe+++ + OH- + *OH

Photosensitizers

Dyes

Methylene
Blue Rose
Bengal
Eosin
Crystal Violet
Acridine
Orange

Pigments

Chlorophyll
Hematoporphyrine
Flavin

Polymerization Reactions

We have seen that conjugated systems of double bonds are not difficult to generate during lipid oxidation. Conjugated bonds react more rapidly than do those that are methylene interrupted. If the content of oxygen is low, there can be polymer formation at elevated temperatures:

Highly saturated oils are utilized for varnish. The same types of reactions can occur in highly heated fats. Can also have the formation of cyclic polymers

The newly formed cyclic polymer may further react with unsaturated lipids to form more complex structures:

If polymers form within the same chain, the products formed can be very toxic. Incorporation of oxygen into the polymers increases their toxicity.

When fatty foods are fried, water is often present. Hydrolysis of triglycerides to yield free fatty acids can occur. This lowers the smoke point of the oil and facilitates foam formation. Unless foods are soaked in used oil for extended periods of time oxidized flavors are usually not a problem with fried foods.

When the products of lipid oxidation are formed in the presence of proteins reactions between the aldehydes formed and the amino groups of the proteins can occur.

If two reactive groups are present there can be crosslinking of proteins. The amino acid group is often associated with the essential amino acid, lysine. Both protein crosslinking and reactions with lysine can lower the protein quality of the food.

Reversion

Linolenic Acid Theory
Isolinolenic Acid Theory
Oxidative Polymer Theory
Phospholipid Theory
Nonsaponifiable Theory
Linoleic Acid Theory

The following have been related to reversion flavor:

2-pentenal
cis hexenal T,C
2,4-heptadienal

Isolenoleic Theory

The compound 9,15 -C18:2 will form beany off-flavors upon aging when added to hydrogenated cottonseed oil. This compound can arise from the hydrogenation of C18:3

Phospholipid Theory

Removal of nearly all phospholipids is essential to produce a finished oil with optimum flavor and color stability. Phospholipids will develop fishy flavors upon heating and can serve as flavor precursors if left in oil prior to deodorization.

Controlling Reversion

Use of metal chelators
Storage in:Dark Under N2 Reduced temperature
Hydrogenate to IV = 110
Removal of all phospholipids
Inactivation of lipoxygenase
Reduction of oxidative polymers