Lipid Oxidation

Edwin N. Frankel

• Publisher: Oily Press
• Year: 2014
• Print ISBN: 9780953194988
• eBook ISBN: 9780857097927
• Edition: 2
• Topic: Technology & Engineering
• Subtopic: Biochemistry
• Index: Technology & Engineering

In this second edition, Edwin Frankel has updated and extended his now well-known book Lipid oxidation which has come to be regarded as the standard work on the subject since the publication of the first edition seven years previously. His main objective is to develop the background necessary for a better understanding of what factors should be considered, and what methods and lipid systems should be employed, to achieve suitable evaluation and control of lipid oxidation in complex foods and biological systems.The oxidation of unsaturated fatty acids is one of the most fundamental reactions in lipid chemistry. When unsaturated lipids are exposed to air, the complex, volatile oxidation compounds that are formed cause rancidity. This decreases the quality of foods that contain natural lipid components as well as foods in which oils are used as ingredients. Furthermore, products of lipid oxidation have been implicated in many vital biological reactions, and evidence has accumulated to show that free radicals and reactive oxygen species participate in tissue injuries and in degenerative disease.Although there have been many significant advances in this challenging field, many important problems remain unsolved. This second edition of Lipid oxidation follows the example of the first edition in offering a summary of the many unsolved problems that need further research. The need to understand lipid oxidation is greater than ever with the increased interest in long-chain polyunsaturated fatty acids, the reformulation of oils to avoid hydrogenation and trans fatty acids, and the enormous attention given to natural phenolic antioxidants, including flavonoids and other phytochemicals.

Chapter 1

Free radical oxidation

Publisher Summary

Autoxidation is the direct reaction of molecular oxygen with organic compounds under mild conditions. Oxygen has a special nature in behaving as a biradical by having two unpaired electrons (‘O–O’) in the ground state and is said to be in a triplet state. The oxidation of lipids proceeds like that of many other organic compounds by a free radical chain mechanism, which can be described in terms of initiation, propagation, and termination processes. These processes often consist of a complex series of sequential and overlapping reactions. Many of the classical mechanistic concepts of lipid oxidation were formulated on the basis of kinetic studies. Later developments in support of the general free radical mechanism of oxidation were based on structural studies of the primary hydroperoxide products. To simplify the kinetics of linoleate oxidation, the reactions were studied at early stages of oxidation, at low levels of conversion, at lower temperatures, and in the presence of an appropriate initiator. Under these conditions, the propagation reactions producing hydroperoxides in high yields are emphasized and the decomposition of hydroperoxides is minimized or considered insignificant. However, the kinetics becomes much more complex when autoxidation is carried out to high conversions, or at elevated temperatures, or in the presence of metals.

A Mechanism

Autoxidation is the direct reaction of molecular oxygen with organic compounds under mild conditions. Oxygen has a special nature in behaving as a biradical by having two unpaired electrons (^·O–O^·) in the ground state and is said to be in a triplet state. The oxidation of lipids proceeds like that of many other organic compounds by a free radical chain mechanism, which can be described in terms of initiation, propagation, and termination processes. These processes often consist of a complex series of sequential and overlapping reactions.

1 Initiation

In the presence of initiators (I), unsaturated lipids (LH) lose a hydrogen radical (H^·) to form lipid free radicals (L^·).

$\text{LH}\underset{{\text{R}}_{\text{I}}}{\overset{\text{I}}{\to }}\text{IH}+{\text{L}}^{ċ}$ (1)

(1)

The direct oxidation of unsaturated lipids by triplet oxygen (³O₂) is spin forbidden because the lipid ground state of singlet multiplicity has an opposite spin direction from that of oxygen of triplet multiplicity. This spin barrier between lipids and oxygen can be readily overcome in the presence of initiators that can produce radicals by different mechanisms.

(a) thermal dissociation of hydroperoxides (LOOH) present as impurities:

$\text{LOOH}\to {\text{LO}}^{ċ}{+}^{ċ}\text{OH}$ (2)

(2)

(b) decomposition of hydroperoxides catalysed by ubiquitous redox metals (M) of variable valency:

$\text{LOOH}+{\text{M}}^{2+}\to {\text{LO}}^{ċ}+{\text{OH}}^{-}+{\text{M}}^{3+}$ (3)

(3)

$\text{LOOH}+{\text{M}}^{3+}\to {\text{LOO}}^{ċ}+{\text{H}}^{+}+{\text{M}}^{2+}$ (4)

(4)

(c) exposure to light in the presence of a sensitizer such as a ketone (RCOR):

$\text{RCOR}+hv\to {\text{RCO}}^{ċ}{+}^{ċ}\text{R}$ (5)

(5)

Because the reaction of unsaturated lipids with singlet oxygen (¹O₂) (Chapter 3) occurs at a significantly greater rate than their reaction with normal triplet oxygen (³O₂), the hydroperoxides that are rapidly formed under photosensiti...

Table of contents

• Cover image
• Title page
• Table of Contents
• Also in the Oily Press Lipid Library
• Copyright
• Preface
• Introduction
• Chapter 1: Free radical oxidation
• Chapter 2: Hydroperoxide formation
• Chapter 3: Photooxidation of unsaturated fats
• Chapter 4: Hydroperoxide decomposition
• Chapter 5: Methods to determine extent of oxidation
• Chapter 6: Research methods for lipid oxidation
• Chapter 7: Stability methods
• Chapter 8: Control of oxidation
• Chapter 9: Antioxidants
• Chapter 10: Oxidation in multiphase systems
• Chapter 11: Foods
• Chapter 12: Frying fats
• Chapter 13: Biological systems
• Glossary
• Abbreviations
• Index