Lipid Oxidation: Theoretical Aspects

Abstract

Lipid oxidation has long been accepted as a classic free radical chain reaction mechanism with initiation, propagation, and termination stages. This view is not inaccurate, but it is incomplete. Initiation is generally treated as if it was spontaneous or random, but in reality, radical initiation is quite specific and varies with the source. Similarly, propagation is considered to involve only hydrogen abstraction where as internal cyclization, addition, scission, and disproportionation reactions are competitive or even dominant under some conditions, leading to variations in oxidation kinetics and, in particular, product mixes. Alternative reaction paths have been recognized for some time, but remain largely ignored in measurements of lipid oxidation. This chapter reviews initiation processes with a focus on differentiating rates, mechanisms, and specificity of radical formation; and it presents evidence for multiple propagation mechanisms that lead to different product mixes. Finally, a new integrated view of lipid oxidation is proposed to account for multiple propagation pathways. Practical application of this information should enable development of analyses that more accurately reflect the true extent of lipid oxidation and also antioxidant approaches that more effectively extend shelf life and preserve desirable food qualities.

The oxidative stability of biodiesel and its impact on the deterioration of metallic and polymeric materials: a review

“…The double allylic hydrogen that lies between two double bonds is more reactive than allylic hydrogen near a double bond because the former has a lower binding energy than the latter. 3 During the termination stage, reactions take place removing radicals formed in the previous stages. The main reactions that occur are recombination reactions between the radicals to form monomers and polymers, and reactions that break the radicals into non-radical substances.…”

“…The products of these reactions are alcohols, ketones, ethers, alkanes, organic acids, aldehydes and oligomers, among others. 3 As biodiesel is oxidized, a series of changes in its properties occurs. Properties like the acid value, peroxide value and viscosity increase, while the iodine value and content of methyl esters decrease.…”

Photo- and Free Radical-Mediated Oxidation of Lipid Components During the Senescence of Phototrophic Organisms

“…Indeed, the mechanism of initiation of lipid radical oxidation, which has been debated for many years, seems to be the homolytic cleavage of photochemically produced hydroperoxides in phytodetritus (Rontani et al, 2003b). Redox-active metal ions are generally considered as the initiators of perhaps greatest importance for lipid oxidation in biological systems (Pokorny, 1987;Schaich, 1992). They may direct the cleavage of hydroperoxides either through alkoxyl or peroxyl radicals.…”

“…Addition of peroxyl or alkoxyl radicals to the double bond becomes competitive only in the case of conjugated, terminal, or trisubstituted double bonds (Schaich, 2005). Effectively, autoxidation of mono-unsaturated fatty acids appears to mainly involve allylic hydrogen abstraction and subsequent oxidation of the allylic radical thus formed.…”

Metal Ions and Hydroperoxide Content: Main Drivers of Coastal Lipid Autoxidation in Riverine Suspended Particulate Matter and Higher Plant Debris

“…In the initiation process, enzymes, heat, light and metal ions may play a role in the generation of radicals. The metal ion-catalyzed homolytic cleavage of photochemically-produced hydroperoxides [1] has been suggested to play an important role in the initiation of autoxidation reactions in phytodetritus [2], although this has not been firmly established experimentally. The chain reaction ends when free radicals collide and exchange electrons to form a new bond, hence rendering them unreactive.…”