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Lipid Oxidation

Lipids are of great importance in human diet. They provide essential fatty acids, supply energy, and assist in absorption of fat-soluble vitamins. Proportional consumption of dietary fats and oils helps maintain healthier mechanism in the body. In addition to their physiological effects, they are organoleptically desirable for food industry and consumers. There is currently a wide range of lipid applications in manufacture of foods, including margarines, vegetable oils, pastries, bakeries, sauces, emulsions, and shortenings.

Lipids are predominantly made up of esters of fatty acids and glycerols, namely, triacylglycerols. They are highly susceptible to degradation, mostly through oxidative or hydrolytic rancidity. The formation of hydrogen peroxides, acting as radical initiators, can be indicative of oxidative rancidity. On the other hand, the release of free fatty acids, following the hydrolysis of triglyceride, can be associated with hydrolytic rancidity. These reactions can adversely exert influence on nutritional, shelf life and sensory profile, rendering the food undesirable and potentially hazardous.

The progression of oxidation essentially relies on the chemical features of fatty acids. The speed of oxidation is higher in free fatty acids than in triacylglycerols. They exhibit activity of pro-oxidation between hydrogen peroxides and carbonyl group that gives rise to the liberation of free radicals from decomposed hydroperoxides.

Autoxidation, a typical form of lipid oxidation that signifies autocatalysis of free radical-based reaction, has been viewed as a major issue in lipid deterioration. The oxidation proceeds through three main steps. These are (i) initiation, (ii) propagation, and (iii) termination. The initiation is substantially reliant on the presence of the activity of hydrogen peroxides. At this stage, the activity of free radical intermediates is initiated via the reaction of peroxides and catalysing metals such as copper and iron. The liberation of monohydroperoxides from peroxy free radicals occurs in the course of initiation and propagation. The basic mechanism is based on the abstraction of hydrogen atoms in single double bond of fatty acids and formation of alkyl radicals.

In principle, the level of oxidation is likely to increase with the increase of allyl groups, within a shorter induction period. At a preliminary stage, the hydrogen peroxides are tasteless and odourless. The oxidation deterioration is progressively accelerated, e.g., through the reaction of metal ions, initiating the cleavage and decomposition of hydroperoxides. As a result, volatile secondary metabolites, such as aldehydes, ketones, acids and alcohols, are generated. The formation of these compounds brings about undesirable flavour and odour in the oxidised food.

In addition to the compositional structure of fatty acids, lipid spoilage may be intensified by presence/activity of other variables, comprising respiratory/metabolic mechanism, enzymatic reaction, storage time/temperature, light exposure, surface area, membrane permeability (in terms of oxygen diffusion), irradiation, and food processing. High temperature, as a major oxidation determinant, is highly detrimental in the entire process of lipid deterioration. On this account, the stored foods are usually controlled at low temperature. However, even at low temperature storage, such as freezing, the quality of foods may be jeopardised due to the potential damage in protective components of protein molecules, leading to excessive dryness. This can result in the leakage of lipid components, disturbing the emulsion system and ultimately increasing the risk of deterioration by active oxygen reaction.

Numerous strategies have been formulated to minimise/ suppress the spoilage in lipid-based foods. The efficacy of these methods varies depending on the nature and compositional characteristics of food products. For example, oxidation may be inhibited by removal of oxygen from food by means of packaging methods (using a vacuum or incorporating glucose oxidase), or by addition of antioxidants to food products.  

Antioxidants, such as polyphenolic compounds, have potential to exhibit bioactivity and their presence in foods helps (i) extend shelf life and increase stability, and (ii) promote nutritional and health benefits. The protective function of antioxidants is attributed to their ability to scavenge free radicals, e.g., via chelating metal ions and donating hydrogen atoms to free radicals, hence enabling stabilisation of radicals via the progress of de-localisation of electron within the ring structure.

Because of the susceptibility/limited bioavailability of endogenous antioxidants, the incorporation of exogenous antioxidants in various food processing has been viewed as an efficient method to promote antioxidant activity and stability of the final product. At present, in place of synthetic antioxidants, due to their possible risk factors, naturally occurring antioxidants are progressively used in food manufacturing as potentially safer and less expensive alternatives.

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By Fereshteh Safarzadehmarkhali – Safarzadeh Markhali -.

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