Fatty autoxidation, secondary product

Oxidative stability of edible oils depends primarily on their fatty acid composition and, to a lesser extent, in the stereospecific distribution of fatty acids in the triacyl-glycerol molecules. The presence of minor components in the oils also affects their oxidative stability. A detailed discussion of oxidative processes in fats and oils is provided elsewhere in this series. Oxidation may occur via different routes and includes autoxidation, photo-oxidation, thermal oxidation, and hydrolytic processes, all of which lead to production of undesirable flavor and products harmful to health. Flavor and odor defects may be detected by sensory analysis or by chemical and instrumental methods. However, chemical and instrumental procedures are often employed in the processing and during usage of edible oils. Indicators of oxidation are those that measure the primary or secondary products of oxidation as well as those from hydrolytic processes or from thermal oxidation, including polymers and polar components (15). 

During lipid oxidation, the primary oxidation products that are formed by the autoxidation of unsaturated lipids are hydroperoxides, which have little or no direct impact on the sensory properties of foods. However, hydroperoxides are degraded to produce additional radicals which further accelerates the oxidation process and produce secondary oxidation products such as aldehydes, ketones, acids and alcohols, of which some are volatiles with very low sensory thresholds and have potentially significant impact on the sensory properties namely odor and flavor [2, 3]. Sensory analysis of food samples are performed by a panel of semi to highly trained personnel under specific quarantined conditions. Any chemical method used to determine lipid oxidation in food must be closely correlated with a sensory panel because the human nose is the most appropriate detector to monitor the odorants resulting from oxidative and non-oxidative degradation processes. The results obtained from sensory analyses provide the closest approximation to the consumers approach. Sensory analyses of smell and taste has been developed in many studies of edible fats and oils and for fatty food quality estimation [1, 4, 5]. 

Autoxidation and photo-oxygenation are two aspects of the non-enzymic reaction between oxygen and unsaturated fatty acids. The enzymic reactions are discussed in Section 10.3. Oxidation of lipids during storage and handling, involving complex substrates and ill-defined reaction conditions, proved difficult to understand. This difficulty is enhanced by the fact that the primary oxidation products are labile and readily converted to secondary oxidation products of several kinds. Understanding of these processes has come from studies of simpler substrates such as methyl oleate or methyl linoleate under clearly defined reaction conditions. 

Saturated and unsaturated hydrocarbons with odd and even numbers of carbon atoms in the molecule (about C11-C35) are present as the primary substances in all vegetable oils and animal fats. Alkanes, alkenes, alkadienes and alkatrienes also arise as oxidation products of unsaturated fatty acids, catalysed by lipoxygenases or by autoxidation of fatty acids during food storage and processing. Only the lower hydrocarbons can play a role as odour-active substances. The main hydrocarbons resulting from oxidation of unsaturated fatty acids are ethane from Hnolenic acid, pentane and butane from Hnoleic acid and hexane and octane from oleic acid. The immediate precursors of hydrocarbons are the fatty acid hydroperoxides (Table 8.4). The unsaturated hydrocarbons are predominantly (Z)-isomers. Numerous other hydrocarbons, including ahcycHc hydrocarbons, appear as secondary hpid oxidation products. 

Terpenic hydrocarbons are stable in the absence of air, but are easily oxidised in air, especially at higher temperatures. Their autoxidation proceeds by similar mechanisms as autoxidation of unsaturated fatty acids and depends greatly on the hydrocarbon structure. The primary autoxidation products are hydroperoxides. In branched hydrocarbons, the hydroperoxyl group mainly occurs in the secondary or tertiary carbon adjacent to the quaternary carbon of the double bond. The final autoxidation products are usually epoxides, alcohols and ketones. The primary site of attack in olefins is the carbon adjacent to the double bond, as in monounsaturated... 

Lipid oxidation in foods is a complex chain of reactions that first consist of the introduction of a functional group containing two concatenated oxygen atoms (peroxides) into unsaturated fatty acids, in a free-radical chain reaction, that afterward gives rise to secondary oxidation products. Different pathways for lipid oxidation have been described radical mechanism or autoxidation, singlet oxygen-mediated mechanism or photooxidation, and enzymatic oxidation. 

After acidification, the lipid fraction is extracted into hexane and the solvent subsequently removed by evaporation. This gives a material consisting of 60-75% (w/w) CLA (depending on the starting materials), with the remainder consisting of unchanged saturated and monounsaturated fatty acids from the starting material. Deodorization of CLA removes volatile compounds such as hexane that may be residual solvent or secondary autoxidation products from CLA. Fractional distillation and crystallization remove metal catalysts (which may stimulate oxidation), as well as undesirable components such as dimers and polymers. The reactor type will affect the amounts of metals found. 

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