Oleic acid, autoxidation

Esters of the fatty acid oleic acid are components of membranes that are subject to autoxidation. Explain which hydrogens of an oleate ester you expect to be abstracted most readily by a radical. Show the structures of the major autoxidation products that would be formed from an oleate ester. 

Compounds that contain allylic and benzylic centres are especially prone to autoxidation, since the radicals formed on oxidation are stabilised by resonance. Oleic acid contains two allylic positions, linoleic acid contains two allylic positions and one double allylic position, while linolenic contains two allylic and two double allylic positions. We would therefore expect linolenic to be the most susceptible acid to oxidation, followed by linoleic and oleic. (The actual relative rates of autoxidation are linolenic (25) > linoleic (12) > oleic (1)). Precautions that can be employed to minimise oxidative deterioration are reducing the oxygen concentration in the container by, for example, the use of an inert atmosphere, and the use of a well-closed and well-filled container. It would also be advisable to store the product at low temperature and in a dark place. 

Autoxidation of oleic acid leads to the 4 main hydroperoxy-octadecenoic acids 8-HPOE, 9-HPOE, lO-HPOE and 11-HPOE (cf. Fig. 3.28). Fission generates a multitude of volatiles. Octanal (30), nonanal (31), decanal (32), 2( )-decenal (33) and 2( )-undecenal (34) belong to the odour-active ones. 

It has been now recognised that die phenolic profile of the foods, along with high intakes of the monounsaturated fatty acids, as oleic acid mainly, confers its health-promoting properties to die Mediterranean diet. In fact, olive oil is the main source of unsaturated acids and polyphenols that constitute a complex mixture in olive fruits and in its derived products. In addition, in O. europaea fruits, phenolic profile and content are important factors to consider in order to evaluate virgin olive oil quality. To remain that they are also partly responsible for autoxidation stability and organoleptic characteristics of olive oil. 

Walnuts are the seeds of Juglans regia L. (Juglandaceae) and are highly nutritious. They are rich in oil that is composed of unsaturated fatty acids that are susceptible to oxidation, such as linoleic acid and oleic acid. Despite the low content of a-tocopherol (an antioxidant) in walnuts, as compared to other types of nut,"" walnuts are fairly stable when preserved this implies that an unknown antioxidant(s) within walnuts inhibits lipid autoxidation. Indeed, an aqueous ethanol extract from commercially available walnuts showed marked superoxide dismutase (SOD)-like... 

Parkanyiova, L., Trojakova, L., Reblova, Z., Zainuddin, A., Nguyen, H.T. T., Sakurai, H., Miyahara, M., and Pokomy, J. 2000. Resistance of high-oleic acid peanut oil against autoxidation under storage and deep frying conditions, Czech J. Food Sci., 18 (Spec.) 125-126. 

Fig. 2. Epoxidation of oleic acid by autoxidation of benzaldehyde to perbenzoic acid in situ. Source Ref. 7.

Fig. 3.20. Autoxidation of oleic acid. Primary reaction products I ll-Hydroperoxyoctadec-9-enoic acid II 9-hydroperoxyoctadec-lO-enoic acid, III lO-hydroperoxyoctadec-8-enoic acid, IV 8-hydroperoxyoctadec-9-enoic acid...

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. 

Table 3.28 shows that the composition of hydroperoxide isomers derived from an unsaturated acid by autoxidation ( 02) differs from that obtained in the reaction with 02- The isomers can be separated by analysis of hydroperoxides using high performance liquid chromatography and, thus, one can distinguish Type I from Type II photooxidation. Such studies have revealed that sensitizers, such as chlorophylls a and b, pheophytins a and b and riboflavin, present in food, promote the Type II oxidation of oleic and linoleic acids. 

The concentration of heavy metal ions that results in fat (oil) shelf-life instability is dependent on the nature of the metal ion and the fatty acid composition of the fat (oil). Edible oils of the linoleic acid type, such as sunflower and com germ oil, should contain less than 0.03 ppm Fe and 0.01 ppm Cu to maintain their stability. The concentration limit is 0.2 ppm for Cu and 2 ppm for Fe in fat with a high content of oleic and/or stearic acids, e. g. butter. Heavy metal ions trigger the autoxidation of unsaturated acyl lipids only when they contain hydroperoxides. That is, the presence of a hydroperoxide group is a prerequisite for metal ion activity, which leads to decomposition of the hydroperoxide group into a free radical ... 

Odor-Active Monocarbonyl Compounds. Model expriments showed that the volatile fractions formed during the autoxidation of oleic, linoleic and linolenic acid contain mainly aldehydes and ketones (Table 3.31). Linoleic acid, a component of all lipids sensitive to autoxidation, is a precursor of hexanal that is predominant in... 

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