Ethyl linoleate, oxidation

As expected, the rate of oxidation was much faster at the higher temperature for both the adsorbed and the bulk samples. At 60°C ethyl linoleate oxidized faster when adsorbed on silica than when In bulk. On the other hand, oxidation of the bulk samples was faster at 180°C than that of the monolayers (Fig. 1). The faster oxidation of linoleate in monolayers at 60°C is probably due to the larger surface and the more facile exposure to oxygen. This, however, appears to be offset by the higher mobility of the substrate molecules and the faster radical transfer in bulk at the higher temperature. 

Figure 1. Rate of ethyl linoleate oxidation on silica and in bulk at 60 C and 180 C. (Reprinted with permission from ref. 39. Copyright 1988 American Oil Chemistry Society.)...

When ethyl linoleate (1.0 mM) was oxidized in distilled water, NaCl solution and cathodic solution under the presence of FeS04 (1.0 mM) and ascorbic acid (20.0 mM), it was easily oxidized in distilled water and in the NaCl solution with about a 60% loss of the substrate after only 24 h of oxidation (Figure 1, (A)). On the other hand, in the cathodic solution, it was very stable to oxidation, more than 90% of the substrate remaining unchanged after more than 100 h of oxidation (4). The antioxidative activity of the cathodic solution toward ethyl linoleate oxidation was also confirmed by measuring the increase in total... 

Antioxidant capacities of common individual curcuminoids were determined in vitro by phosphomolybdenum and linoleic acid peroxidation methods. Antioxidant capacities expressed as ascorbic acid equivalents (pmol/g) were 3099 for curcumin, 2833 for demethoxycurcumin, and 2677 for bisdemethoxycurcumin at concentrations of 50 ppm. The same order of antioxidant activity (curcumin > demethoxycurcumin > bisdemethoxycurcumin) was observed when compared with BHT (buty-lated hydroxyl toluene) in linoleic peroxidation tests. The antioxidant activity of curcumin in the presence of ethyl linoleate was demonstrated and six reaction products were identified and structurally characterized. The mechanism proposed for this activity consisted of an oxidative coupling reaction at the 3 position of the curcumin with the lipid and a subsequent intramolecular Diels-Alder reaction. ... 

Hydroperoxide Levels. In thermally oxidized fats hydroperoxides are usually very low. At higher temperatures, oxidation proceeds rapidly and the rate of hydroperoxide decomposition exceeds that of hydroperoxide formation (17,18). For example, when ethyl linoleate was oxidized at 70°C, peroxide content reached a maximum of 1777 meq/kg after 6 hr then decreased to 283 meq/kg after 70 hr. At 250°C, on the other hand, peroxide value reached a maximum of only 198 meq/kg after 10 min, and was zero after 30 min. 

During the oxidation to form hydroperoxides, the natural cis,cis unsaturation of linoleate is converted to cis, trans and trans, trans isomers. Privett and co-workers (10) concluded that at least 90% of linoleate hydroperoxide preparations are conjugated. When the oxidation is conducted at 0°C the hydroperoxides are predominately cis, trans isomers, but room temperature oxidation produces a large amount of trans, trans unsaturation (11, 12). Ethyl or methyl linoleate hydroperoxides are relatively low melting and as a result purification by crystallization is difficult. Bailey and Barlow (13) prepared high melting p-phenylphenacyl linoleate, oxidized the ester in benzene solution, and isolated virtually pure hydroperoxide by crystallization. Infrared spectra of the 99% purity p-phenylphenacyl linoleate hydroperoxide correspond to a trans, trans conjugated isomer. 

Bolland, J.L. Kinetic studies in the chemistry of rubber and related materials. VI. Benzyl peroxide-catalyzed oxidation of ethyl linoleate. Trans. Faraday Soc. 1948, 44, 669-677. 

Feeding of casein treated with oxidized ethyl linoleate, at 9% protein level, caused depressed growth rate and enlargement of the liver (266). Such effects could not be the result of the oxidation of sulfhydryl groups only, but may be caused by additional reactions as discussed under the section on Cross-Linking in this chapter. 

Modern kinetic investigations of antioxidant action began with the investigations of Bolland and ten Haave [151,152] on inhibited oxidation of ethyl linoleate and with the broad theoretical and experimental studies of Waters and his coworkers [153—155]. Bolland and ten Haave proposed that inhibition resulted from chain-breaking by the faster reaction of R02 with antioxidant, AH, than with hydrocarbon RH to give an unreac-tive radical A which then terminates with R02- or A, viz. 

Bolland and ten Haave [151] found that the oxidation of ethyl linoleate inhibited by hydroquinone was described by the relation... 

The oxidation rate of emulsions of ethyl linoleate (EL) diluted with n-tetradecane in the presence of Tween 20 was studied in [301], The emulsions had an initial droplet diameter of 0.3 pm and total oil contents of 5 wt.% (EL-tetradecane). At 1% EL in oil, oxidation proceeds at a slow rate. At 20% EL in oil, the oxidation rate was rapid initially and then slowed down with time. In the absence of tetradecane, the oxidation rate was slow at first, and then increased with time. In all cases, the oxidation remained high in the presence of emulsifier. 

Pokorny, J., Davidek, J., Chocholata, V, Panek, J., Bulantova, H., Janitz, W., Valentova, H., and Vierecklova, M. 1990. Interactions of oxidized ethyl linoleate with collagen,... 

Linoleic and linolenic acid are the oxygen acceptors in the linseed oil. The heat of oxidation of linolenic acid was given to the author as 172 cal/g, which ties in with the figures of about 53 kcal/g mole for ethyl linoleate or linolenate of 308.51 and 306.49 m. w. The heat of polymerization of linseed oil has been reported as 230 cal/g. 

Hydroxycinnamates as well as their conjugates may act as powerful antioxidants. In edible plants they have received much attention as protecting agents against oxidative deterioration of food. Antioxidant mechanism studies on ferulic acid and its coupling products with linoleate on the molecular level have been conducted recently. A radical scavenging reaction occurred at the 3 -position of the ferulate radical with four types of peroxyl radicals of ethyl linoleate. The produced peroxides subsequently underwent intramolecular rearrangement to afford stable tricyclic peroxides (Masuda et al. 2006). 

Figure 1. Oxidative stability of ethyl linoleate in an aqueous micelles, (A) Decrease in unoxidized linoleate. (B) Formation of peroxides.

The flavour of distillates from apple and pear is characterised by typical aroma compounds from these fruits formed by enzymatic degradation of fatty acids to C6-fragments like hexanol, trans-2-hexenol, as well as ethyl esters and acetates of hexanoic acid. In distillates of pears, especially of the variety Bartlett pear, the characteristic pear flavour is mainly dominated by the ethyl and methyl esters of frans-2-czs-4-decadienoic acid and trans-2-trans-A-decadienoic acid [27-29], The biogenesis of these monounsaturated, diunsaturated, and triunsaturated esters may be explained by -oxidation of unsaturated linoleic and linolenic acid in the fruits. The sesquiterpene compound a-farnesene, which is formed during postharvest ripening and storage of Bartlett pears [28], shows that quality and intensity of distilled pear spirits is mainly influenced by the quality and degree of ripeness of the fruits. 

The products of lipid oxidation in monolayers were also studied. Wu and coworkers (41) concluded that epoxides rather than hydroperoxides might be the major intermediates in the oxidation of unsaturated fatty acids adsorbed on silica, presumably because of the proximity of the substrate chains on the silica surface. In our work with ethyl oleate, linoleate and linolenate which were thermally oxidized on silica, the major decomposition products found were those typical of hydroperoxide decomposition (39). However, the decomposition patterns in monolayers were simpler and quantitatively different from those of bulk samples. For example, bulk samples produced significantly more ethyl octanoate than those of silica, whereas silica samples produced more ethyl 9-oxononanoate than those of bulk. This trend was consistent regardless of temperature, heating period or degree of oxidation. The fact that the same pattern of volatiles was found at both 60°C and 180°C implies that the same mode of decomposition occurs over this temperature range. 

More than 300 compounds had been identified in cocoa volatiles, 10% of which were carbonyl compounds (59,60). Acetaldehyde, 2-methylpropanal, 3-methylbutanal, 2-methylbutanal, phenylacetaldhyde and propanal were products of Strecker degradation of alanine, valine, leucine, isoleucine, phenyl-acetaldehyde, and a-aminobutyric acid, respectively. Eckey (61) reported that raw cocoa beans contain about 50-55% fats, which consisted of palmitic (26.2%), stearic (34.4%), oleic (37.3%), and linoleic (2.1%) acids. During roasting cocoa beans these acids were oxidized and the following carbonyl compounds might be produced - oleic 2-propenal, butanal, valeraldehyde, hexanal, heptanal, octanal, nonanal, decanal, and 2-alkenals of Cg to C-q. Linoleic ethanal, propanal, pentanal, hexanal, 2-alkenals of to C q, 2,4-alkadienals of Cg to C-q, methyl ethyl ketone and hexen-1,6-dial. Carbonyl compounds play a major role in the formation of cocoa flavor components. 

Fats and oils (triglycerides) from plants and animals are renewable sources of chemicals, but the amounts of the chemicals made from them are small compared with those made from petroleum and natural gas. This may change if biodiesel fuel (e.g., ethyl oleate) made by the alcoholysis of oils becomes common. Such esters may be useful as environmentally friendly solvents.50 Unsaturated oils, such as linseed oil, are the basis of oil-based paints, which cure by cross-linking through oxidation by air. Soaps are the potassium or sodium salts of the long-chain fatty acids obtained by the hydrolysis of the triglycerides. The dibasic dimer fatty acids obtained by the dimerization of oleic and linoleic acids (both Cig acids) are made into oligomeric fatty amides which are used to cure epoxy resins. The un-... 

A large proportion of the volatiles identified in vegetable oils are derived from the cleavage reactions of the hydroperoxides of oleate, linoleate, and linolenate (Section D). A wide range of hydrocarbons (ethane, propane, pentane and hexane) appears to be formed in soybean oil oxidized to low peroxide values. A number of volatiles identified in vegetable oils that are not expected as primary cleavage products of monohydroperoxides include dialdehydes, ketones, ethyl esters, nonane, decane, undecane, 2-pentylfuran, lactone, benzene, benzaldehyde and acetophenone. Some of these volatiles may be derived from secondary oxidation products, but the origin of many volatiles still remains obscure. However, studies of volatile decomposition products should be interpreted with caution, because the conditions used for isolation and identification may cause artifacts, especially when fats are subjected to elevated temperatures. 

Figure 6.7. HPLC of hydroperoxy epidioxide isomers from photosensitized-oxidized linoleate with a microporous 10 /im silica Partisil 10 column and 6 4 1 hexane methylene chloride ethyl acetate (v/v) as mobile solvent, refractive index detector. From Neff et al. (1982). Courtesy of the American Oil Chemists Society.

Acyl-CoA originates from the 3-oxidation of fatty acids and also occasionally from amino acid metabolism. Figure 5.30 shows an example of how ethyl (E,Z)-2,4-decadienoate, an important aroma constituent of pears, is synthesized from linoleic acid. 

Figure 5. Oxidative stability of ethyl docosahexaenoate in super pure water (open triangle), NaCl solution (open circle),cathodic solution just after electrolysis (solid circle) and cathodic solution after incubation for 7 days (solid triangle). (A) Decrease in unoxidized linoleate. (B) Formation of peroxides.

The reason for auto-oxidation is the presence of double bonds in the chains of many fatty compounds. The auto-oxidation of unsaturated fatty compounds proceeds with different rates depending on the number and position of double bonds (Frankel, 2005). The positions allylic to double bonds are especially susceptible to oxidation. The bix-aUylic positions in common polyunsaturated fatty acids, such as linoleic add (double bonds at C-9 and C-12, giving one te-allylic position at C-11) and linolenic add (double bonds at C-9, C-12, and C-15, giving two te-allylic positions at C-11 and C-14), are even more prone to autoxida-tion than allylic positions. The relative rates of oxidation given in the literature (Frankel, 2005) are 1 for oleates (methyl, ethyl esters), 41 for Unoleates, and 98 for linolenates. This is essential because most biodiesel fuels contain significant... 

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