Hydroperoxide-Epidioxides

Peroxy radicals which contain isolated P,y double bonds are prone to cyclization reactions in competition with reactions leading to monohydroperoxides. A hydroperoxide-epidioxide results through attachment of a second oxygen molecule and abstraction of a hydrogen atom  

Peroxy radicals with isolated P,y double bonds are formed as intermediary products after aut-oxidation and photooxidation (reaction with singlet O2) of unsaturated fatty acids having two or more double bonds. 

For this reason the 10- and 12-peroxy radicals obtained from linoleic acid readily form hydroperoxy-epidioxides. While such radicals are only minor products in autoxidation, in photooxidation they are generated as intermediary products in yields similar to the 9- and 13-peroxy radicals, which do not cyclize. Ring formation by 10- and 12-peroxy radicals decreases formation of the corresponding monohydroperoxides (Table 3.28 reaction with 02). 

Among the peroxy radicals of linolenic acid which are formed by autoxidation, the isolated P,y double bond system exists only for the 

13- peroxy radicals of linolenic acid to form hydroperoxy-epidioxides results in the formation of less monohydroperoxide of the corresponding isomers as opposed to the 9- and 16-isomers (Table 3.28). 

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Review of all the scission reactions responsible for the hundreds of volatile products in lipid oxidation is beyond the scope of this chapter. The reader is referred to the available reviews (3, 314, 340, 341, 347) for further details. The scission pattern of hydroperoxide epidioxides from linoleic acid is included here to show how the decompositions can become quite complex (Figure 14), and lists of typical products resulting from scission reactions of oleic, linoleic, and linolenic acids are presented in Table 12. 

Malonic Aldehyde, This dialdehyde is preferentially formed by autoxidation of fatty acids with three or more double bonds. The compound is odorless. In food it may be bound to proteins by a double condensation, crosslinking the proteins (cf. 3.7.2.4.3). Malonic aldehyde is formed from a-linolenic acid by a modified reaction pathway, as outlined under the formation of hydroperoxide-epidioxide (cf. 3.7.2.1.3). However, a bicyclic compound is formed here as an intermediary product that readily fragments to malonic aldehyde ... 

The peaks in the chromatograms most likely represent not only primary but also secondary oxidation products of TAGs. According to Neff and co-workers (140), the minor peaks eluting before the main peaks of standard compounds illustrate secondary oxidation products, such as hy-droperoxy epidioxides or bis- or tri.v-hydroperoxides. 

Ozone adds directly to double bonds in fatty acids to form ozonides (183-185). These decompose to lipid alkoxyl and peroxyl radicals that abstract hydrogens to initiate radical chains (186). In the process, internal rearrangements within the original lipid molecule(s) yield hydroxy epoxides and hydroxy epidioxides with 1,3- and 1,4-cyclic hydroperoxides ... 

Hydrogen abstraction also increases at elevated temperature as thermal energy decreases bond dissociation energy. Typical H abstraction rates for ROO at room temperature are < 1 M s, but this increases to 10 -10" L M s at 65°C (223). For example, in linolenic acid autoxidized neat at room temperature to PV 1113, products were not quantified, but estimates from intensities of HPLC peaks gave about 40% LnOOH, 12% dihydroperoxides, 12% hydroperoxy epidioxides, and 4% epoxides (228). At 40°C, H abstraction occurred more as a secondary process. Hydroperoxides per se were still the main products, but fewer were present as mono- and dihydroperoxides (36% total) and more had formed after cyclization or addition (31%). Data are not available to distinguish whether this... 

The singlet oxygen is 1450 times more reactive than molecular oxygen. It is inserted at the end carbon of a double bond, which is shifted to an allylic position in the trans configuration. The resulting hydroperoxides have an aUyhc trans double bond, which renders them different from hydroperoxides formed during autoxidation. Hydroperoxides formed during photooxidation are more easily cychzed than hydroperoxy epidioxides (Frankel, 1998). 

Trilinolenin produced on autoxidation 1(3)- and 2-monohydroperoxides, 1,2- and 1,3-bis-hydroperoxides and tris-hydroperoxides by sequential oxidation (Figure 2.17). Trilinolenin produced also significant amounts of hydroperoxy epidioxides, formed by 1,3-cycUzation (Section C). The... 

Figure 2.18. Oxidation of four synthetic triacylglycerols containing linoleate (L) and linolenate (Ln) in different specific positions at 40°C (Miyashita et al, 1990). Formation of oxidation products was determined by HPLC as total peak area of hydroperoxides and hydroperoxy epidioxides. With permission of AOCS press.

The formation of hydroperoxy epidioxides in significant yields during autoxi-dation of methyl linolenate was discussed previously (Chapter 2, Figure 2.10). Autoxidation studies with pure c/5, raw5 -9-linolenate hydroperoxide (prepared enzymatically) led to the formation of a mixture of 16% 16-hydroperoxy... 

In addition to the monohydroperoxides, unsaturated aldehydes and ketones undergo autoxidation and provide additional sources of volatile compounds. Nonvolatile secondary products that undergo further decomposition into volatile products include dimers and oligomers, hydroperoxy epoxides, hydroperoxy epidioxides and dihydroperoxides. These secondary products contain one or more hydroperoxide groups and decompose the same way as monohydroperoxides to produce similar volatile materials. As with the monohydroperoxides, the multitude of volatile compounds formed from secondary products are important contributors to the flavor quality of lipid-containing foods. 

The hydroperoxy epidioxides formed from photosensitized oxidized methyl linoleate are important precursors of volatile compounds, which are similar to those formed from the corresponding monohydroperoxides. Thus, 13-hydroperoxy-10,12-epidioxy-tra 5 -8-enoic acid produces hexanal and methyl lO-oxo-8-decenoate as major volatiles (Figure 4.24). The 9-hydroperoxy-10,12-epidioxy-rrans-13-enoic acid produces 2-heptenal and methyl 9-oxononanoate. Other minor volatile products include two volatiles common to those formed from the monohydroperoxides, pentane and methyl octanoate, and two that are unique, 2-heptanone and 3-octene-2-one. The hydroperoxy epidioxides formed from autoxidized methyl linolenate produce the volatiles expected from the cleavage reactions of linolenate hydroperoxides, and significant amounts of the unique compound 3,5-octadiene-2-one. This vinyl ketone has a low threshold value or minimum detectable level, and may contribute to the flavor impact of fats containing oxidized linolenate (Chapter 5). 

Table 6.1. Chromatographic separation of hydroperoxides and hydroperoxy epidioxides on solid-phase extraction (SPE) column ...

Autoxidized methyl esters (200 mg) Eluting solvents, ml (etherthexane, v/v) Hydroperoxide absorptivity (A i ) Hydroperoxy epidioxides (A ... 

Literature values 26,000-28,600 for pure cis,trans and trans,trans isomers of methyl tinoleate 9- and 13-hydroperoxides (Chan and Levett, 1977) 24,600 for isomeric mixture of cis,Irons and trans,trans methyl linolenate hydroperoxides (Frankel et al., 1961a), and 24,200-28700 for pure cis,trans and Irons,irons isomers of methyl linolenate hydroperoxy epidioxides (Neff el a/., 1981). 

Analyses of oxidized trilinolenin by reversed phase HPLC with a 5 m C-18 column showed that monohydroperoxides and hydroperoxy epidioxides are the only products formed initially. Bis- and tris-hydroperoxides were detected... 

Figure 6.8. Normal phase HPLC of hydroxyoctadecatrienoate isomers to study the effect of a-tocopherol in inhibiting the formation of trans, trans-hydroperoxides and hydroperoxy epidioxides in autoxidized methyl linolenate with a 5 jam Partisil-5 column, and 0.4% ethanol in hexane (v/v) as eluting solvent, UV detector at 234 nm. (i) no a-tocopherol added, (ii) + 0.05% a-tocopherol, (iii) + 0.5% a-tocopherol, (iv) + 5% a-tocopherol. From Peers et al (1981). Courtesy of Society of Chemical Industry.

Figure 6.11. Autoxidation of trilinolenin at 40°C. Analyses by reversed-phase HPLC as described in Figure 6.10. Mono-OOH, monohydroperoxides OOH Epi, hydroperoxy epidioxides di-OOH, dihydroperoxides Bis-OOH, bis-hydroperoxides Tris-OOH, tris-hydroperoxides OOH Bicyclic, hydroperoxy bicycloendoperoxides. From Frankel et al (1990). Courtesy of the American Oil Chemists Society.

Figure 6.16. Mass fragment ions obtained by chemical ionization-mass spectrometry (CI-MS) analysis of 9- and 13-hydroperoxides from oxidized methyl linoleate, from Plattner and Gardner (1985) (note both hydroperoxide isomers produced intense ions at m/z 309 and 311 in the isobutane spectrum), and 9-hydroperoxy epidioxide and 9,16-dihydroperoxide from oxidized methyl linolenate, fromFrankel etal (1986).

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