Hydroperoxy epidioxides

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.

Figure 3.6. Formation of five- and six-membered cyclic peroxides by photosensitized oxidation of 9-and 16-hydroperoxy epidioxides of methyl linolenate (Neff and Frankel, 1984). 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... 

Figure 4.3. Secondary oxidation of methyl linolenate. In the absence of methyl linolenate, oxidation of pure 9-OOH and 13-OOH produced mixtures of hydroperoxy epidioxides by rearrangement of the linolenate hydroperoxyl radicals (Coxon etal, 1981). See text.

Figure 4.4. Formation of malonaldehyde from hydroperoxy epidioxides and bicyclo-endoperoxides of methyl linolenate (Pryor et al, 1976).

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). 

Figure 4.24. Volatile decomposition products from hydroperoxy epidioxides of methyl linoleate oxidized with singlet oxygen (Frankel etal, 1982).

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 6.6. Normal phase HPLC isomers of hydroperoxy epidioxide isomers from autoxidized linolenate on microporous 10 jum silica Partisil 10 column, 0.3% ethanol in hexane (v/v) as eluting solvent, variable wavelength detector at 212 nm. From Frankel et al (1981). Courtesy of the American Oil Chemists Society.

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).

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). 

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