Hydroperoxide cleavage hexanal

Galliard, T., et al. 1976. The formation of cw-3-none-nal, frarw-2-nonenal and hexanal from linoleic acid hydroperoxide isomers by a hydroperoxide cleavage enzyme system in cucumber (Cucumis sativus) fruits. Biochim. Biophys. Acta 441 181-192. 

Yeast-derived saturated short-medium chain and branched-chain aldehydes are formed from sugar metabolism, fatty acid metabolism and branched-chain amino acid metabolism (Fig 8D.7). In addition, hexanal, as well as hexenal isomers, are formed during the pre-fermentative stages of winemaking by the sequential action of grape lipoxygenase and hydroperoxide cleavage enzyme on linoleic and linolenic acid, respectively (Crouzet 1986). 

Aldehyde Formation. Several investigators observed a marked dominance of hexanal in the volatile products of low-temperature oxidation. At the higher temperatures, however, 2,4-decadienal was the major aldehyde formed (19,20,21). Both aldehydes are typical scission products of linoleate hydroperoxides. Swoboda and Lea (20) explained this difference on the basis of a selective further oxidation of the dienal at the higher temperature, while Kimoto and Gaddis (19) speculated that the carbon-carbon bond between the carbonyl group and the double bond (Type B) is the most vulnerable to cleavage under moderate conditions of autoxidation, while scission at the carbon-carbon bond away from the olefinic linkage (Type A) is favored under stress such as heat or alkali. 

The 13-hydroperoxide of linoleate would thus produce more hexanal at lower temperatures while 2,4-decadienal from the 9-hydroperoxide isomer predominates at elevated temperatures. Although our quantitative work with propyl linoleate (21) supports this rationale, i.e., a temperature-dependent preferential cleavage, the pattern was not as clear when linoleate was oxidized at three different temperatures (18). The more unsaturated substrate oxidized much faster and many of the oxidation products, themselves polyunsaturated, readily underwent further decomposition. 

Hexanal and 2,4-decadienal are the primary oxidation products of linoleate. The autoxidation of linoleate generates 9- and 13-hydroperoxides of linoleate. Cleavage of 13-hydroperoxide will lead to hexanal and breakdown of 9-hydroperoxide will lead to 2,4-decadienal (9). Subsequent moisture-mediated retro-aldol reaction of 2,4-decadienal will produce 2-octenal, hexanal, and acetaldehyde (10). 2,4-Deca-dienal is known to be one of the most important flavor contributors to deep-fat fried foods (11). 

Although LOX from tomato fruits forms predominantly 9-hydroperoxides from linoleic and linolenic acids (Matthew et al., 1977), the cleavage enzyme from tomato does not attack these positional isomers but, rather, is specific for the 13-hydroperoxy isomers, producing hexanal or c/5-3-hexanal, respectively (Galliard and Matthew, 1977). Thus one can rationalize the formation of both Cg and C9 volatiles aldehydes in cucumber extracts with less specificity of LOX and cleavage enzymes and the absence of C9 volatiles in tomato with the substrate specificity of the cleavage enzyme. 

With autoxidized linoleate, 2,4-decadienal and methyl octanoate are produced by homolytic cleavage A on the 9-hydroperoxide, and 3-nonenal and 9-oxononanoate by cleavage B hexanal, pentane, 1-pentanol, and pentanal are produced from the 13-hydroperoxide (Figure 4.12). With linoleate subjected to photosensitized oxidation, the 9- and 13-hydroperoxides produced less 2,4-decadienal, methyl octanoate and pentane, and similar amounts of hexanal. The unique unconjugated 10-hydroperoxide produced 9-oxononanoate and methyl lO-oxo-8-decenoate, and the 12-hydroperoxide produced a significant amount of 2-heptenal (Figure 4.13). 

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). 

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. 

The biosynthetic route 1 We found that the precursors of green odor are a-iinolenic and iinoleic acids as confirmed by C-labelling experiments with tea chloro-plasts. The 13-(5)-hydroperoxide was isolated as an intermediate in the formation of (3Z)-hexenal, key compound, in green odor (see scheme, ADH Alcohol dehydro-gnase, IF Isomerization factor). In this chapter, the subject shall be forcused on the cleavage reaction of 13-(5)-hydroperoxide (co6-hydroperoxide) to (3Z)-hexenai or n-hexanal. 

However, in the water-free fat or oil phase of food, the homolytic cleavage of hydroperoxides presented above is the predominant reaction mechanism. Since option A of the cleavage reaction is excluded (Fig. 3.26), some other reactions should be assumed to occur to account for formation of hexanal and other aldehydes from hnoleic acid. The further oxidation reactions of monohydroperoxides and carbonyl compounds are among the possibilities. 

Linoleic and linolenic acids in fruits and vegetables are subjected to oxidative degradation by lipoxygenase alone or in combination with a hydroperoxide lyase, as outlined in sections 3.7.2.2 and 3.7.2.3. The oxidative cleavage yields oxo acids, aldehydes and allyl alcohols. Among the aldehydes formed, hexanal, (E)-2-hexenal, (Z)-3-hexenal and/or (E)-2-nonenal, (Z)-3-nonenal, (E,Z)-2,6-nonadienal and (Z,Z)-3,6-nonadienal are important for aroma. 

In addition to the previoulsy mentioned less volatile higher hydrocarbons, hydrocarbons with a short chain, such as pentane and hexane, may also be found in fats. They form by cleavage of fatty acid hydroperoxides produced by oxidation of unsaturated fatty acids. These hydrocarbons, not ranked among the Upid accompanying compounds, are easily removed by heating, so they do not occur in freshly refined oils. 

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