Product specificity conjugated hydroperoxides

To improve our understanding of the complex oxidation reactions in meat and fish products, evaluations should be carried out with each product stored under realistic conditions. Lipid oxidation should be followed by several and more reliable methods than the TEA test. These methods should measure specific lipid oxidation products (e.g. hydroperoxides or conjugated dienes) acting as precursors of rancid flavors, and specific volatile products (e.g. propanal, hexanal and other volatiles associated with fish flavors) that serve as reliable indicators of flavor deterioration. Instrumental analyses of volatile oxidation products should be supported and confirmed by sensory evaluations. 

The products of the aerobic reactions catalyzed by LOXs are conjugated ci5,tran5-pentadienyl hydroperoxides with the trans double bond next to the hydroperoxide. They differ in the position of the hydroperoxide group which, for linoleic acid, can be either the C-9 or C-13. Most LOXs catalyze the formation of one particular positional isomer (Table III). As well as positional specificity, the enzymes are also enantiospecific with respect to the new chiral center generated. In the cases that have been studied, the 9-hydroperoxide formed has a 9R configuration while the 13-isomer has a 135 configuration. 

The difference in volatile products in plants (aldehydes) and mushrooms (allyl alcohols) is due to the different substrate and reaction specificity of HPL. In the first case, in hydroperoxides with conjugated diene systems (Fig. 3.31a), the bond between the C-atom bearing the HOO-group and the C-atom of the diene system is cleaved. In the second case (Fig. 3.31b), cleavage of hydroperoxides with isolated double bonds occurs in the opposite direction between the C-atom... 

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