Furan from hydroperoxide decomposition

A variety of compounds such as hydrocarbons, alcohols, furans, aldehydes, ketones, and acid compounds are formed as secondary oxidation products and are responsible for the undesirable flavors and odors associated with rancid fat. The off-flavor properties of these compounds depend on the structure, concentration, threshold values, and the tested system. Aliphatic aldehydes are the most important volatile breakdown products because they are major contributors to unpleasant odors and flavors in food products. The peroxidation pathway from linoleic acid to various volatiles is determined in several researchs, - by using various techniques (Gas chromatography mass spectrometry, GC-MS, and electron spin resonance spectroscopy, ESR), identified the volatile aldehydes that are produced during the oxidation of sunflower oil. In both cases, hexanal was the major aldehyde product of hydroperoxide decomposition, whereas pentanal, 2-heptenal, 2-octenal, 2-nonenal, 2,4-nonadienal, and 2,4-decadienal were also identified. 

As with oleate and linoleate, some volatile decomposition compounds are formed from linolenate hydroperoxides that cannot be explained by the classical A and B cleavage mechanisms, including acetaldehyde, butanal, 2-butyl furan, methyl heptanoate, 4,5-epoxyhepta-2-enal, methyl nonanoate, methyl 8-oxooctanoate, and methyl lO-oxo-8-decenoate. Some of these minor volatile oxidation products can be attributed to further oxidation of unsaturated aldehydes. Other factors contribute to the complexity of volatile products formed from hydroperoxides, including temperature of oxidation, metal catalysts, stability of volatile products and competing secondary reactions including dimerization, cyclization, epoxidation and dihydroperoxidation (Section E). 

A specific type of rancidity, known as reversion in the United States, occurs during the oxidation of soybean oil. Reversion is caused by some furan derivatives, such as 2-pentylfuran or (Z)- and ( )-2-(pent-2-en-l-yl)furan and other decomposition products of hydroperoxides. Pentylfuran arises (Figure 3.48) as a by-product of the decomposition of 9-hydroperoxyoctadeca-10,12-dienoic acid derived from linoleic acid via the terminal radical. The content of linoleic acid in soybean oil is about 51% of the total fatty acids. Similarly, both isomers (cis and trans) of 2-(pent-2-en-l-yl) furan (3-139) arise by decomposition of 9-hydroperoxyoctadeca-10,12,15-trienoic acid formed by autoxidation of linolenic acid. 

Flavour reversion is characteristic of soybean oil, and sometimes also occurs in other oils containing Knolenic acid, for example rapeseed oil. It manifests itself when the oil still contains fairly low amounts of fatty acid hydroperoxides. Smells resembling grass and beans are due to flavour reversion, and are caused by different compounds resulting from the decomposition of hydroperoxides, including various derivatives of furan. Oil, in which this defect is apparent, can have this smell removed during refining, but the defect re-appears after a certain amount of time (hence the name... 

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