Volatile autoxidation

From the volatile autoxidation products which contain the methyl end of the linoleic acid molecule, the formation of 2,4-decadienal and pentane can be explained by reaction 3.72. 

Trichloroethylene [79-01-6J, trichloroethene, CHCL=CCL2, commonly called "tri," is a colorless, sweet smelling (chloroformlike odor), volatile Hquid and a powerhil solvent for a large number of natural and synthetic substances. It is nonflammable under conditions of recommended use. In the absence of stabilizers, it is slowly decomposed (autoxidized) by air. The oxidation products are acidic and corrosive. Stabilizers are added to all commercial grades. Trichloroethylene is moderately toxic and has narcotic properties. 

In plant tissues, various enzymes convert the hydroperoxides produced by LOX to other products, some of which are important as flavor compounds. These enzymes include hydroperoxide lyase, which catalyzes the formation of aldehydes and oxo acids hydroperoxide-dependent peroxygenase and epoxygenase, which catalyze the formation of epoxy and hydroxy fatty acids, and hydroperoxide isomerase, which catalyzes the formation of epoxyhydroxy fatty acids and trihydroxy fatty acids. LOX produces flavor volatiles similar to those produced during autoxidation, although the relative proportions of the products may vary widely, depending on the specificity of the enzyme and the reaction conditions. 

A widespread method for determining the induction period for autoxidation of oils and fats consists of passing a continuous stream of air through the heated sample and collecting the volatile acids evolved in a water trap, where they are determined on a real time basis. The time plot usually presents a flat appearance for a certain period and then takes off in an accelerated manner. This test is the basis of several national and international standards (e.g. AOCS Cd 12b-92—oil stability index" ISO 6886—accelerated oxidation test for oxidative stability of fats and oils ) and the design of the Rancimat equipment, where the end determination is based on conductivity measurements . In addition to oxidation stability as determined by the Rancimat method and POV, which negatively affects virgin olive oil stability, other nonstandard properties were proposed for better assessment of the quality of this oil, namely LC determination of Vitamin E (21), colorimetric determination of total polar phenols and UVD of total chlorophyll. ... 

Butenes were subjected to photosensitized reaction with molecular oxygen in methanol. 1-Butene proved unreactive. A single hydroperoxide, l-butene-3-hydroperoxide, was produced from 2-butene and isolated by preparative gas chromatography, Thermal and catalyzed decomposition of pure hydroperoxide in benzene and other solvents did not result in formation of any acetaldehyde or propionaldehyde. The absence of these aldehydes suggests that they arise by an addition mechanism in the autoxidation of butenes where they are important products. l-Butene-3-hydroperoxide in the absence of catalyst is converted predominantly to methyl vinyl ketone and a smaller quantity of methyl vinyl carbinol —volatile products usually not detected in important quantities in the autoxidation of butene. 

Volatile compounds formed by anabolic or catabolic pathways include fatty acid derivatives, terpenes and phenolics. In contrast, volatile compounds formed during tissue damage are typically formed through enzymatic degradation and/ or autoxidation reactions of primary and/or secondary metabolites and includes lipids, amino acids, glucosinolates, terpenoids and phenolics. 

The volatiles produced by the LOX pathway and autoxidation are typically volatile aldehydes and alcohols responsible for fresh and green sensorial notes. In the LOX pathway these volatile compounds are produced in response to stress, during ripening or after damage of the plant tissue. The pathway is illustrated in Scheme 7.2. Precursors of the LOX (EC 1.13.11.12) catalysed reactions are Cis-polyunsaturated fatty acids with a (Z,Z)-l,4-pentadiene moiety such as linoleic and a-linolenic acids that are typically oxidised into 9-, 10- or 13-hydro-peroxides depending on the specificity of the LOX catalyst. These compounds are then cleaved by hydroperoxide lyase (HPL) into mainly C, C9 and Cio aldehydes, which can then be reduced into the corresponding alcohols by alcohol dehydrogenase (ADH EC 1.1.1.1) (Scheme 7.2) [21, 22]. The production of volatile compounds by the LOX pathway depends, however, on the plants as they have different sets of enzymes, pH in the cells, fatty acid composition of cell walls, etc. 

Many of the compounds derived from enzyme-catalysed oxidative breakdown of unsaturated fatty acids may also be produced by autoxidation [23]. While the enzymatically produced hydroperoxides in most cases yield one hydroperoxide as the dominant product, non-enzymatic oxidation of unsaturated fatty acids yields a mixture of hydroperoxides which differ in the position of the peroxide group and in the geometrical isomerism of the double bonds [24]. As the number of double bonds increases, the number of oxidation and oxygen-addition sites increases proportionally and thus the number of possible volatile... 

Products of the LOX pathway or compounds formed by autoxidation of fatty acids (Scheme 7.2) are also important for leek aroma [31, 163]. Volatile compounds of the LOX pathway are not pronounced in the aroma profile of freshly cut leeks owing to a high content of thiosulfinates and thiopropanal-S-oxide [30]. In processed leeks that have been stored for a long time (frozen storage), however, these aliphatic aldehydes and alcohols have a greater impact on the aroma profile owing to volatilisation and transformations of sulfur compounds [31, 165]. The most important volatiles produced from fatty acids and perceived by GC-O of raw or cooked leeks are pentanal, hexanal, decanal and l-octen-3-ol (Table 7.5) [31, 35, 148, 163, 164]. 

Day, E. A. and Lillard, D. A. 1960. Autoxidation of milk lipids. I. Identification of volatile monocarbonyl compounds from autoxidized milk fat. J. Dairy Sci. 43, 585-597. 

Ullrich, F. and Grosch, W. 1987. Identification of the most intense volatile flavour compounds formed during autoxidation of linoleic acid. Z. Lebensm. Unters. Forsch. A. 184 277-282. 

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. 

Routine procedures to assay the extent of oxidation in lipids and lipid-containing foods should be simple, reliable and sensitive. Results from routine procedures should ideally correlate well with results obtained from sensory taste panels. St. Angelo (1996) has described volatile compound profiles formed during lipid oxidation in different groups of food products. However, because of the complexity of lipid oxidation, no single test can be equally useful at all stages of the oxidative process. The methods should be capable of detecting autoxidation before the onset of off-flavor. This is particularly true in the case of milk products where a low level of oxidation can lead to off-flavor. 

Fat portion of meats, particularly their phospholipid components, undergo autoxidation/degradation (2) and produce an overwhelming number of volatiles. Fats also serve as a depot of fat-soluble compounds that volatilize on heating and strongly affect flavor. Since compositional characteristics of lipids in meats, vary from one species to another, these factors may be responsible for the development of some species-specific flavor notes in cooked meats (8.9 ). Obviously presence of 4-methyloctanoic and 4-... 

In our opinion, the predominant contribution to flavor seems to come from sulfurous and carbonyl-containing volatiles. While many of the sulfur-containing volatiles are known to have meaty aromas, volatile carbonyl compounds generally are formed by lipid autoxidation/degradation and do not possess meaty flavor notes. However, it has been indicated that the carbonyl compounds are responsible for the "chickeny" aroma of cooked chicken (17). Thus, lipid autoxldatlon appears to yield the character impact compounds for chicken (18). 

As oxidation normally proceeds very slowly at the initial stage, the time to reach a sudden increase in oxidation rate is referred to as the induction period (6). Lipid hydroperoxides have been identified as primary products of autoxidation decomposition of hydroperoxides yields aldehydes, ketones, alcohols, hydrocarbons, volatile organic acids, and epoxy compounds, known as secondary oxidation products. These compounds, together with free radicals, constitute the bases for measurement of oxidative deterioration of food lipids. This chapter aims to explore current methods for measuring lipid oxidation in food lipids. 

Hydroperoxides of unsaturated fatty acids formed by autoxidation are very unstable and break down into a wide variety of volatile flavor compounds as well as nonvolatile products. It is widely accepted that hydroperoxide decomposition involves homolytic cleavage of the -OOH group, giving rise to an alkoxy radical and a hydroxy radical (5). 

Aldehydes Of the volatiles produced by the breakdown of the alkoxy radicals, aldehydes are the most significant flavor compounds. Aldehydes can be produced by scission of the hpid molecules on either side of the radical. The products formed by these scission reactions depend on the fatty acids present, the hydroperoxide isomers formed, and the stability of the decomposition products. Temperature, time of heating, and degree of autoxidation are variables that affect thermal oxidation (7). 

Some volatile aldehydes formed by autoxidation of unsaturated fatty acids are listed in Table 1. The aromas of aldehydes are generally described as green, painty, metallic, beany, and rancid, and they are often responsible for the undesirable flavors in fats and oils. Hexanal has long been used as an index of oxidative deterioration in foods. Some aldehydes, particularly the unsaturated aldehydes, are very potent flavor compounds. Table 2 fists aroma characteristics of some common aldehydes found in fats and oils (8). 

TABLE 1. Some Volatile Aldehydes Obtained from Autoxidation of Unsaturated Fatty Acids (6). 

The development of a characteristic, objectionable, beany, grassy, and hay-like flavor in soybean oil, commonly known as reversion flavor, is a classic problem of the food industry. Soybean oil tends to develop this objectionable flavor when its peroxide value is still as low as a few meq/kg, whereas other vegetable oils, such as cottonseed, com, and sunflower, do not (15, 51). Smouse and Chang (52) identified 71 compounds in the volatiles of a typical reverted-but-not-rancid soybean oil. They reported that 2-pentylfuran formed from the autoxidation of linoleic acid, which is the major fatty acid of soybean oil, and contributes significantly to the beany and grassy flavor of soybean oil. Other compounds identified in the reverted soybean oil also have fatty acids as their precursors. For example, the green bean flavor is caused by c/i-3-hexenal, which is formed by the autoxidation of linolenic acid that usually constitutes 2-11% in soybean oil. Linoleic acid oxidized to l-octen-3-ol, which is characterized by its mushroom-like flavor (53). 

The high oxidation rates of EPA and DHA and the instability of their hydroperoxides caused the rapid formation of secondary products such as volatile aldehydes and other compounds, which, in turn, impart flavor reversion in fish oils (56). The hydroperoxides produced from autoxidation of EPA (73) and DHA (74) have been identified but not quantified. They form eight and ten isomers, respectively. Noble and Nawar (75) analyzed the volatile compounds in autoxidized DHA and identified a number of aldehydes. Most of the aldehydes identified could be explained by the p-scission of alkoxy radicals generated by the homolytic cleavage of each isomer of the hydroperoxides as shown in Figure 9. 

Methyl tert-butyl ether, MTBE, and ethyl tert-butyl ether, ETBE, are added to gasoline in order to increase the efficiency of gasoline combustion, which reduces the quantity of volatile organic compounds (VOCs) that escape into the atmosphere and cause smog. The chemical industry is very interested in using MTBE as a substitute for THF and diethyl ether, because autoxidation of THF and diethyl ether is a major safety problem for chemical companies. The industry is less interested in ETBE as a solvent substitute. 

Autoxidation of oleic acid leads to the 4 main hydroperoxy-octadecenoic acids 8-HPOE, 9-HPOE, lO-HPOE and 11-HPOE (cf. Fig. 3.28). Fission generates a multitude of volatiles. Octanal (30), nonanal (31), decanal (32), 2( )-decenal (33) and 2( )-undecenal (34) belong to the odour-active ones. 

Freshly stir-fried Chinese food has a much better flavor quality than after it is aged. The main change in volatile constituents of stir-fried bell peppers during aging is the production of volatile carbonyl compounds from autoxidative breakdown of unsaturated fatty acids (Wu et al., 1986). 

To understand the sensitivity of the extracts to artifact formation it is informative to review the volatiles in kiwifruit. Quantitatively, peroxidation products of unsaturated fatty acids (11, 12), which include (E)-2-hexenal (77.87%), (E)-2-hexen-l-ol (6.80%), 1-hexanol (3.40%), hexanal (1.78%), (Z)-2-hexenal (0.87%), (E)-3-hexen-l-ol (0.32%) and (Z)-3-hexen-l-ol (0.17%), constitute over 90% of the total volatiles. Other major constituents include the esters, methyl butanoate (2.54%) and ethyl butanoate (3.52%). The presence of large amounts of saturated and unsaturated aldehydes in the extract is noteworthy since they are quite susceptible to free-radical oxidation. We therefore expected that at least some of the artifacts were the products of autoxidation. 

Flavors and aromas commonly associated with seafoods have been intensively investigated in the past forty years ( l-7), but the chemical basis of these flavors has proven elusive and difficult to establish. Oxidized fish oils can be described as painty, rancid or cod-liver-oil like (j ), and certain volatile carbonyls arising from the autoxidation of polyunsaturated fatty acids have emerged as the principal contributors to this type of fish-like aroma ( 3, 5, 9-10). Since oxidized butterfat (9, 11-12) and oxidized soybean and linseed oils (13) also can develop similar painty, fish-like aromas, confusion has arisen over the compounds and processes that lead to fish-like aromas. Some have believed that the aromas of fish simply result from the random autoxidation of the polyunsaturated fatty acids of fish lipids (14-17). This view has often been retained because no single compound appears to exhibit an unmistakable fish aroma. Still, evidence has been developed which indicates that a relatively complex mixture of autoxidatively-derived volatiles, including the 2,4-heptadienals, the 2,4-decadienals, and the 2,4,7-decatrienals together elicit unmistakable, oxidized fish-oil aromas (3, 9, 18). Additionally, reports also suggest that contributions from (Z -4-heptenal may add characteristic notes to the cold-store flavor of certain fish, especially cod (4-5). 

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