Oxidatively derived volatiles

Table III. Effect of a lipoxygenase-specific inhibitor on the occurrence of enzymically- and oxidatively-derived volatiles in fish.

Phospholipids contribute specific aroma to heated milk, meat and other cooked foods through lipid oxidation derived volatile compounds and interaction with Maillard reaction products. Most of the aroma significant volatiles from soybean lecithin are derived from lipid decomposition and Maillard reaction products including short-chain fatty acids, 2-heptanone, hexanal, and short-chain branched aldehydes formed by Strecker degradation (reactions of a-dicarbonyl compounds with amino acids). The most odor-active volatiles identified from aqueous dispersions of phosphatidylcholine and phos-phatidylethanolamine include fra 5 -4,5-epoxy-c/5-2-decenal, fran5,fran5-2,4-decadienal, hexanal, fra 5, d5, d5 -2,4,7-tridecatrienal (Table 11.9). Upon heating, these phospholipids produced cis- and franj-2-decenal and fra 5-2-undecenal. Besides fatty acid composition, other unknown factors apparently affect the formation of carbonyl compounds from heated phospholipids. 

F Boukobza, PJ Dunphy, AJ Taylor. Measurement of lipid oxidation-derived volatiles in fresh tomatoes. Postharvest Biol Technol 23 117-131, 2001. 

The behaviour of sesquiterpenes, which are less volatile (fi-caryophyllene bp760 262°C), is quite different. Initially their migration from the matrix to the gaseous phase is predominant but beyond a certain temperature, which depends on the boiling point, the migration from the fibre to the gaseous phase predominates. For example, the optimal trapping temperature is lower (40°C) for fi-caryophyllene than for its oxide derivative (60°C). 

Erom HRTEM studies, it is proposed that the majority of the bismuth molybdate phases can be derived from the fluorite structure, in which both the cation and anion vacancies can be accommodated within the fluorite framework (Buttrey et al 1987). Several industrial processes containing multicomponent bismuth molybdates may suffer loss of Mo oxides by volatilization under operating conditions, resulting in the loss of catalytic activity. Monitoring the catalyst microstructure using EM is therefore crucial to ensuring the continuity of these catalytic processes. 

The study of manoyl oxide derivatives i.e. 7 and 8 in, Fig (7), (i.e ent-hydroxy and en/-acetoxy-3(3-manoyl oxides) isolated from Cistus creticus, by GC-MS resulted in only one peak indicative of the purity of the products [33]. From the H-NMR data it is clear that the 13-epi isomer was present in both derivatives [58,139]. The chromatographic data of the compounds 7 and 8 were recently published [33,63]. Hence, investigations have proven that, apart from the 13-epi isomer, there are more isomers with varying intensities, which correspond to isomers that arise from the different configuration of C-8 chiral center [33]. This isomer showing a different configuration at C-8 has been isolated from the volatile leaf oil of Alaska (yellow) cedar and its structure has been confirmed using spectroscopic methods as well as chemical reactions [150],... 

Oxo complexes are important aspects of the high valent chemistry of Nb and Ta. Much of the interest derives from their use as soluble reactivity models of metal oxo fragments relevant to heterogeneous oxidation catalysis. Volatile or hydrolysable alkoxo species have been explored as precursors for oxide films. 

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). 

The definition of oxidized fish oil-like aromas still leave fresh fish aromas undefined. Various freshly harvested fish have distinguishing aromas, but they also are characterized by a common plant-like, seaweed-like aroma. Thus, compounds and reaction pathways different from random autoxidation appear likely and reasonable. Even conflicting descriptions of fishy odors, i.e., including roles for volatile amines (2 19) and sulfur compounds (20-22), can be accommodated by the hypothesis that previously unrecognized biochemical reactions yield characterizing fresh fish aromas. These premises led to investigations (23-26) which have resulted in the identification of a group of enzymically-derived volatile aroma compounds that contribute fresh, plant-like aromas to freshly harvested fish (Table I). 

In addition to the enzymically-derived volatile aroma compounds (Table I), low levels of autoxidatively-derived carbonyls can also be detected in harvested fish held a day on ice, and these volatiles are listed in Table II. The oxidatively-derived carbonyls modify the fresh plant-like aromas of fresh fish by providing oxidized-oil-like, staling fish-type odor notes (3-5, 9). The formation of hexanal in freshly harvested fish appears to be enzymic because the concentration of this compound can be diminished by lipoxygenase inhibitors (25). However, when fresh fish are stored on ice or are held under frozen storage, hexanal concentrations also increase because of autoxidative processes (40). 

Lipid-derived volatile compoimds dominate the flavor profile of pork cooked at temperatures below 100°C. The large numbers of heterocyclic compounds reported in the aroma volatiles of pork are associated with roasted meat rather than boiled meat where the temperature does not exceed 100 C (34,35). Of flie volatiles produced by lipid oxidation, aldehydes are the most significant flavor compounds (35,36). Octanal, nonanal, and 2-undecenal are oxidation products from oleic acid, and hexanal, 2-nonenal, and 2,4-decadienal are major volatile oxidation products of linoleic acid. 

Lipid oxidation starts in raw beef and continues during cooking. Mottran et al. (21) demonstrated that the intramuscular lipids (not the adipose tissues) are responsible for the formation of most of the lipid-derived volatiles. Intramuscular lipids consist of marbling fat made primarily of triglycerols and structural or membrane lipids made of phospholipids. The phospholipids contain relatively high amount of unsaturated fatty acids more prone to oxidation. In beef, the intramuscular tissue phospholipids are sufficient in imparting a full meaty aroma (22). 

Liquids burn in the vapour phase, so that a liquid has to evaporate in order to form a combustible vapour layer above the surface of the liquid. A liquid is thermally stable when its molecules in the volatilized vapour phase are identical to those in the liquid phase. Most combustible liquids are thermally unstable i. e. their chemical constitution is changed by heating. In the vapour phase, therefore, molecules of the original liquid are accompanied by their thermal decomposition products as well as by their oxidized derivatives. This complex vapour mixture is mixed with air, resulting in a combustible composition within certain concentration limits just as for gases as described above. 

As compared with other platinum metals, osmium and iridium have some specific features to which they owe their names. Osmium derives from the Greek osme for smell since osmium oxide is volatile and has a peculiar smell. Iridium got its name from the variety of colouring of its salts (from the Greek iris for rainbow ). A painter could have prepared an entire palette from iridium paints if they were not so expensive. These unusual properties promoted the discovery of these platinum metals. 

The hydroperoxides of oleate subjected to photosensitized oxidation not only produced the volatiles expected from the 9- and 10-isomers, but also those derived from the 8- and 11-isomers. These results are explained by the interconversion between the 9- and 10-hydroperoxides of oleate formed by photosensitized oxidation into a mixture of 8-, 9-, 10-, and 11-hydroperoxides (Figure 4.11). Although both types of hydroperoxide produce the same major volatiles, the photosensitized oxidation-derived hydroperoxides produced more of the volatiles derived from the 9- and 10-hydroperoxide isomers (octane, 1-octanol, 2-decenal, and methyl octanoate). Small amounts of volatile... 

Excess molybdenum, as derived from Reactions 4 and 6, can oxidize to volatile M0O3 or can be incorporated into the carbide lattice, forming solid solutions. The reactions 3-7 discussed above are feasible and the products, according to the Zr-Si binary phase diagram, are liquid at the sintering temperature applied. 

It has been stated that many halogen-free compounds, e.g., certain derivatives of pyridine and quinoline, purines, acid amides and cyano compounds, when ignited on copper oxide impart a green colour to the dame, presumably owing to the formation of volatile cuprous cyanide. The test is therefore not always trustworthy. The test is not given by duorides. 

Other volatile compounds of elements can be used to transport samples into the plasma flame. For example, hydride reduction of mercury compounds gives the element (Hg), which is very volatile. Osmium can be oxidized to its volatile tetroxide (OSO4), and some elements can be measured as their volatile acetylacetonate (acac) derivatives, as with Zn(acac)2. 

Preparation and Properties of Organophosphines. AUphatic phosphines can be gases, volatile Hquids, or oils. Aromatic phosphines frequentiy are crystalline, although many are oils. Some physical properties are Hsted in Table 14. The most characteristic chemical properties of phosphines include their susceptabiUty to oxidation and their nucleophilicity. The most common derivatives of the phosphines include halophosphines, phosphine oxides, metal complexes of phosphines, and phosphonium salts. Phosphines are also raw materials in the preparation of derivatives, ie, derivatives of the isomers phosphinic acid, HP(OH)2, and phosphonous acid, H2P(=0)0H. 

Ketones are more stable to oxidation than aldehydes and can be purified from oxidisable impurities by refluxing with potassium permanganate until the colour persists, followed by shaking with sodium carbonate (to remove acidic impurities) and distilling. Traces of water can be removed with type 4A Linde molecular sieves. Ketones which are solids can be purified by crystallisation from alcohol, toluene, or petroleum ether, and are usually sufficiently volatile for sublimation in vacuum. Ketones can be further purified via their bisulfite, semicarbazone or oxime derivatives (vide supra). The bisulfite addition compounds are formed only by aldehydes and methyl ketones but they are readily hydrolysed in dilute acid or alkali. 

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