Lipid decomposition volatiles

Lipid decomposition volatiles. Reactions of sugar and amino acids give rise to odor profiles that are, at best, common to all cooked or roasted meats. The water soluble materials extracted from chicken, pork, or beef give reasonably similar meat flavor. To develop a species specific aroma one needs to study the lipid fraction and the volatiles produced from those lipids. The work of Hornstein and Crowe (10) reported that the free fatty acids and carbonyls generated by heating will establish the specific species flavor profiles. 

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. 

The volatiles identified in cooked chicken were the same as those of beef, but with higher levels of volatiles derived from unsaturated lipids (2,4-decadienal, and 7-dodecalactone), and lower beef aroma compound bis-(2-methyl-3-furyl)-disulfide. Chicken meat has a higher level of linoleic acid than beef and would be expected to contribute more volatile lipid decomposition products. 

A simple method for assessing lipid oxidation is measuring the headspace concentration of hexanal by capillary GLC. Also, the total volatiles appearing in the chromatogram up to hexanal can be taken as oxidation index. The method was applied to determine the amounts of lipid peroxides present in rat liver cells. Enhancement of the hexanal concentration can be achieved on adding ascorbic acid (22), that reduces Fe(ni) present in the matrix to Fe(II), which catalyzes decomposition of hydroperoxides to aldehydes. Significant correlations are found between hexanal concentrations and various oxidation indices, such as TBARS (Section IV.D.2)" . ... 

Frankel, E.N. and Gardner, H.W. 1989. Effect of a-tocopherol on the volatile thermal decomposition products of methyl linoleate hydroperoxides. Lipids 24 603-608. 

When lipids are heated a large variety of compounds are formed. Hydrolysis of the ester linkage occurs, liberating glycerol and volatile fatty acids. It 1s well known that thermal decomposition of triglycerides generates a series of compounds including alkanals, alkenals, alkadlenals, methyl ketones, lactones, and hydrocarbons. 

The products of lipid oxidation in monolayers were also studied. Wu and coworkers (41) concluded that epoxides rather than hydroperoxides might be the major intermediates in the oxidation of unsaturated fatty acids adsorbed on silica, presumably because of the proximity of the substrate chains on the silica surface. In our work with ethyl oleate, linoleate and linolenate which were thermally oxidized on silica, the major decomposition products found were those typical of hydroperoxide decomposition (39). However, the decomposition patterns in monolayers were simpler and quantitatively different from those of bulk samples. For example, bulk samples produced significantly more ethyl octanoate than those of silica, whereas silica samples produced more ethyl 9-oxononanoate than those of bulk. This trend was consistent regardless of temperature, heating period or degree of oxidation. The fact that the same pattern of volatiles was found at both 60°C and 180°C implies that the same mode of decomposition occurs over this temperature range. 

Volatile compounds generated by model systems of zeln, corn amylopectin and corn oil extruded at barrel temepratures of 120°C and 165°C were analyzed by GC and GC/MS. The largest quantities of lipid oxidation products were detected in systems containing all three components. In each system, the quantity of 2,4-deca-dienal was low relative to the quantities of hexanal, heptanal and benzaldehyde. Identification of the Maillard reaction products, 2-methyl-3(or 6)-pentyl-pyrazine, 2-methyl-3(or 6)-hexylpyrazine and 2,5-di-methyl-3-pentylpyrazine, suggested that lipid-derived aldehydes might be involved in the formation of substituted pyrazines. 4-Methylthiazole was identified as a major decomposition product of thiamin when corn meal containing 0.5% thiamin was extruded at a final temperature of 180°C. 

Review of all the scission reactions responsible for the hundreds of volatile products in lipid oxidation is beyond the scope of this chapter. The reader is referred to the available reviews (3, 314, 340, 341, 347) for further details. The scission pattern of hydroperoxide epidioxides from linoleic acid is included here to show how the decompositions can become quite complex (Figure 14), and lists of typical products resulting from scission reactions of oleic, linoleic, and linolenic acids are presented in Table 12. 

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. 

The primary oxidation products (hydroperoxides) are unstable and susceptible to decomposistion. A complex mixture of volatile, nonvolatile, and polymeric secondary oxidation products is formed through decomposition reactions, providing various indices of lipid oxidation (5). Secondary oxidation products include aldehydes, ketones, alcohols, hydrocarbons, volatile organic acids, and epoxy compounds, among others. Methods for assessing lipid oxidation based on their formation are discussed in this section. 

Analysis Measure relatively low levels of oxidation (below 1%) and include measurement of initial or primary products of lipid oxidation (e.g., hydroperoxides, conjugated dienes) as well as secondary decomposition products of lipid oxidation (e.g., carbonyls, volatiles, dialdehydes). 

The major precursors in meat flavors are die water-soluble components such as carbohydrates, nucleotides, thiamine, peptides, amino acids, and the lipids, and Maillard reaction and lipid oxidation are the main reactions that convert these precursors in aroma volatiles. The thermal decomposition of amino acids and peptides, and the caramelization of sugars normally require temperatures over 150C for aroma generation. Such temperatures are higher than those normally encountered in meat cooking. During cooking of meat, thermal oxidation of lipids results in the formation of many volatile compounds. The oxidative breakdown of acyl lipids involve a free radical mechanism and the formation of... 

Carbonyl compounds in oxidized lipids are the secondary oxidation products resulting from the decomposition of the hydroperoxides. They can be quantified by the reaction with 2,4-dinitrophenylhydrazine and the resulting colored hydrazones are measured spectrophotometrically at 430-460 nm. The carbonyl value is directly related to sensory evaluation, because many of the carbonyl molecules are those responsible for off-flavor in oxidized oil. The anisidine value is a measure of carbonyl compounds that have medium molecular weight and are less volatile (Frankel 1998). It can be used to discover something about the prior oxidation or processing history of an oil. 

Because phospholipids enhance the decomposition of lipid hydroperoxides, they stimulate the formation of flavor-active volatiles in meat and meat products (Mot-tram, 1999). 

A large body of scientific evidence suggests that the loss of food palatability as a result of lipid oxidation is due to the production of short chain compounds from the decomposition of the hydroperoxides. The volatile compounds produced from the oxidation of edible oils are influenced by the composition of the hydroperoxides and the positions of oxidative cleavage of double bonds in the fatty acids." ... 

In such a system, the rate of oxidation is influenced by the emulsion composition (relative concentrations of substrate and emulsifier) and especially, by the partition of the emulsifier between the interface and the water phases. Other factors influencing lipid oxidation in emulsions are particle size of the oil droplets, the ratio of oxidizable to non-oxidizable compounds in the emulsion droplets, and the packing properties of the surface-active molecules. In addition, the amount and composition of the oil phase in an emulsion are important factors that influence oxidative stability, formation of volatiles, and partition of the decomposition products, between the oil and water phase. 

In actual lipid oxidation, one cannot overlook the critical role of trace metals, which complicate the kinetic sequences of initiation and decomposition of lipid hydroperoxides. These metals catalyse both initiation of free radicals and decomposition of hydroperoxides, which become particularly significant with polyunsaturated lipids containing more than two double bonds. With these polyunsaturated lipids, although the yields of hydroperoxides are reduced in the presence of metals, they produce volatile decomposition products that have a serious impact on flavor deterioration. In foods and biological systems, the mixture of trace metals and hydroperoxides is the most important initiator that plays a key part in the development of free radical oxidation and rancidity. The use of artificial azo compounds as initiators to study free radical oxidation is therefore not relevant. 

Frankel, E.N., Neff, W.E., Selke, E. and Weisleder, D. Photosensitized oxidation of methyl linoleate. Secondary and volatile thermal decomposition products. Lipids 17, 11-18 (1982). 

Flavor deterioration of food lipids is caused mainly by the presence of volatile lipid oxidation products that have an impact on flavor at extremely low concentrations, often at the parts per billion (ppb) levels. An understanding of the sources of volatile oxidation products provides the basis for improved methods to control and evaluate flavor deterioration. The decomposition of lipid hydroperoxides produces carbonyl compounds, alcohols and hydrocarbons under various conditions of elevated temperatures and in the presence of metal catalysts. 

Volatile decomposition products from autoxidized linoleic acid and methyl linoleate were characterized for their intense aroma and flavor impact by capillary gas chromatography-olfactometry. This technique involves sniffing the gas chromatograph effluent after stepwise dilution of the volatile extract. The most intense volatiles included hexanal, c/ -2-octenal, /ra. s-2-nonenal, l-octene-3-one, 3-octene-2-one and trans-l-ociQmX (Table 4.2). This analytical approach does not, however, consider the effects of complex interactions of volatiles occurring in mixtures produced in oxidized food lipids. 

In addition to the monohydroperoxides, unsaturated aldehydes and ketones undergo autoxidation and provide additional sources of volatile compounds. Nonvolatile secondary products that undergo further decomposition into volatile products include dimers and oligomers, hydroperoxy epoxides, hydroperoxy epidioxides and dihydroperoxides. These secondary products contain one or more hydroperoxide groups and decompose the same way as monohydroperoxides to produce similar volatile materials. As with the monohydroperoxides, the multitude of volatile compounds formed from secondary products are important contributors to the flavor quality of lipid-containing foods. 

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