Polyunsaturated fatty acids decomposition

AH the bis- and tri-unsaturated prostanoids display sensitivity to atmospheric oxygen similar to that of polyunsaturated fatty acids and Hpids. As a result, exposure to the air causes gradual decomposition although the crystalline prostanoids ate less prone to oxygenation reactions than PG oils or solutions. 

The accumulation of hydroperoxides and their subsequent decomposition to alkoxyl and peroxyl radicals can accelerate the chain reaction of polyunsaturated fatty-acid p>eroxidation leading to oxidative damage to cells and membranes as well as lipoproteins. It is well-recognized that transition metals or haem proteins, through their... 

Mukai, F.H. Goldstein, B.D. (1976) Mutagenicity of malonaldehyde, a decomposition product of peroxidized polyunsaturated fatty acids. Science, 191, 868-869... 

Direct Measurements—UVNIS Techniques. The conjugated diene (CD) formed among the polyene hydroperoxide products that are formed as a result of oxidation of polyunsaturated fatty acids (PUFAs) have a UV absorbance that can be monitored to follow the progress of the oxidation. The effect of antioxidants on the suppressed rate of product formation can be followed with time. For example, conjugated dienes from oxidation of linoleate lipid molecules absorb at 234 nm and can be monitored directly , or else after HPLC separation (via normal phase or reverse phase j of the individual isomers. In order to use these findings to calculate the antioxidant activity of phenols and relate it to oxygen uptake studies (equations 7 and 14), one also has to make a correction to account for loss of absorbance due to loss (from decomposition) of hydroperoxides (equation 22) . 

Peroxide value (P V) is the most commonly used measurement of lipid oxidation. The standard iodometric method requires a relatively large sample (5 g) when the lipid is only slightly oxidized. The ferric thiocyanate method, based on the oxidation of ferrous to ferric ion, involves colorimetric measurement of ferric thiocyanate. This method is more sensitive than the iodometric method and requires a relatively small sample (0.1 g). The PV is a useful measure for samples with low levels of oxidation and when the hydroperoxides are not decomposed. During prolonged oxidation, a maximum PV is reached and the value then begins to decrease due to peroxide degradation. This maximum value occurs early for soybean and rapeseed oil, due to the more rapid decomposition of the hydroperoxides of the polyunsaturated fatty acids. 

During the last steps of lipid oxidation, the fatty acid chains breakdown to give aldehydes (hexanal, propanal, malondialdehyde), depending on the lipid structure. These compounds react with thiobarbituric acid to give coloured compounds the measurement of which at 535 nm can be used to follow the oxidation process in its terminal phase [91]. In addition, hexanal, which is an important decomposition product of n-6 polyunsaturated fatty acid peroxidation in rat liver samples, human red blood cell membranes, and human LDL (low density lipoproteins), can be measured by headspace gas chromatography [92]. Malondialdehyde, another important decomposition product, can also be analysed by GC (Gas Chromatography) [93], and, after reaction with urea to give 2-hydroxypyrimidine, by HPLC [94]. 

These decomposition reactions become especially important with polyunsaturated fatty acids and at elevated temperatures or in the presence of metal catalysts (see Chapter 4). 

Because the 12,15-diene has a 1,4-diene system, it oxidizes like linoleate to form two conjugated dienoic 12- and 16-hydroperoxides. However, in contrast to linoleate, the external 16-hydroperoxide is formed at a higher concentration than the internal 12-hydroperoxide, (with normalized concentrations of 58% and 42%) (Figure 2.12). Therefore, both the 9,15- and 12,15-dienes produce allylic radicals in which the terminal carbons 16 and 17, closest to the end of the fatty acid chain, are the most reactive with oxygen. The same preference of oxygen attack at the terminal double bond position is also observed in other polyunsaturated fatty acids with n-3 double bonds (linolenate, eicosapentaenoic and docosahexaenoic acids), and n-6 double bonds (arachidonic acid). Volatile decomposition products derived from hydroperoxides containing an n-3 double bond are particularly significant for their impact on flavor (Chapter 4). 

The chemistry of antioxidant activity of nitrite is complex and not completely understood. Although much of the literature evidence for this activity is based on the notoriously imreliable TBA test (see Chapter 5), there is more specific evidence based on GC volatile analyses indicating significant reduction of hexanal and other decomposition products of polyunsaturated fatty acids. A number of mechanisms have been suggested for the antioxidant activity of nitrite it inhibits heme-catalysed lipid oxidation by forming a complex with the iron porphyrins, stabilizes the polyunsaturated lipids in membranes, chelates catalytic metals, forms inactive complexes with nonheme iron and copper, or low molecular weight iron fractions, produces... 

Thermal oxidation of unsaturated fats is accompanied by considerable isomerization of double bonds, leading to products containing trans double bonds and conjugated double bond systems. Non-volatile decomposition products are formed by thermal oxidation, hydrolysis and cyclization. A large number of hydroxy, alkoxy-substituted, epoxy and keto compounds are produced by oxidation, free fatty acids and mono- and diacylglycerols by hydrolysis, and cyclic monomers by cyclization of polyunsaturated fatty acids in heated fats. Aldehydo glycerides (also referred to as core aldehydes ) and keto glycerides... 

The second edition includes important developments in the characterization by capillary gas chromatography-olfactometry of aroma and flavor impact of volatile decomposition products from polyunsaturated fatty acids and esters. Discussions are included on various mechanisms for the formation from linoleate of 4-hydroxy-2-nonenal, which has received much attention in the biochemical literature because of its cytotoxic properties, and its occurrence in oxidized LDL and in vegetable oils heated at frying temperatures. Some of the volatiles produced from fish oils are responsible for major problems in then-utilization, because they produce very powerful fishy odors and flavors perceptible at extremely low levels of oxidation. 

After bleaching, some oils or fats which contain polyunsaturated fatty acids show an increase in absorbance at 270 nm. This is due to decomposition of hydroperoxides, formed by autoxidation, into oxo-dienes (I) and fatty acids with three double bonds (II) ... 

A solution of 1-2 % (v/v) concentrated sulfuric acid in methanol transesterifies lipids in the same manner and at much the same rate as methanolic hydrogen chloride. It is very easy to prepare, and it is thus the author s preferred reagent for esterification of free fatty acids, but utilised at a temperature below reflux. If the reagent is used carelessly, some decomposition of polyunsaturated fatty acids may occur. 

Mildness and Miscellaneous Reactions. The mildness of the lipases has been particularly well suited in transformations involving labile compounds that are likely to undergo decomposition when conventional chemical methods are applied, such as the long-chain polyunsaturated to-3-type fatty acids and highly labile prostaglandin precursor derivatives. Under mild conditions, lipase was exploited to hydrolyze the peracetal protected hydroperoxy derivative in eq 18 to afford the corresponding acid without affecting the peracetal protection moiety. ... 

The large number of precursors of volatile decomposition products affecting the flavor of oils has been discussed in Chapter 4. Only qualitative information is available on the relative oxidative stability of hydroperoxides, aldehydes and secondary oxidation products. As observed with the unsaturated fatty ester precursors, the stability of hydroperoxides and unsaturated aldehydes decreases with higher unsaturation. Different hydroperoxides of unsaturated lipids, acting as precursors of volatile flavor compounds, decompose at different temperatures. Hydroperoxides of linolenate and long-chain n-3 PUFA decompose more readily and at lower temperatures than hydroperoxides of linoleate and oleate. Similarly, the alkadienals are less stable than alkenals, which in turn are less stable than alkanals. The short-chain fatty acids produced by oxidation of unsaturated aldehydes will further decrease the oxidative stability of polyunsaturated oils. For secondary products, dimers are less stable than dihydroperoxides, which are less stable than cyclic peroxides. 

Reaction (14) producing stable hydroxy fatty acid derivatives is important, because it inhibits the decomposition of hydroperoxides into aldehydes and other volatile products that decrease the oxidative and flavor stability of polyunsaturated food lipids and contribute to biological damage (see Chapter 13). 

Nowadays, food scientists are including in their research and product development equations the production of snacks with minimal fat calories and fat decomposition products to counter contents known to have adverse effects on human health. At the same time they seek to produce snacks with similar sensory properties (flavor, texture, and overall acceptability) compared to the norm. A healthful balance of fatty acids (saturated monounsaturated polyunsaturated and omega 6 omega 3) and the production of snacks free of trans fatty acids are taken into consideration. 

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