Autoxidation of saturated fatty acids

It has been suggested that autoxidation of saturated fatty acids and aldehydes occurs through a free-radical mechanism (13, 14). Supporting evidence of a radical chain mechanism was provided by Palamand and Dieckmann (15) who studied the autoxidation of hexanal. The reaction involves peroxycarboxylic acid as an intermediate (16) and probably proceeds via the mechanism shown in Figure 1. 

Btodnitz, M.H. Autoxidation of saturated fatty acids. A review. J. Agric. Food Chem. 1968, 16, 994-999. 

Fig. 3.37. Autoxidation of saturated fatty acids. Postulated reaction steps involved in formation of methyl ketones...

Autoxidation of saturated hydrocarbons is, like autoxidation of saturated fatty acids, important at higher temperatures (around 150 °C). The final odorous products formed are mainly fatty acids, their lactones, alcohols and ketones with fewer carbon atoms in the... 

The hydroperoxides formed in the autoxidation of unsaturated fatty acids are unstable and readily decompose. The main products of hydroperoxide decomposition are saturated and unsaturated aldehydes. The mechanism suggested for the formation of aldehydes involves cleavage of the isomeric hydroperoxide (I) to the alkoxyl radical (II), which undergoes carbon-to-carbon fission to form the aldehyde (III) (Frankel et al. 1961). 

Milk is characterized as having a pleasing, slightly sweet taste with no unpleasant after-taste (Bassette et al., 1986). However, its bland taste makes it susceptible to a variety of flavor defects. Autoxidation of unsaturated fatty acids gives rise to unstable hydroperoxides, which decompose to a wide range of carbonyl products, many of which can contribute to off-flavors in dairy products. The principal decomposition products of hydroperoxides are saturated and unsaturated aldehydes (Frankel et al., 1961), with lesser amounts of unsaturated ketones (Stark and Forss, 1962), saturated and unsaturated hydrocarabons (Forss et al., 1961), semialdehydes (Frankel et al., 1961) and saturated and unsaturated alcohols (Hoffman, 1962 Stark and Forss, 1966). 

A product with a higher melting point is necessary for consumer acceptance. In addition, triacylglycerides that are more unsaturated tend to spoil more rapidly. This spoilage is due to oxidation caused by radical reactions. (This is an example of the au-toxidation process described in Section 21.8 and the Focus On box Vitamin E and Lipid Autoxidation on page 937. The hydrogens on the allylic carbons of unsaturated fatty acid residues are more readily abstracted because the resulting radicals are stabilized by resonance, so these compounds oxidize and spoil faster.) However, there is a trade-off, because it has been demonstrated that saturated fats have more deleterious health consequences than unsaturated fats do. 

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. 

The selectivity of autoxidation decreases above 60 °C since the hydroperoxides formed are subjected to homolysis giving hydroxy and alkoxy radicals (Reaction RS-4 in Fig. 3.19) which, due to their high reactivity, can abstract H-atoms even from saturated fatty acids. 

Saturated and unsaturated hydrocarbons with odd and even numbers of carbon atoms in the molecule (about C11-C35) are present as the primary substances in all vegetable oils and animal fats. Alkanes, alkenes, alkadienes and alkatrienes also arise as oxidation products of unsaturated fatty acids, catalysed by lipoxygenases or by autoxidation of fatty acids during food storage and processing. Only the lower hydrocarbons can play a role as odour-active substances. The main hydrocarbons resulting from oxidation of unsaturated fatty acids are ethane from Hnolenic acid, pentane and butane from Hnoleic acid and hexane and octane from oleic acid. The immediate precursors of hydrocarbons are the fatty acid hydroperoxides (Table 8.4). The unsaturated hydrocarbons are predominantly (Z)-isomers. Numerous other hydrocarbons, including ahcycHc hydrocarbons, appear as secondary hpid oxidation products. 

The secretion of the red hartebeest is characterized by its high aldehyde content. Of an estimated 100 detectable constituents, 25 are saturated and unsaturated aliphatic aldehydes [138]. Because the aldehydes are highly susceptible to autoxidation, the secretion could therefore only be used for shortterm territorial marking. On the other hand, the conversion of the aldehydes to carboxylic acids could also be transmitting information with a date stamp . In this regard, it is debatable whether fatty acids, which are almost ubiquitous in the animal world, really are such major carriers of semiochemical information in all of the many species in which they are purported to fulfill this role. 

In addition to the major fatty acids, milk also contains many minor polyunsaturated acids (Kurtz 1974) hence the autoxidation of dairy products can lead to a multitude of saturated and unsaturated aldehydes. 

Litwinienko, G, Daniluk, A., and Kasprzyska-Guttman, T. 2000. Study on Autoxidation Kinetics of Fats by Differential Scanning Calorimetry. 1 Saturated C12-C18 Fatty Acids and Their Esters. Ind. Eng. Chem. Res., 39,7-12. 

One additional H abstraction reaction must be mentioned. Internal 1,5 (Reaction 54) or 1,6 (Reaction 55) hydrogen abstraction generates an alcohol and a radical (21) in a position that may or may not be normal for autoxidation. Intramolecular H abstraction involving a six-membered transition state (Reaction 55) has been identified in saturated alkyls with long side chains (304). Occurrence of the corresponding reaction in unsaturated fatty acids would produce oxidation at sites previously attributed to HO attack (314). 

Oxidation of unsaturated acyl chains of lipids is a major route to volatile compounds during cooking of fat-containing food of either animal or vegetable origin. The unsaturated fatty acids, readily susceptible to the attack by oxygen, form hydroperoxides which in themselves are odorless and tasteless. The compounds that influence the flavor of the product result from a further breakdown of these hydroperoxides, and, normally, include saturated and unsaturated aldehydes, alcohols, and ketones. The carbonyl compounds resulting from autoxidation impart specific flavors that are normally detrimental to food products (Table 9.3). It should be pointed out, however, that they may also contribute to the desirable characteristic flavor of foods [48]. 

Earlier applications of ESR to the study of lipid free radicals and autoxidation were reviewed by Chapman (1965). More recently. Brown and Wiithrich (1977) have shown an elegant example of a spin label study of lipid oxidation catalysed by haem proteins, and Bascetta etal. (1983, 1984) have used ESR to study the abstraction of hydrogen from saturated, olefinic and acetylenic fatty acids and esters. 

Unsaturated lipids produce qualitatively similar products when thermally oxidized or autoxidized at low temperatures. These include a series of aldehydes, ketones, acids, esters, alcohols, hydrocarbons, lactones, cyclic compounds, dimers and polymers. However, quantitative pattern of the decomposition products formed at high temperatures is different from that of autoxidation, varying widely depending on the nature of the substrate and parameters of heat treatment (Nawar, 1985 Pokorny, 1989). Unsaturated fatty acids are much more susceptible to oxidation than their saturated analogs. According to Frankel (1980), at 25 to 80 °C, relative proportions of isomeric hydroperoxides isolated from each substrate varies with the oxidation temperature, however, their qualitative pattern remains the same. At oxidation temperatures higher than 80°C, isolation and quantitation of hydroperoxide intermediates is difficult due to their extreme heat sensitivity. Furthermore, the primary decomposition products are unstable and rapidly undergo further oxidative decomposition. As the oxidative process continues, a variety of possible reaction mecha-... 

After acidification, the lipid fraction is extracted into hexane and the solvent subsequently removed by evaporation. This gives a material consisting of 60-75% (w/w) CLA (depending on the starting materials), with the remainder consisting of unchanged saturated and monounsaturated fatty acids from the starting material. Deodorization of CLA removes volatile compounds such as hexane that may be residual solvent or secondary autoxidation products from CLA. Fractional distillation and crystallization remove metal catalysts (which may stimulate oxidation), as well as undesirable components such as dimers and polymers. The reactor type will affect the amounts of metals found. 

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