Lipids thermal oxidation

Both intermuscular (depot) lipids and intramuscular (tissue) lipids play important roles. Table I shows the di-f-ference in the aldehydes produced -from thermally oxidized lipids -from meat o-f di-f-ferent species ( 6). ... 

Complex mixtures are produced by non-enzymatic browning reactions between thermally oxidized lipids and amines, amino acids and proteins (see Chapter 11.B.4). Interactions between aldehydes, epoxides, hydroxy ketones, and dicarbonyls with proteins cause browning that has been related with losses of lysine, histidine, and methionine. Schiff base formation results in polymerization to form brown macromolecules. Interactions between epoxyalkenals formed at elevated temperatures and reactive groups of proteins produce protein pyrroles polymers and volatile heterocyclic compounds. Much of the published research in this complex chemical area was based on model systems. More stmctural information is needed however with real foods subjected to frying conditions. 

Chapter 12 on Frying contains new material on complex interaction compounds produced by non-enzymatic browning reactions between thermally oxidized lipids and amines, amino acids and proteins. This chapter... 

MEDINA I, SATUE-GRACIA M T, GERMAN J B and FRANKEL E N (1999) Comparison of natural polyphenol antioxidants from extra virgin olive oil with synthetic antioxidants in tuna lipids during thermal oxidation, JAgric Food Chem, 47, 4873-9. 

The bulk of our knowledge regarding thermal oxidation has been derived from studies with model systems of fatty acids and their derivatives, or with individual natural oils (2,3,6,12,13,14,15,16). However, in biological systems as complex as food, lipids usually exist in a complicated environment markedly different from that of the single phase model system. In cell membranes, for example, the lipid molecules are highly ordered, relatively restricted in distance and mobility, and closely associated with different neighboring molecules, e.g., other lipids, protein, cholesterol, water, pro- and antioxidants. What influence does such an environment have on the oxidation of the lipids at elevated temperature Even in less organized systems, e.g., depot fat from animal or plant, the lipids... 

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. 

Autoxidation is the most common process leading to oxidative deterioration and is defined as the spontaneous reaction of atmospheric oxygen with lipids (3). The process can be accelerated at higher temperatures, such as those experienced during deep-fat frying, which is called thermal oxidation, with increases in free fatty acid and polar matter contents, foaming, color, and viscosity (4). Lfnsaturated fatty acids... 

The most important precursors for lipid oxidation are unsaturated fats and fatty acids like oleic (18 1), linoleic (18 2), linolenic (18 3) and arachidonic acid (20 4). The more unsaturated ones are more prone to oxidation. Lipid peroxidation and the subsequent reactions generate a variety of volatile compounds, many of which are odour-active, especially the aldehydes. That is why lipid oxidation is also a major mechanism for thermal aroma generation and contributes in a great measure to the flavour of fat-containing food. Lipid oxidation also takes place under storage conditions and excessive peroxidation is responsible for negative aroma changes of food like rancidity, warmed-over flavour, cardboard odour and metallic off-notes. 

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... 

The effect of lipids in the Maillard reaction has been studied by many authors who cooked or roasted mixtures of amino acids and reducing sugars in various vegetable oils. The thermal oxidative degradation of lipids generates lower molecules, for instance aldehydes, that contribute to the formation of heterocyclic volatile compounds. 

The biological properties of thermally oxidized fats have been studied for many years. Evaluation of laboratory-heated and commercially-used fats in diets for animals have included feed consumption, weight gain, and feed efficiency ( l-4), pathology ( 5- ), organ weights (, 10), enzyme assays (11), and total lipid... 

Moderately thermally oxidized soybean oil (130°C, air flow-through) of peroxide value (PV) = 75 mEq 02/kg diet (compared with the control of 9.5 Eq 02/kg diet) was fed to rats for 40 days (Eder and Kirchgessner, 1999). The study showed no adverse effects on liver, heart, kidney, or adipose tissue FA composition, and even a reduction in the osmotic fragility of erythrocytes and hepatic lipogenesis. However, the moderately oxidized oil slightly reduced the vitamin E status in the tissues. A slightly increased susceptibility of LDL to lipid peroxidation, and an increased concentration of thiobarbituric acid reactive substances (TEARS) in LDL, were also observed. 

The mechanism of initiation of lipid oxidation has been debated for many years. The most likely initiation process is the metal-catalysed decomposition of preformed hydroperoxides. The thermal oxidation of unsaturated lipids is usually autocatalytic and involves initiation by decomposition of hydroperoxides, which is generally considered metal-catalysed because it is very difficult or nearly impossible to eliminate trace metals that act as potent catalysts for reactions (3) and (4). 

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-... 

Bhalerao, V.R., Inoue, M. and Kummerow, F.A. (1963) Fatty acid composition of lymph lipids from rats fed fresh thermally oxidized fats. J. Dairy Sci. 46, 176-180. 

Jethmalani, S.M., Viswanathan, G., Bandyopadhyay, C., Noronha, J.M. (1989) Effect of ingestion of thermally oxidized edible oils on plasma lipids, lipoproteins and postheparin lipolytic activity of rats. Indian J. Exp. Biol. 27, 1052-1055. 

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