Edible oils autoxidation

ECN (effective carbon number), 689, 690, 708 Edible oils analysis, 636, 701 autoxidation, 614, 623, 661-2 peroxide value, 657, 658, 660-3 EDTA... 

A process with potential practical applicability is the hydrogenation of edible oils. Reduction of multiply unsaturated triglycerides with hydrogen over Ni-based catalysts is frequently used to gain autoxidative stability of edible oils. According to the Polanyi-Horiuti mechanism, multiple 1,2 or 1,4 diadsorption of the fatty acid tail with exclusively c/s-configuration around the double bonds causes cis-trans isomerisation, whilst the number of double bonds is being reduced. The trans-fatty acid chains have adverse effects on the human metabolism and must be minimized. 

The most important chemical reactions for triglycerides (fats and edible oils) are hydrolysis, methanolysis, and interesterification. The other reactions, such as hydrogenation, isomerization, polymerization, and autoxidation that are primarily relevant to the processing of edible oils and fats are also discussed in this section. 

In order to avoid the autoxidation problem it has become a common practice in the edible oil industry to construct reaction vessels of stainless steel. Citric acid is often added as a metal sequestrant to effectively inactivate trace metal ions. 

During this process, the position and geometry of the double bond may change. The hydroperoxide mixtures produced by autoxidation and photooxidation are not the same, indicating that different mechanisms are involved. Free radical oxidation can be promoted or inhibited. Deliberate promotion speeds the polymerization of drying oils, and strenuous efforts are made to inhibit the onset of rancidity in edible oils. Frankel has recently reviewed this topic in depth (41) see also (1) for an extensive discussion of oxidation of food lipids. 

Carotenoids are present in edible oils at different levels. These are powerful antioxidants against both autoxidation and photo-oxidation. Therefore, attempts have been made to retain them or recover them, as in the case of palm oil. However, carotenoids may be degraded to colorless products at high temperatures exceeding 150°C. 

Oxidative stability of edible oils depends primarily on their fatty acid composition and, to a lesser extent, in the stereospecific distribution of fatty acids in the triacyl-glycerol molecules. The presence of minor components in the oils also affects their oxidative stability. A detailed discussion of oxidative processes in fats and oils is provided elsewhere in this series. Oxidation may occur via different routes and includes autoxidation, photo-oxidation, thermal oxidation, and hydrolytic processes, all of which lead to production of undesirable flavor and products harmful to health. Flavor and odor defects may be detected by sensory analysis or by chemical and instrumental methods. However, chemical and instrumental procedures are often employed in the processing and during usage of edible oils. Indicators of oxidation are those that measure the primary or secondary products of oxidation as well as those from hydrolytic processes or from thermal oxidation, including polymers and polar components (15). 

Another potential area of application of FTIR spectroscopy is in the determination of the oxidative status or stability of an oil. Autoxidation is a major deteriorative reaction affecting edible fats and oils, and it is of major concern to processors and consumers from the standpoint of oil quality, as the oxidative breakdown products cause marked off flavours in an oil. A wide range of end products are associated with the autoxidative deterioration of fats and oils, the most important being hydroperoxides, alcohols, and aldehydes. Moisture, hydrocarbons, free fatty acids and esters, ketones, lactones, furans, and other minor products may also be produced, with the free fatty acids becoming more important in thermally stressed oils. In addition, there is significant cis to trans isomerisation and conjugation of double bonds in the hydroperoxides formed as an oil oxidises. 

The first stage of autoxidation of a pure oil is easily traceable by applying conventional wet chemistry, for example determination of the peroxide value. Peroxides are known to be heat labile compounds, which undergo breakdown at elevated temperatures to form simple hydrocarbons. Thus ethane and pentane are the predominant breakdown products of linolenate and linoleate peroxides, respectively (Evans et al., 1967). Scholz and Ptak (1966) and Evans et al. (1969) proposed a gas chromatographic method to measure rancidity in edible oils whereby the undiluted oil is injected directly into the hot injector (250°C). At these temperatures lipid peroxides are... 

The concentration of heavy metal ions that results in fat (oil) shelf-life instability is dependent on the nature of the metal ion and the fatty acid composition of the fat (oil). Edible oils of the linoleic acid type, such as sunflower and com germ oil, should contain less than 0.03 ppm Fe and 0.01 ppm Cu to maintain their stability. The concentration limit is 0.2 ppm for Cu and 2 ppm for Fe in fat with a high content of oleic and/or stearic acids, e. g. butter. Heavy metal ions trigger the autoxidation of unsaturated acyl lipids only when they contain hydroperoxides. That is, the presence of a hydroperoxide group is a prerequisite for metal ion activity, which leads to decomposition of the hydroperoxide group into a free radical ... 

Sea mammals, whales and seals, and fish of the herring family serve as sources of marine oils. These oils t) ically contain highly unsaturated fatty acids with 4—6 allyl groups, such as (double bond positions are given in brackets) 18 4 (6, 9, 12, 15) 20 5 (5, 8, 11, 14, 17) 22 5 (7, 10, 13, 16, 19) and 22 6 (4, 7, 10, 13, 16, 19) (Table 14.4). Since these acids are readily susceptible to autoxidation, marine oils are not utilized directly as edible oils, but only after hydrogenation of double bonds and refining. 

Rapeseed Canola) oil is used as an edible oil. It is susceptible to autoxidation because of its relatively high content of linolenic acid. It is saturated by hyrogenation to a melting point of 32-34 °C and, with its stability and melting properties, resembles coconut oil. 

Linseed Oil. Flax, used for fiber and seed production and the subsequent processing of the seed into linseed oil, is grown mainly in Canada, China and India (cf. Table 14.0). Due to its high content of linolenic acid (cf. Table 14.11), the oil readily autoxidizes, one of the processes by which some bitter substances are created. Since autoxidation involving polymerization reactions proceeds rapidly, the oil solidifies Jast drying oir). Therefore, it is used as a base for oil paints, varnishes and linoleum manufacturing, etc. A comparatively negligible amount, particularly of the coldpressed oil, is utilized as an edible oil. 

During lipid oxidation, the primary oxidation products that are formed by the autoxidation of unsaturated lipids are hydroperoxides, which have little or no direct impact on the sensory properties of foods. However, hydroperoxides are degraded to produce additional radicals which further accelerates the oxidation process and produce secondary oxidation products such as aldehydes, ketones, acids and alcohols, of which some are volatiles with very low sensory thresholds and have potentially significant impact on the sensory properties namely odor and flavor [2, 3]. Sensory analysis of food samples are performed by a panel of semi to highly trained personnel under specific quarantined conditions. Any chemical method used to determine lipid oxidation in food must be closely correlated with a sensory panel because the human nose is the most appropriate detector to monitor the odorants resulting from oxidative and non-oxidative degradation processes. The results obtained from sensory analyses provide the closest approximation to the consumers approach. Sensory analyses of smell and taste has been developed in many studies of edible fats and oils and for fatty food quality estimation [1, 4, 5]. 

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