Oxidative stability of vegetable oils

Gertz, C. Klostermann, S. Kochhar, S.P. 2000. Testing and comparing oxidative stability of vegetable oils and fats at frying temperature. Eur. J. Lipid Sci. Tech. 102 543-551. 

Thermal analysis methods, such as pressurized differential scanning calorimetry (PDSC), are popular for the determination of oxidative stabilities of vegetable oils (33, 36, 37). 

The detection and quantification of tocopherols, carotenoids, and chlorophylls in vegetable oil were effectively used for authentication pnrposes. The presence of tocopherols, carotenoids, and chlorophylls influence the oxidative stability of vegetable oils and their potential health benefits. Puspitasari-Nienaber et demonstrated the application of a rapid and reliable analysis method of direct injection of C-30 RP-NPLC with electrochemical detection for the simultaneous analysis of the above mentioned substances. Aliquots of vegetable oils were dissolved in appropriate solvents and injected directly without saponification, thus preventing sample loss or component degradation. Thus the effective separation of tocopherols, carotenoids, and chlorophylls was achieved. 

Quality of soybean oil is affected by a number of factors such as quality of soybeans, content of moisture, storage temperature and processing conditions, exposure to air, prooxidants, light, minor compounds such as free fatty acids, mono- and diacylglycerols, phospholipids, oxidized triacylglycerols, tocopherols, triterpene alcohols, phytosterols, isoflavonones, and possibly some unknown compounds contained in soybean oil. Some of these minor compounds affect the oxidative stability of vegetable oils. 

Matthaus, B. W. 1996. Determination of the oxidative stability of vegetable oils by Rancimat and conductivity and chemiluminescence measurements. J. Am. Oil Chem. Soc. 73 1039-1043. 

In edible oils with high content of imsaturated fatty acids, for example, Brazil nut oil, there is a reduction in oxidative stability due to the presence of monounsatu-rated and polyunsaturated double bonds, such as those present in oleic, linoleic, and linolenic acids. Studies reported effects of unsaturations on the oxidative stability of vegetable oils with high oleic add content and its relationship with the addition of antioxidants [8]. 

Subsequent work by Zambiazi and Przybylski (1998) also showed that fatty acid composition could only explain half of the oxidative stability of vegetable oils including canola oil. The other half was attributed to the amount and composition of endogenous minor components which can shorten or extend the shelf-life of an oil. Such endogenous components were later discussed by Przybylski and Eskin (2006) and included tocopherols, mono- and diacylglycerols, free fatty acids, phospholipids, chlorophylls and derivatives, carotenoids, phytosterols, phenolic compounds and trace metals. In addition, the position that the fatty acid occupies in the triacylglyc-erol can also affect stability. For example, the location of linolenic and linoleic acids on the sn-2 position has been reported to cause faster oxidation and lower stability compared to the same fatty adds on ml- and sn-3 positions. In contrast oleic acid at the sn-2 position proved stabler compared to its location on sn-1 and sn-3 positions (Neffetal., 1994,1997). 

Zambiazi, R.C. and Przybylski, R. 1998. Effect of endogenous minor components on the oxidative stability of vegetable oils. Lipid. Technol. 10 58-62. 

On the other hand, hydrogen at the allylic carbon-8 or carbon-11 of OA can be abstracted under the presence of free radicals and other initiators to produce two kinds of delocalized three-carbon alkyl radicals (Porter et al., 1995). And then, oxygen attack at the end-carbon positions of these intermediates prodnces a mixture of four kinds of isomeric OA mono-hydroperoxides (Frankel, 1998a). When the reactivity of OA and SFA snch as PA is compared, SFA shonld be less reactive to oxidation than OA becanse of no double bond in its molecnle. However, results obtained in the comparative study on the oxidative stability of vegetable oil TAG were more complicated. 

Initially, kinematic viscosities at 40°C and pour points of the fluids were determined (results shown in Table 8). It appears that low-temperature properties of vegetable oils are much more inferior to those of synthetic basestocks or even mineral oil. Cold storage properties of vegetable oils do not appreciably respond to the pour point depressants (PPD), as opposed to mineral oils. This is consistent with earlier observations (35). Oxidative stabilities of the oils are compared in Table 9. 

Oxidation effects Characterization of lipid oxidation by ID and 2D NMR Formation of cyclic fatty acids Analysis of non-volatile components produced upon heating of fatty acid esters Oxidative stability of sesame oil Effects of hydrogenation in vegetable oils 1999 (233, 235) 1997 (234) 1998 (236) 1997 (237) 1996 (238) 1998 (239, 240)... 

Soybean oil is miscible with many non-polar organic solvents. The solubility characteristics of vegetable oils in various solvents can be estimated from their dielectric constants or solubility parameters (Sipos and Szuhaj 1996a). Anhydrous or aqueous ethanol is not a good solvent for soybean oil at ambient temperature. Solubility increases with temperature until the critical solution temperature is reached, at which point the oil and ethanol become miscible. The solubility of oxygen in soybean oil contributes to the oxidative stability of the oil. It varies from 1.3 to 3.2ml/100ml in refined and crude oils. The solubility of water in soybean oil is about 0.071% at —1°C and 0.141% at 32°C (Perkins 1995b). 

Gertz Ch. and Kochhar, S.P. (2001) A new method to determine oxidative stability of vegetable fats and oils at simulated frying temperature. Oleagineux Corps Gras Lipides, 8, 82—88. 

Erhan, S.Z. B.K. Sharma J.M. Perez. Oxidation and low temperature stability of vegetable oil-based lubricants. Ind. Crop Prod. 2006,24, 292-299. 

The production of oxidative dimers and triglyceride oligopolymers by oxidation prior to deodorization decreases the stability of oils. Flavor and oxidative stability of soybean oil correlates with the concentration of oxidative dimers determined chromatographicaUy. The oxidative stabihty of vegetable oils can therefore be markedly improved, if any oxidation products present in partially processed oils can either be removed or chemically reduced to stable hydroxides before deodorization. 

Soybean oil has a high linoleic acid content of around 50.8% and 7% of linolenic acid and is thus susceptible to oxidation. Adverse effects caused by oxidation of vegetable oils include loss of essential fatty acids, production of off-flavors and toxic compounds (Sonntag, 1979). The oxidative stability becomes an important quality control parameter for the manufactures and users of vegetable oils. Although the oxidative stability of soybean oil has been greatly improved through the concerted efforts of universities, industries... 

---