Comparative study of the autoxidation of TAG containing conjugated and nonconjugated C₁₈ PUFA

Abstract

Four TAG containing linoleate (soybean-TAG), α-linoleate (perilla-TAG), conjugated linoleate (CLA-TAG), and conjugated linolenate (bitter gourd-TAG) were oxidized in the bulk phase at 50°C. The effects of α-tocopherol and Trolox (6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid) on the oxidation of these TAG were also studied. Progress of oxidation was determined by measuring the oxygen consumption, peroxide formation, and polymer formation. The rates of oxygen consumption and polymer formation of bitter gourd-TAG and CLA-TAG were faster than those of perilla-TAG and soybean-TAG, respectively. The present results revealed that the main oxidation products of bitter gourd-TAG and CLA-TAG were dimers and polymers, whereas hydroperoxides were the main products in the oxidation of perilla-TAG and soybean-TAG. These differences in the oxidation of TAG were characterized mainly by oxidation of the main PUFA contained in each TAG, namely, linoleate (56.1%) for soybean-TAG, α-linolenate (54.5%) for perilla-TAG, conjugated linoleate (69.5%) for CLA-TAG, and conjugated linolenate (61.6%) for bitter gourd-TAG, respectively. α-Tocopherol and Trolox inhibited the oxidation of TAG. The inhibitory effect of these antioxidants was more effective against the oxidation of CLA-TAG and bitter gourd-TAG than that of soybean-TAG and perilla-TAG, respectively. This was probably due to the high rate at which the antioxidants inhibited the dimerization and polymerization of CLA-TAG and bitter gourd-TAG.Conjugated PUFA are unique FA that contain conjugated double bonds in their molecules. Among these conjugated FA, CLA are well known, and many papers have been published on the biological activities of CLA (1-3). Conjugated linolenic acids (CLN) also reportedly show cytotoxic effects on mouse and human tumor cells (4); and the anticarcinogenic effect of CLN has been confirmed in an in vivo system (5). Seed oils from some kinds of edible plants contain a very high level (30-70%) of CLN (6). Therefore, these seed oils may be used for functional foods and as nutraceuticals.With the increasing interest in CLA and CLN, more information on the oxidative stability of these conjugated PUFA is required. Oxidation of PUFA not only produces rancid flavors in foods but also can decrease their nutritional quality and safety. Therefore, it is important to establish effective control methods against the oxidation of conjugated PUFA. Early studies (7) revealed differences between the oxidation of conjugated PUFA and their corresponding nonconjugated PUFA. Allen et al. (8) reported that the autoxidation of CLA formed less peroxide than the autoxidation of LA. In addition, Holman (7) found that polymers form with the oxidation of conjugated PUFA. Recently, Yurawecz et al. (9) reviewed the pathway of CLA oxidation, including hydroperoxide formation, and Hämäläinen et al. (10) showed the presence of monohydroperoxides in oxidized CLA. However, the oxidation of CLA and CLN is still not well understood, and less is known about the effects of antioxidants...

Pursuit of Oxidation Behavior for Conjugated Polyenoyl Glycerols and Establishment of Their Novel Oxidation Prevention Method

“…Furthermore, comparison of the increments of AnV and PC content which contains polymerized compounds revealed that the formation of PC content in secondary oxidation products occurred more rapidly in conjugated oils than in non-conjugated oils. The information described above corresponds quite well with the results of Suzuki et al 13) , which means that dimmer and polymer contents increased more rapidly with time in conjugated oil than in non-conjugated oil.…”

“…To reconfirm the report by Suzuki et al 13) in non-aqueous autoxidation test, CLA-TAG, LA-TAG, CLN-TAG, and LN-TAG ( ) were incubated in a 30 thermostatic chamber for autoxidation in the dark and under fluorescent light irradiation at 960 lux. The hydroperoxide, carbonyl compound, and polar compound (PC), polymer contents were measured over time using potentiometric titration of the peroxide value (PV), colorimetric determination of anisidine value (AnV), TLC-FID and GPC analyses, respectively.…”

“…To date, there have been several reports on the oxidative stabilities of conjugated fatty acids. Suzuki et al 13) reported that the autoxidative stabilities of CLA-TAG (containing 34.30% 9c,11t-CLA and 35.2% 10t,12c-CLA) and bitter gourd (Momordica charantia) seed oil (containing about 61.6% 9c,11t,13t-CLN) were lower than those of soybean oil and perilla oil, which are nonconjugated oils. Moreover, Lee et al 14) reported that t-butylhydroquinone conferred the most potent antioxidant activity on CLA of the seven synthetic antioxidants.…”

Autoxidation of Conjugated Linoleic Acid Methyl Ester in the Presence of α‐Tocopherol: The Hydroperoxide Pathway

“…Tsuzuki et al (2004) have, for example, reported that 9c,11t-CLA oxidizes at the same rate as linolenic acid (18:3; 9c,12c,15c) when monitored by substrate consumption, whereas when monitored by oxygen consumption it oxidizes at a similar rate to linoleic acid, and linolenic acid at a comparable rate to -eleostearic acid (18:3; 9c,11t,13t). Moreover, Van den Berg et al (1995) and Suzuki et al (2004) have demonstrated that peroxide value (PV) measurements produce incorrect results in kinetic studies when compared with result obtained by following substrate consumption.…”

“…It is well established by PV and oxygen consumption measurements (Allen et al 1949), by MS (Muizebelt et al 2000), and by HPSEC (Luna et al 2007) that the autoxidation profile of CLA differs from that of its nonconjugated counterpart. Peroxide products accumulate more slowly and only in small amounts compared to linoleic acid (Allen et al 1949;Suzuki et al 2004). CLA absorb less oxygen per mole of oxidized substrate than linoleic acid (Allen et al 1949;Tsuzuki et al 2004) and produces mainly polymeric products (Luna et al 2007).…”