Synergetic and antagonistic role of natural antioxidant in the autoxidation of soybean oil
Introduction
Soybean oil, an acyltriglyceride, one of the most popular vegetable oils, is an ingredient widely used in cooking and also in the food industry [1]. SBO contains many unsaturated fatty acids such as oleic, linoleic, and linolenic acids so that it can be easily oxidized by oxygen which has a triplet electronic configuration in ground state [2].
Therefore SBO has α-hydrogen, allylic, bisallylic hydrogen, methylene hydrogen and methyl groups. In general, double bonds present in fatty acids can offer several possibility to chemically modify the structure to improve some of their properties. One example is a hydrogenation of double bond [3] for solidification of SBO. Epoxidation is another example of widely used modification. Epoxidized soybean oil (ESBO) is commercially available and is used as a plasticizer for polyvinyl chloride [4] and can further be used to a new biocomposite [5], [6] because SBO and its epoxide are cross linked with mono-, and di-amine [5].
From the food and health-related standpoint, the presence of double bonds in fatty acids, however, are troublesome. They are vulnerable to oxidation even when stored at low temperatures and used in kitchen as frying oil which often renders SBO more prone to rancidity and quality decline for food use (autoxidation or lipid peroxidation) [7]. Therefore, many studies were carried out for the stabilization of substrates, and similar effects of α- and γ-tocopherol on formation of hydroperoxide from methyllinoleate [8], alkyl substituted ascorbic acids as free radical quenchers [9], and Icariin as antioxidant against free radical induced peroxidation of linoleic acid [10].
In China, Japan and Korea, the vegetable oil is used mainly for frying and cooking at high temperature often to the boiling temperature. Therefore the interacting behaviors between cooking oil and antioxidant could be of interest to investigate. Also autoxidations at the high temperature and high pressure were well studied and industrialized [7], [11]. Among them are cumyl process (90–130 °C, 5–10 atm) to produce acetone and phenol, and cyclohexane oxidation (125–165 °C, 8–15 atm) to cyclohexanone and cyclohexanol (so called KA oil), ethylene oxide production (250–300 °C, 10–20 atm), maleic anhydride from butene (350–450 °C, 2–3 atm) and/or benzene (400–450 °C, 2–5 atm), and many others.
In the food industry, extensive efforts have been made towards preventing or minimizing oxidation of SBO. In this regard, an antioxidant is very attractive due to its capability of slowing or preventing the oxidation of other molecules. Antioxidant is now widely accepted as a supplementary dietary ingredient. In medicine, it is also reported to have a potential efficacy in preventing diseases such as inflammation, cancer and diabetes [12]. Antioxidants have been used as preservatives in the food and, cosmetics industry, and preventing for the degradation of rubber and petroleum fuels.
Antioxidants are also called inhibitors, since they slow down or inhibit the oxidation of substrates having double bonds [13]. Antioxidants are often categorized into two classes, i.e., radical trapping and peroxide decomposer based on their mechanism in preventing oxidation [14]. Phenols and secondary aromatic amines are well known as the radical traps and phosphate are known to be a good peroxide decomposer.
In the present study, we have investigated the antioxidants effect such as ascorbic acid and α-tocopherol, compared with 2,6-di-t-butyl-4-cresol (BHT) and 4-hydroxy-2,2,6,6-tetramethyl-piperidine-N-oxyl (4-hydroxy-TEMPO) at different concentrations (1.0, 0.7, 0.5, and 0.3 wt%). The two vitamins are well known as powerful radical capturer in human body. It was of special interest to know, the role of ascorbic acid, which has four hydroxyl groups in comparison with petrochemical inhibitors like BHT and 4-hydroxy-TEMPO.
In the previous work of Lee et al. [15], only the inhibiting effect of the phenol derivatives was investigated. In this work, the inhibiting effect of some natural alcohols, e.g., ascorbic acid and α-tocopherol were added to obtain further information by comparing with petrochemical antioxidants.
The authors also investigated the reaction phenomena in different concentrations of natural inhibitors and identified the oxidation products and the composition of SBO by means of LC–ESI-MS/MS.
Section snippets
Soybean oil purification [3]
In order to eliminate interfering water, coloring agent, and antioxidant from the purchased soybean oil, (Zhonglyang group, Beijing, China. Fortune trade mark), the oil was purified by the following procedures. Into a 1 L three necked flask which equipped with a Dean Stark separator, water cooling condenser and mechanical stirrer, a mixture of 800 g SBO and 40 g of active carbon was added and refluxed (160–180 °C) for about 1 h with constant stirring. The mixture was filtered while still warming.
Autoxidation of SBO in the presence of different concentration of antioxidants
Fig. 1, Fig. 2 showed the active oxygen concentration in the different concentration of antioxidants, ascorbic acid and 2,6-di-t-butyl-4-cresol (BHT). Also, the behavior of α-tocopherol was similar to ascorbic acid while the behavior of N-oxyl was similar to that of 2,6-di-t-butyl-4-cresol. In general, active oxygen concentrations obtained with the addition of antioxidant show time-profiles similar to that obtained for pure SBO. At all antioxidant concentrations, the amount of active oxygen
Conclusion
In the investigation of antioxidant effect of two naturals, ascorbic acid and α-tocopherol and two synthetics, BHT and 4-hydroxy-TEMPO, the ascorbic acid showed the synergetic and antagonistic effect according to the concentration while 4-hydroxy-TEMPO showed only the antagonistic effect. The synergetic effect at lower concentration is attributable to the preferred inhibiting ability through less inter- and/or intra-hydrogen bonding tendency of ascorbic acid than in high concentration.
The
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