Fatty acid composition of Oenothera biennis seed oil during storage. Antioxidant activity

Abstract

In continuation of research on the lipid composition of seed oil from Oenothera biennis growing in Krasnodar Territory [1], we studied the effect of long-term (12 months) storage on the content of the principal fatty acids palmitic (16:0), stearic (18:0), oleic (18:1), linoleic (18:2), and -linolenic (18:3). An oil sample from Oenothera biennis seeds (~1 mL) was placed in a tube (~2 mL), closed with a stopper, and stored in a refrigerator at 8°C. A sample was taken each month. The fatty acid composition was determined (Table 1). The composition and content of fatty acids in the samples were practically constant and within the uncertainty limits of the experiment. The presence or absence of oxidized lipids was determined by hydrolyzing a sample of oil that was stored for one year, isolating the fatty acids, and preparing the methyl esters, which were analyzed for the presence of oxidized fatty acids by analytical TLC (ATLC) using solvents 1–3. The models were methyl esters of fatty acids from castor oil that contained monohydroxyand dihydroxy-fatty acids. However, spots corresponding to oxidized fatty acids were not observed in the studied sample. Oil of O. biennis is known to contain tocopherols [2, 3], which we identified previously in the neutral lipids [1]. GC/MS of the unsaponified fraction of O. biennis oil identified the natural antioxidants -, -, and -tocopherol from their mass spectra using the AMDIS software [4]. The antioxidant activity of the oil was analyzed using a model system based on radical-chain oxidation of methyloleate initiated by azodiisobutyronitrile (AIBN). The advantage of this model system is that the oxidation process in it is very similar to the lipid oxidation process in living systems. Studies of natural compounds using this system and analogous apparatus are rather rare. Figure 1 shows typical kinetic curves for oxygen absorption during initiated oxidation of methyloleate without an inhibitor and with O. biennis seed oil. It can be seen from the curves that the reaction occurred with an induction period. The rate of uninhibited oxidation of methyloleate and the oxidation rate of methyloleate inhibited by the oil components after complete consumption of the inhibitor were the same. Therefore, our experimental conditions did not reveal the contribution from oxidation of the oil itself. The reduced oxidation rate in the initial portion (induction period) indicated that an additional pathway for consumption of peroxide radicals had opened.