Effect of herbal extracts obtained from pomegranate (Punica granatum L.) peel and green coffee (Coffea Arabica) and their formulations on the oxidative stability of walnut oil

In this thesis study, powder (E) and lyophilized (L) methanol extracts of pomegranate (Punica granatum L.) peel (PE, PL) and green coffee (Coffea arabica) (CE, CL) and their formulations (PE-CE, PL-CL) as antioxidants were used to test the thermal oxidation stability of unrefined walnut oil (Juglans regia L) under accelerated storage conditions (18 days at 60 °C). Free fatty acidity (FFA), peroxide value (PV), conjugated diene (CDE), conjugated triene (CTE), thiobarbituric acid reactive substances (TBARS) and photometric color index (FCI) were selected as quality parameters. Additionally, the protective effects of antioxidants were examined by looking at the oxidative stability index (OSI), fatty acid composition, ratio of fatty acids such as linoleic/oleic (L/O) and linoleinic/linoleic (Ln/L) and calculated oxidation value (COV) in walnut oil samples supplemented with plant-derived antioxidants and their suitability were compared with gallic acid (GA) and alpha-tocopherol (T) standards as well as synthetic antioxidant butylated hydroxytoluene (BHT). Variance analysis was performed using a Randomized Parcel Factorial Trial design, with a total of 140 samples including 10 antioxidants (A) (C (control), GA, T, BHT, PL, PE, CL, CE, PL-CL, and PE-CE), 7 storage periods (SP), and 2 replications (R). The significant averages were compared with the Tukey Multiple Comparison Test. Among all the antioxidants, PL (natural antioxidant) has the highest levels of anthocyanin (ACN) at 55.11 mg/100 g, total phenolics (TP) at 19350.99 mg GAE/100 g, DPPH-radical scavenging activity (DPPH-RSA) at 84.06%, and Trolox equivalent antioxidant capacity (TEAC) at 761.60 μg TE/mg. On the other hand, the PL-CL formulation showed the highest ferric ion reducing antioxidant parameter (FRAP) at 447.43 μg TE/mg, and ABTS radical scavenging activity at 341.85 μmol TE/g. The walnut oil used in the research has an average total tocopherol (TT) amount of 166 mg/kg. Additionally, the average values for FFA, AN (acidity number), PV, TBARS, CDE, CTE and FCI were 0.835%, 1.658 mg KOH/g, 0.685 meq O2/g, 0.392 mg/kg, 1.090, 3.619 and 46.79 respectively. In the walnut oil examples, of the total detectable 10 fatty acids, cis-linoleic acid (18:2), which constitutes an average of 52.96%, is the dominant fatty acid, followed by cis-oleic acid (18:1), linolenic acid (18:3), cis-11-eicosenoic acid (20:1), gama-linolenic acid (18:3), trans-linolelaidic acid (18:3) and palmitoleic acid (16:1), respectively. Moreover, while palmitic acid (16:0) and stearic acid (18:0) were found in the highest amounts among saturated fatty acids, behenic (22:0) fatty acid was also detected at a detectable level. According to the results of Variance Analysis, it was determined that A, SP factors and AxSP interaction were statistically effective on the FFA, PV, TBARS, CDE, CTE and FCI values of walnut oil (P <0.01). As a result of accelerated thermal oxidation, FFA, PV, TBARS, CDE, CTE and FCI values increased relatively less in antioxidant-added oil samples compared to the C sample. On the other hand, OSI, fatty acid profile, ratio of fatty acids (L/O, Ln/L) and COV showed different changes according to all antioxidants. While the antioxidant effect was highest in GA, this was generally followed by PL-CL, PE-CE, BHT, PL, PE, T, CL and CE antioxidants although the order may vary. The study concluded that PE, PL except for CE and CL plant extracts and their formulations (PE-CE and PL-CL), as natural antioxidant sources, are effective in preventing the oxidation of walnut oil. These extracts can be used successfully in many different oxidation-sensitive foods as a natural alternative to synthetic antioxidants. Keywords: Antioxidant, Antioxidant activity, Green coffee, Herbal extract, Juglans regia L., Lyophilization, Oxidative stability, Pomegranate peel, Rancimat, Walnut oil.