Investigation of antioxidant features of Thompson orange peels as a natural antioxidant

This research was conducted to investigate the antioxidant feature of Thompson orange peels as a natural antioxidant, and to compare this feature with those of synthetic antioxidants. For this purpose, the solvent methanol was first used to obtain the peel extract and the total phenolic compounds were then determined by using the Folin–Ciocalteumethod. Different concentrations of the extract were added to antioxidant free canola oil to study the antioxidant activity of the extract, and the effects of these concentrations of the extract were evaluated through measuring the peroxide values, the iodine values and the percentages of free fatty acids in oil samples stored for 5 months at 45°C. Furthermore, the oil stability test (Rancimat) was performed and all of the treatments were compared with oil samples containing the synthetic antioxidant TBHQ. Results obtained showed that the effects of the different concentrations of the extract on the peroxide values were significant at the 1% probability level. It was also found that the phenolic and polyphenolic compounds (and the other antioxidant substances) in the peels were more effective when higher concentrations of the extract were used. The measurements of the indices investigated the peroxide value (PV), the iodine value (IV), the percentage of free fatty acids (FFA%) and the oil stability index (OSI) indicated that the antioxidant activity of the extract increased when it was used at higher concentrations, and hence the canola oil became more stable. The maximum antioxidant activity of the extract was observed when it was used at 3000 ppm. It was also found that the 1000 ppm concentration of the extract was not able to control the percentage of free fatty acids when the oil was kept for a longer period of time. However, the 2000 ppm and the 3000 ppm concentrations of the extract were similar to the 100 ppm concentration of the synthetic antioxidant THBQ with regards to preventing an increase in the percentage of free fatty acids in canola oil. Therefore, orange peels can be considered as a suitable source of natural antioxidants due to their content of phenolic compounds.