Antioxidative effect of Urtica dioica extract on quality characteristics of rapeseed oil and comparison with TBHQ during deep frying

Using synthetic antioxidants to protect oils from oxidative deterioration due to the possibility of toxic and carcinogenic effects is limited. Thus, it is important to study the natural antioxidants for use as alternatives for synthetic ones. The objective of this study was to evaluate the effect of Urtica dioica leaves extract as a natural antioxidant on the oxidative stability of rapeseed oil and compared its effect at two levels (100 and 800 ppm) with synthetic antioxidant TBHQ at 100 ppm. Hence, the phenolic and flavonoid compounds content, antioxidative activity and extraction efficiency of methanolic extract of Urtica dioica leaves determined. Also, effect of both natural and synthetic antioxidant was determined by measuring peroxide value during 96h and iodine value, acid value and anisidine value during 48h deep frying. Results showed that extraction efficiency was 11.95%, amount of phenolic compounds extracted by methanol were 87.127±6.096mg gallic acid equivalent/g dry sample, flavonoid compounds 91± 2.2 mg/g dry sample and antioxidant capacity was 0.303±0.025mg. Results of measuring peroxide value after 96h showed that control oil without any antioxidant had the highest peroxide value (3.384 meq o2/kg oil) and oil containing 100 ppm TBHQ had the lowest (1.794 meq o2/kg oil). In addition, investigating results of acid, anisidine and iodine values during 48h deep frying showed that oil containing 100 ppm TBHQ was more effective than oils containing natural antioxidant Urtica dioica in 100 and 800 ppm on stability of rapeseed oil. Efficiency of TBHQ in stabilizing the Rapeseed oil during deep frying can be attributed to high practicality concentration in comparison to Urtica dioica extract.