Impact of cooking on nutritional contents of kenger in terms of antioxidants

Abstract

Kenger (Gundelia tournefortii) grown in Sanliurfa and neighbouring provinces in Turkiye, is known for its health promoting potential due to the presence of bioactive compounds as phenolics and antioxidant vitamins. However, such compounds in food may be affected by thermal treatment as cooking. This study investigated the impact of common cooking techniques (boiling and stir-frying in oil) on phenolics, vitamin C content and antioxidant activity of kenger. Phenolic compounds were analysed using LC–MS/MS and DPPH inhibition. Vitamin C was quantified using HPLC and antioxidant activities were assessed by DPPH inhibition, ABTS and FRAP methods. Major phenolic compounds in raw samples were vanillic (18.755 ± 0.606 µg/g) and fumaric (16.211 ± 0.524 µg/g) acids. In boiled kenger, significant loss of fumaric (5.789 ± 0.187 µg/g), in stir-fried kenger, loss of vanillic (15.604 ± 0.504 µg/g) and fumaric (8.113 ± 0.262 µg/g) acids were noted (p < 0.05). Vitamin C content of raw kenger was 7.104 ± 0.074 µg/g but decreased to 6.812 ± 0.22 µg/g in boiled and 6.898 ± 0.072 µg/g in stir-fried samples. The radical scavenging potentials of sample extracts at different concentrations (25 mg/mL, 50 mg/mL, 75 mg/mL, 100 mg/mL,150 mg/mL and 300 mg/mL) were tested by DPPH, FRAP and ABTS methods. At 300 mg/mL, antioxidant activity in raw kenger was calculated as 1.350 ± 0.0007 μmoles/mg/mL (FRAP), 0.731 ± 0.0008 μg (EC50 DPPH) and 52 ± 3.45% TEAC (ABTS). In general, antioxidant activities for all methods increased with rising concentration of the sample extract. Antioxidant activity of boiled samples decreased significantly as per FRAP and increased for stir-fried kenger at higher sample concentrations as per FRAP and ABTS, possibly due to chemical changes during high thermal treatment in oil. Correlation between cooking methods was not significant (p > 0.05) in terms of phenolic compounds and vitamin C. Correlation between antioxidant activity assessing methods were only significant for 75 and 100 mg/mL sample concentration (rho = -0.90 p < 0.05). In conclusion, boiled and stir-fried kenger is suggested as optional consumption methods. Inclusion of condiments, herbs, olive oil, yoghurt during service would not only increase palatibility but also add to the health benefits.