Effect of microwave and conventional oven heating on phenolic constituents, fatty acids, minerals and antioxidant potential of fennel seed

Abstract

In plant species, the phenolic compounds usually exist in bound forms. Microwave and conventional oven heating were applied on fennel seed to see their effect on its selected phenolic constituents, fatty acids, minerals, total phenol content (TPC), total flavonoid content (TFC), and the antioxidant potential. The impact of microwave power (300 W, 500 W, 700 W), oven temperature (100 °C, 125 °C, 150 °C), and treatment time (5 min, 10 min, 15 min for microwave; 20 min, 40 min, 60 min for conventional oven) was investigated. The amounts of individual phenolic compounds, fatty acids, TPC, TFC and the antioxidant activity of fennel seed were increased by increasing the applied microwave power up to 500 W and the treatment time up to 10 min, but 720 W and 15 min treatment time caused reduction in the contents of tested compounds and the antioxidant potential. As compared to all the microwave treatments, heating at 500 W for 5 min gave a significantly higher content of fatty acids, phenolic compounds, TPC, TFC and also a higher antioxidant activity of fennel seed. Oven heating showed a similar trend as the microwave heating, and among all the oven heating treatments, the samples heated at 125 °C for 20 min showed a significantly higher antioxidant activity and a higher contents of the tested compounds of fennel seed but those were still significantly lower as compared to that of the sample heated by microwave at 500 W for 5 min. The mineral contents were decreased upon both the microwave and oven heating. As compared to the oven heating, microwave heating yielded better results even at shorter treatment time for enhancing the antioxidant activity and phenolic and fatty acid contents of fennel seed. Based on these results, the microwaves at moderate power and treatment time might be considered for heating/roasting of fennel seeds.