Ultrasound-induced lipid peroxidation: Effects on phenol content and extraction kinetics and antioxidant activity of Tartary buckwheat (Fagopyrum tataricum) water extract

Lipoxidation caused by ultrasound treatment was investigated. Its effects on total phenol content (TPC), total flavonoid content (TFC), antioxidant activity (AA) and phenols extraction kinetics in Tartary buckwheat extracts were also studied. Malondialdehyde (MDA) levels and peroxide value (PV) were used to measure lipoxidation. Kinetic modeling of phenol extraction was also done according to Fick's second law of diffusion. Increasing treatment time increased TPC, TFC and AA significantly (p < 0.05) and increments in MDA and PV were significant after 45 min. Increases in MDA and PV correlated with reductions in TPC, TFC and AA. Phenol diffusion followed a kinetic curve that agrees with the classical solid-liquid extraction models, with a rapid initial phase and a slow phase at the end. The effective diffusion coefficients (D-e), both De-rapid and De-slow, for phenols positively correlated with time (5-60 min) and temperature (20-80 degrees C). TPC increased when extraction temperature and time increased with De-rapid from 3.30 to 7.97 x 1011 m(2)s(-1) from 20 to 80 degrees C and 2.88 to 7.47 x 10(11) m(2)s(-1) from 5 to 60 min. The De-slow phase also increased from 2.02 to 5.21 x 10(12) m(2)s(-1) and 2.85 to 4.33 x 10(12) m(2)s(-1), respectively, for the same range of extraction temperatures and times. It is recommended that ultrasound assisted extraction of phenols from polyunsaturated fatty acid-rich sources are done at temperatures below 65 degrees C and not for extended periods to preserve bioactivity and minimize lipoxidation.