Thermal stability and degradation kinetics of the phenolics of Trigonella-foenum graecum L. leaf extracts

In this study, thermal stability and degradation kinetics of the phenolics of the aqueous fenugreek leaf extracts were determined. Thermal degradations of total phenolics, total flavonoids, antioxidant activity and total saponins of the fenugreek leaf extracts were examined at different pH values (3.0, 6.0 and 9.0) and different temperatures (60, 70, 80, 90 and 100°C) for time. Moreover, degradation kinetics of the total phenolics were explained by first-order reaction kinetics. Half-life values, free energy and activation energy of the extracts for total phenolic compounds were calculated. According to the results, the extracts showed better thermal stability at pH 3.0 than the other pH values at the selected temperatures concerning total phenolics, total flavonoids, antioxidant activity and total saponins. The degradation of the total phenolics, total flavonoids and antioxidant activity followed similar trends. The phenolic extract of the fenugreek leaves had high thermal stability. The extract had antioxidant activity despite applying eight hours of thermal treatment at 100°C. Kinetic constants (k) were 0.151-0.435 h-1, 0.181-0.491 h-1 and 0.197-0.634 h-1 at pH 3.0, pH 6.0 and pH 9.0, respectively. Activation and free energy values for the degradation of fenugreek phenolics were calculated in the range of 26.02-29.97 kJ/mol and 109.31-120.07 kJ/mol, respectively. The half-life values of total phenolics treated at 60-100°C were 1.59-4.59 h, 1.41-3.83 h, and 1.09-3.52 h for pH 3.0, 6.0 and 9.0, respectively.