Thai guava leaves were extracted, using 50% ethanol in water. The DPPH radical scavenging activity of the extract was determined along with its total phenolic content using caffeic acid as a standard. It has been found that the extract of Thai guava leaves possessed high antioxidant activity with EC50 values of 3.31 (r2=1) mcg /mL and total phenolic contents of 35.39±2.63%w/w. Since the dried extract was hygroscopic under high humidity, its preparation was formulated by combining 3.35% w/w Aerosil® with the extract before drying. This provided a more physically stable extract product. The stability evaluation of the extract product was carried out under accelerated conditions (45, 60 and 70oC, 75% RH). Quercetin in the extract was determined as a marker using HPLC. Under accelerated conditions, the overall loss of quercetin in the extract product displayed a non-linear profile. The initial phase of degradation exhibited an apparent second-order kinetic behavior where the half-life was dependent on the initial concentration and the degradation rate constant at each condition. The initial lag time was also observed in the degradation profile, suggesting the protective effect of the additive in formulation. No degradation was observed in the product heated at 100oC for 3 minutes. Although the EC50 of the extract product increased to about 10 mcg/mL under accelerated conditions, it is still better than the activity of the positive control, BHT (EC50 19.92 mcg/mL).
Acknowledgements: Thailand Research Fund, Thailand
References: 1. Qian, H., Nihorumbere, V. (2004), J. Zhejiang University Science 5: 676–683. 2. Yamasaki, K. et al. (1994), Chem.Pharm. Bull. 42: 1663–1665.
