Assessment of total flavonoid contents and ferric reducing antioxidant power of several Green and oolong tea products in Vietnam Available to Purchase

Tea plants were discovered thousands of years ago in South-East Asia, and humankind has been drinking tea for thousands of years due to its medical benefits to prevent and treat a lot of diseases. Tea leaves are specific for their antioxidant capacities due to the presence of a wide range of flavonoid and other phenolic compounds. In this study, spectrophotometric methods were evaluated for determining the total flavonoid contents (TFCs) and ferric reducing antioxidant power (FRAP) in various green and oolong tea products collected in two specific tea-tree plantation regions in the mountainous area (Yen Bai Province-YB, the North) and Central Highland (Da Lat-DL and Lam Ha-LH, Lam Dong Province, the South) of Vietnam. The ultrasound-assisted solid-liquid extraction at 70 °C was employed to prepare the tea extracts with 70% v/v methanol in deionized water as the extraction solvent. Quercetin and trolox were used to build up the calibration curves for TFCs and FRAP, respectively. The results indicated that oolong tea products performed lower TFCs (expressed as milligrams of quercetin equivalents per gram dried weight or mg QE g⁻¹ DW) than green tea (68.06±0.76 to 79.89±0.48 mg QE g⁻¹ DW vs.117.4±1.7 to 244.1±3.9 mg QE g⁻¹ DW) due to the differences in tea plant species and the oxidation levels during the fermentation period. Among the collected dried tea products, the FRAP values (expressed as micromoles of trolox equivalents per gram or µmol TE g⁻¹ DW) were high and followed the decreasing order of YB-Green (2138±23 to 2688±40 µmol TE g⁻¹ DW)>DL-Green (2232±60 to 2349±63 µmol TE g⁻¹ DW)>LH-Oolong (1745.2±5.0 to 1947±45 µmol TE g⁻¹ DW)>DL-Oolong tea products (1551±23 to 1679±25 µmol TE g⁻¹ DW). It could be concluded that green and oolong tea products are rich sources of flavonoids and antioxidants, and the total flavonoid contents could remarkably contribute to the antioxidant capacities (r=0.841).