Determination of the bioactive compounds of fresh tea leaf plucked at different harvest periods by optimization and investigation of bioaccessibility of polyphenols

In this study, the effect of different plucking years and periods on total polyphenol (TP), radical scavenging activity (RSA), total flavonoid (TF) and ascorbic acid (AA) of fresh tea leaves extracts were evaluated. The extraction of polyphenols from fresh tea leaves was optimized by using the response surface methodology (RSM). The results showed that in order to obtain the highest TP, the optimal extraction conditions were found to be material-to-solvent (M/S) ratio of 1/60,86, ethanol concentration of 84,65% and extraction time of 50 min. As a result of the extraction of polyphenols from tea leaves under the optimal conditions, the leaves plucked in September 2020 had the highest TP, RSA and AA. In addition, the analysis of individual phenolic compounds of tea leaves was performed by high performance liquid chromatography (HPLC) and the result indicated that the leaves plucked in September 2020 contained the highest amount of EGCG content (57,52 mg/g dry weight (DW)). After the in-vitro digestion of the same samples, TP and RSA values showed a significant decrease, mostly at the intestinal phase (p<0,05). However, the bioaccessibility of polyphenols was determined as 48,98%. The AA identification and quantification of tea leaves from different periods of two years were performed by HPLC. The obtained results revealed that leaves plucked in May for 2019 (38,55 mg/100 g DW) and those in September for 2020 (41,92 mg/100 g DW) had the highest AA contents. Additionally, in this study, the spectrophotometric methods used for TP content and RSA of the samples were compared to the modifications of these methods which are most widely used in the literature and significant differences were found between the methods regarding TP and RSA. The TP results obtained spectrophotometrically were compared with the results obtained by HPLC and it was concluded that only one of the five different methods applied (FCR method 1) provided the best fit and therefore had the most reliability.