Production of oolong tea with different oxidation degree from Turkish tea and determination of some quality and antidiabetic properties of these teas

Different types of teas such as black tea, green tea, oolong tea, white tea and pu-erh tea are produced from the fresh tea leaves and shoots of the tea plant (Camellia Sinensis (L.) O. Kuntze). They are originated from China and then spread all over the world. While oxidation of phenolic compounds is not allowed in green and white teas, phenolic compounds are oxidized in black tea. For oolong tea, on the other hand, a partial oxidation is required Oolong tea, which is widely consumed in Southeast Asian countries, has started to be recognized, preferred, and sought in the western world. Within the scope of this research, all production was carried out at Atatürk Tea and Horticultural Research Institute in Rize during the three flushing periods of the 2018 production season. Green tea, oolong tea with four different oxidation levels and black tea were produced using fresh tea leaves, which harvested as 2.5 – 3.5 leaves from the Ali Riza Erten clone (Tuglali 10) in the tea garden of the Institute. The productions were realized as two replicas in each flushing period. A batch originating from Taiwan was used as a control sample of oolong tea. A fresh tea leaf sample was also included for comparison. Moisture content and water activity, total phenolic content, total flavonoid content, total antioxidant activity (DPPH and ABTS methods), caffeine and polyphenolic composition, phenolic acid analysis, chlorophyll amount, color analysis, theaflavin, tearubigin, total color and brightness analysis, volatile component analysis were performed on tea samples produced in the study. In addition, antidiabetic properties and sensory characteristics of the teas produced were determined. It was observed that the total phenolic content of oolong teas showed similar properties as the green tea. It was determined that the total phenolic content of oolong teas decreased with the increase in oxidation time and the lowest amount was found in black tea. In addition, the total phenolic content of the teas produced in the third flushing period was lower than the first and second flushing periods. The phenolic content and antioxidant activity of the samples were found to be directly related to the oxidation time in oolong tea production. Individual catechins decreased with increasing oxidation time, whereas theaflavins increased with increasing oxidation time. In the teas produced within the scope of the study, 73, 68 and 63 volatile components were detected in the first, second and third flushing periods, respectively. Among the volatile components detected, hexanal, linanool, nonanal, limonene, beta-cyclocitral and beta-ionone were observed as the main volatiles and were detected in all three flushing periods and in all tea types. Tea was found to have a good inhibitory effect on both α-amylase and α-glucosidase compared to acarbose, resulting in decreased absorption and digestion of starch. Furthermore, oolong tea has been found to be an important beverage for reducing the risk of diabetes, as well as controlling it. In conclusion, the oolong teas produced in this research showed significantly different characteristics, mostly depending on the degree of oxidation, and when compared to control oolong tea samples from Taiwan, it was shown that high-quality teas can be produced. The experience and expertise gained during this study led to the conclusion that oolong tea produced using Turkish tea, which is a new product for our country, can compete with oolong teas, as well as other teas in the market.