A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Food Engineering and Bioprocess Technology, School of Environment, Resources and Development

Thesis (M. Sc.) - Asian Institute of Technology, 2013

The aim of this study was to evaluate the antioxidant and antimicrobial activities of extracts from underutilized black tea from Vietnam. Response surface methodology was applied to obtain a higher total phenolic content (TPC) in crude tea extract by using ultrasonic assisted extraction. The results of the response TPCand independent factorswere found to fit the second-order polynomial equations that obtained by applying in a multiple regression analysis, and relationship between all the independent factors: different concentrations of ethanol and temperature and time were studied. It found that all the independent factors were significant terms for extraction, and ethanol concentrations and temperature were the most significant terms to obtain a high TPC. Antimicrobial activities of water and ethanol extracts were studies. It found that both water and ethanol extracts had significant effect against the growth of Staphylococcusaureus.Different extraction methods hadsignificant effect on antioxidant capacities and quality attributes of tea drinks. Tea extracted from lower temperature (60 °C) with water bath has a higher sensory score and overall acceptability, however with a lower antioxidant capacity compared tothe tea extracted from higher temperature (80 °C) with water bath. Ultrasonic assisted extraction combined with lower temperature (60 °C)can improve the antioxidant capacity of the tea drinks