1 st Mae Fah Luang University International Conference 2012 1 MOISTURE ADSORPTION ISOTHERMS OF ASSAM GREEN TEA POWDER Natthawuddhi Donlao 1, * and Suwit Siriwattanayotin 2 1 School of Agro-Industry, Mae Fah Luang University, Thasud, Chaing Rai 57100, Thailand 2 Department of Food Engineering, Faculty of Engineering, King Mongkut’s University of Technology Thonburi, 126 Pracha-u-tit Road, Bangkok 10140, Thailand *e-mail: natthawuddhi.don@mfu.ac.th Abstract Moisture adsorption isotherms of Assam green tea powder were determined at 25, 35 and 45C. A water activity analyzer (Novasina ® model LabMaster-a w ) was used to detecting the equilibrium over the relative humidity range of 30-70%. The sorption isotherms of Assam green tea powder were typical type III sigmoid curves, according to BET classification. The equilibrium moisture content data were fitted to six mathematical models. From the estimated parameters of different models, it was indicated that the Peleg model gives the best fit to the experimental data and it is the most suitable model for describing the relationships between equilibrium moisture content (EMC) and equilibrium relative humidity (ERH) of Assam green tea powder over the entire range of temperature. The net isosteric heat of sorption was determined by the Clausius-Clapeyron equation and was found to be between 45.2 and 45.6 kJ/mol at moisture levels ranged 0.05-0.25 g/g dry matter. Keywords : Assam green tea powder, adsorption isotherm, isosteric heat of sorption, equilibrium moisture content. Introduction The northern part of Thailand is the major tea growing area and includes the provinces of Chiangrai, Chiangmai, Lampang, Phrae and Mae Hong Son. One notable tea growing area is Doi Mae Salong in Chiangrai where there is a tea plantation of 4,500 rais. Different kinds of tea are produced here, and the province of Chiangrai produces about 200 tons of tea a year. About 70% of the tea production is consumed in Thailand, while the other 30% is exported. Much of the tea produced in Thailand is Assam tea which is the local plant of the northern part. Assam tea production has a high volume of 83% and 17% for Chinese tea (Department of Agricultural Extension Thailand, 2003). There are many kinds of tea and tea products and one interesting product is ‘instant tea’ or ‘tea powder’. It is widely used for application in the ingredients of food, snacks, beverages, cosmetics and drugs. Instant green tea is one of the most widely consumed beverages worldwide because of its aroma, stimulatory effect, health benefits and convenience in use (Sinija and Mishra, 2008). Because of its processing, instant tea decreases the transportation and storage costs due to a decrease in weight and volume. In addition, value added and prolonged shelf-life are interesting points for the manufacturer. The moisture sorption isotherms of the tea powder are very useful for predicting its stability during storage and selecting appropriate packaging materials (Sinija and Mishra, 2008). Moisture sorption isotherms show the relation between water activity and change in moisture