The Effects of Roasting Conditions on the Physical Appearance Traits and Aroma and Taste Components of Roasted Stem Tea

Roasted stem tea, the product of roasting green tea stems, has a characteristic aroma and appearance. In this study, we evaluated the effects of different roasting conditions on the physical appearance, aroma, and taste of roasted stem tea. The real-time moving paths of color parameter L*a*b* were similarly changed from yellow-green to yellow-red with roasting at 180 °C, 200 °C, and 220 °C for each second, indicating that L*a*b* can be a measurement of roasting degree. Bulk density data and scanning electron microscope images revealed that tea stem swelling was optimal under high-temperature roasting conditions. Higher roasting temperatures provide heat that penetrates deeper within the tea stems. Regarding aroma, the higher the roasting temperatures, the more pyrazine and geraniol are produced. The higher internal heat promoted higher pyrazine and geraniol production, thereby improving the aroma of the roasted stem tea. Catechins and amino acids were rapidly decreased under high-temperature roasting.