When Heat Elevates Tea

The science of roasting: how fire transforms tea from chemistry to aroma.

Roasting transforms tea beyond taste—it reshapes its chemistry. From Wuyi’s rocky cliffs to Kyoto’s roasteries, heat awakens hidden dimensions of aroma. Under the influence of fire, sugars, amino acids, and polyphenols rearrange, forming new molecules that evoke caramel, toasted nuts, or cocoa. 

Fire as Catalyst

Between 100 and 160 °C, the Maillard reaction begins: a cascade between reducing sugars and amino acids, the same that browns bread or coffee. In tea, this process yields pyrazines (about 80–150 µg/g depending on roast), furans, and aldehydes, compounds responsible for roasted, nutty, and woody notes. The total volatile content can nearly double between light and heavy roasting—from roughly 200 to 350 µg/g in Dong Ding oolong—according to recent analyses. 

Chemistry Meets Texture

The transformations extend beyond aroma. A light roast preserves floral and grassy nuances, while prolonged heating reduces catechins by 30–60 %, softening bitterness and astringency. L-theanine, the molecule of umami sweetness, can decrease by 20–40 %. This shift allows mellow, round sensations to emerge. Meanwhile, aromatic aldehydes  and melanoidins, products of Maillard chemistry—accumulate, contributing not only color but also mild antioxidant activity, though weaker than that of the original polyphenols. 

The Craft of Heat

Master roasters in Taiwan and China often alternate heating and cooling cycles over several days, maintaining temperatures below 130 °C to stabilize aroma and avoid burnt notes. Too much fire brings smoke and bitterness; too little leaves the liquor green and unfinished. Precision and intuition coexist in every batch. 

Tradition, Science, and the Modern Flame

Japan’s hōjicha pushes roasting beyond 150 °C, yielding a bronze liquor with sweet, woody tones and minimal caffeine. In China, Wuyi rock teas exemplify how heat and minerality merge into depth and fragrance. Since 2021, researchers have explored thermal control methods to standardize volatile profiles—an attempt to bring the art of roasting into the realm of quantifiable science. 

Bibliography

- HUANG, J., et al. “Effect of different roasting levels on aroma and chemical profiles of Wuyi rock tea.” Food Chemistry, 2021, 344 : 128626. 
- LI, Y., et al. “Impact of roasting degree on volatile formation in Dong Ding oolong tea.” Journal of Food Science and Technology, 2022, 59 (8) : 3175–3185. 
- XU, X., et al. “Changes in amino acids and sugars during roasting of green tea and their relationship with Maillard reactions.” LWT – Food Science and Technology, 2023, 184 : 115231. 
- MURAMATSU, T., et al. “Roasting optimization and sensory quality of Japanese Hōjicha tea.” Journal of Agricultural and Food Chemistry, 2024, 72 (4) : 1452–1463. 
- LIN, C., et al. “Thermal treatment and antioxidant properties of oolong teas.” Food Research International, 2021, 147 : 110498.