Black tea has one of the most complex aromas of anything we eat or drink. It starts out as simple, green-tasting young leaves; then the combination of enzyme action and heat generates hundreds of volatile molecules. Among the more prominent aromas are the flowery and fruity ones; roses, orange flowers, violets, apricots, and raspberries are in there, along with fresh-crushed leaves, dried hay, and caramel. German flavor chemists recently identified a previously unknown note: a molecule that also contributes to the characteristic sweet aroma of oats. Something for tea lovers to sniff for and enjoy.
In the course of their analysis, the chemists also discovered a new reason for the traditional practice of brewing black tea in a preheated teapot with water just off the boil. High-temperature brewing does the best job of extracting aroma compounds from the tea leaves, but it also generates even larger quantities of certain aromatics than were in the leaves to begin with! It may do so by breaking off small, aromatic portions from much larger molecules. By contrast, we brew green teas in water far below the boil (around 150 degrees F, 65 degrees C) to preserve their more delicate, simple aroma and color.
Schuh, C. and P. Schieberle, J.Agric Food Chem. 2006, 54(3) 916.