Pu-er is the only class of tea that is both oxidized and fermented. The terms “fermentation” and “oxidation” have been used interchangeably in the tea trade in part because the Chinese historically only used one word to refer to both processes. In reality, oxidation is a purely chemical reaction (elements in the leaf react with and absorb a controlled amount of oxygen to create a darker brown-red color and characteristic aroma to the resulting tea) whereas fermentation involves microbes and the absence of oxygen.
Sheng “raw” Pu-er leaves are plucked and placed to wither in the sun. The withered tea is then fired almost completely to stop most of its enzymatic processes and oxidation. A complete firing would eliminate all internal moisture which would inhibit oxidation and fermentation. The tea is then rolled and laid out to dry at which time it begins to oxidize, and is finally steamed and pressed (unless it is going to be made into loose-leaf Pu-er) into a variety of shapes (bricks, nests, and cakes) and then undergoes natural post-fermentation (aging). Sheng Pu-er has a much “greener” flavor than Shou Pu-er, and mellows and darkens over time.
Shou “ripe” Pu-er leaves are processed in the same way as Sheng “raw” Pu-er leaves in the beginning stages, but instead of being steamed and pressed into a variety of shapes and then allowed to ferment naturally, the tea is placed in a warm and humidity controlled room where it is wet-piled and tossed and turned (oxidized and fermented) at least once a week for a period of up to two months. After each tossing and turning, the piles of tea are covered with a canvas blanket to activate and encourage microbial activity and the fermentation process. After the initial fermentation process is deemed to be complete, the tea is dried, quickly steamed, and pressed into a variety of shapes and then continues to undergo slight post-fermentation (aging). Shou Pu-er is artificially aged, and while the flavor changes over time, not nearly as much as naturally aged Sheng Pu-er. Tea artisans developed this accelerated aging process in the 1970s to satisfy the worldwide demand for ready-to-drink Pu-er tea, and to quickly reproduce much of the naturally aged “ripe” Pu-er that was destroyed during China’s Cultural Revolution. Shou Pu-er has a strong, earthy flavor right from the beginning.
Pu-er is the only tea which is intentionally aged—the other classes are always better fresh. Rare Sheng Pu-ers can be more than 30 years old.