Fermentation

The tea industry has historically used 'fermentation' loosely to describe oxidation in black tea production, but technically these are different processes. Oxidation is an enzymatic reaction between polyphenol oxidase and catechins in the presence of oxygen — no microorganisms are involved. True fermentation involves living microbes (bacteria, fungi, yeasts) metabolizing tea compounds into new substances. Pu-erh's wo dui process is the most prominent example: Aspergillus, Penicillium, and Saccharomyces species transform the leaf chemistry over months, producing the earthy, smooth character of shu pu-erh. Natural aging of sheng pu-erh also involves slow microbial activity over years and decades. Other fermented teas include Anhua dark tea from Hunan (which develops a probiotic 'golden flower' fungus, Eurotium cristatum), Liubao from Guangxi, and Japanese goishicha, a rare pickled tea from Shikoku.