Pu'er tea is produced using two methods: artificial heap fermentation (ripe tea) and natural fermentation (raw tea). Artificial fermentation is a modern tea-making technique invented in Yunnan, involving the rapid fermentation of sun-dried Green Tea through heap fermentation, followed by steaming and pressing into solid shapes or left loose. Natural fermentation is a traditional method that involves steaming and pressing sun-dried green tea into solid shapes to preserve its aging potential.
Pu'er Ripe Tea: Heap Fermentation
Heap fermentation is a process used in making Pu'er ripe tea and is critical in determining the quality of the tea.
A certain amount of sun-dried green tea is piled up to maintain a stable temperature within the pile, then the piled tea is moistened to initiate fermentation.
The height of the heap can be adjusted as needed, usually reaching around one meter. As fermentation begins, the pile needs to be turned to manage temperature and fermentation conditions. Each master has their own judgment, but the core principle is to create an environment conducive to the activity of microorganisms in the tea while inhibiting the growth of unwanted bacteria, ensuring even and smooth fermentation.
During turning, if moisture levels are insufficient, they need to be replenished, as a certain level of moisture is required for continuous fermentation. After several turnings, the tea gradually ripens. The master will determine when it's time to disassemble the heap based on the degree of maturity. Before disassembling, channels are made to allow rapid moisture loss, stopping the fermentation. If the tea still contains more than 10% moisture after disassembling, fermentation will continue uncontrollably, leading to poor quality and often failure. Thus, managing moisture levels and timing during the disassembly process are significant tests for the master.
Yunnan, located at the border, has vast land with low population density and has been relatively untouched by human activity, preserving its natural ecological environment. The large-Leaf tea Trees grow alongside various flora and fauna in the primeval forests, retaining a primitive “wild” character. The tea from these ancient trees is rich and potent; drinking the sun-dried green tea directly can be challenging for many people's digestive systems. To enjoy the milder Pu'er raw tea, years or even decades of aging and transformation are required, presenting a significant challenge in storage. Any mishandling could lead to contamination and spoilage, rendering the tea useless.
To enable the enjoyment of a milder Pu'er tea in a shorter timeframe, the process of heap fermentation was developed. This involves creating an environment suitable for microbial activity, allowing the tea to rapidly ferment and transform.
Pu'er Raw Tea: Natural Fermentation
The “aging” of Pu'er raw tea involves the non-enzymatic autoxidation of the tea's internal substances under specific temperature conditions.
Through the preserved active substances in the original leaves, amino compounds significantly decrease, causing the fresh flavor to disappear and revealing a mature aroma; the oxidation of polyphenols reduces bitterness and astringency, resulting in a smoother taste; the oxidation of theaflavins and thearubigins leads to the accumulation of undialyzable high polymers (theabrownins), giving the tea a reddish-brown hue.
The fermentation of raw tea is slow in the first few years but becomes more noticeable after a decade.
During post-fermentation, due to the involvement of microorganisms, acids are produced (e.g., citric acid by black Aspergillus), as well as alcohols and lipids. Esters (aromatic esters) are the primary source of aromatic compounds in the world.
Over years of annual asexual reproduction during the summer and autumn and dormancy in Winter, beneficial microorganisms in Pu'er raw tea produce and retain aromatic compounds, gradually forming the characteristic qualities of Pu'er tea being “more fragrant with age and having a reddish-brown clarity.”