Sun-drying is one of the most critical processes in the production of Yunnan Pu'er Tea and a term repeatedly mentioned in national standards. If we liken the production process of Pu'er tea to a symphony, the sun-drying step would be its most magnificent finale.

The Importance of Sun-Drying in Pu'er Tea

As early as before the Shang and Zhou dynasties, Chinese laborers knew how to use sunlight to dry fresh leaves into tea. From the Qin and Han dynasties through to the Yuan and Ming dynasties, although steaming and pan-firing methods were invented, traditional sun-drying techniques continued to be used, such as in Fujian Rock tea and Guangdong Phoenix single-origin teas.

The importance of sun-drying lies in the fact that it brings about photochemical changes, wet-heat reactions, and microbial effects on the tea. Although these microscopic phenomena are not widely known, they are crucial for the formation of the quality of sun-dried tea.

A photochemical reaction occurs when a substance's molecules absorb external photons or electromagnetic waves (usually limited to ultraviolet and visible light wavelengths), triggering a chemical reaction.

When these light waves penetrate the tea leaves and propagate continuously, they are constantly absorbed and transformed into heat. Combined with the remaining moisture inside the leaves, this creates the conditions for a wet-heat reaction, causing the following significant changes:

The first change is the evaporation of water. The photon energy produced by collisions between light and other molecules causes the tea to lose moisture through the penetration of light and accumulation of heat, resulting in the natural drying of the tea leaves from a macroscopic perspective.

The second change is the generation of aroma. Photochemical reactions cause molecular decomposition or rearrangement and stimulate intermolecular collisions, leading to new chemical changes. Many of the diverse aromas found in sun-dried tea come from the newly generated substances.

The third change is an improvement in quality. The wet-heat effect during sun-drying is exceptionally powerful, causing the internal substances of the tea to rapidly oxidize, polymerize, and degrade. This helps to compensate for any deficiencies in the initial fixation process, further eliminating the “raw and astringent taste” and reducing various bitter compounds, making the tea more “sweet, mellow, and balanced.”

The fourth change is a transformation in appearance. During sun-drying, continuous exposure to sunlight causes the cellular sap within the leaves to start flowing and its concentration to increase. Some low-boiling-point aromatic compounds evaporate, and enzymes will continue to degrade chlorophyll in the leaves until it completely disappears.

At the same time, polyphenol oxidase comes into contact with the phenolic groups in the tea, causing enzymatic reactions and generating phloretin, catechins, epicatechins, and a series of flavonoids (primarily dimers to heptamers) based on the epicatechin skeleton structure.

The specific biochemical process involves the original chlorophyllase in the tea leaves hydrolyzing chlorophyll into phytyl alcohol and pheophorbide, which is then oxidized under the catalytic action of polyphenol oxidase into unstable ortho-quinones. These compounds undergo non-enzymatic oxidation reactions, causing the fresh leaves to transform from deep green to gray-yellow, gray-white, or gray-brown.

The fifth change is the elimination of harmful bacteria. As the tea absorbs heat, it has both antibacterial and microbial activation effects, eliminating most pathogenic microorganisms and ensuring the safety of drinking Pu'er tea.

The final change is the inoculation of microorganisms. The previous kneading allows air-borne microorganisms to enter the interior of the tea leaves. With solar radiation temperatures ranging from approximately 15°C to 40°C, which is the optimal temperature range for aerobic fermentation, microorganisms can further react with the enzymes and other substances originally present in the tea, completing aerobic fermentation.

Different processing locations have different environmental conditions, and thus different types of airborne microorganisms and bacterial communities. These microorganisms inoculated into the Pu'er tea will significantly impact the flavor during the post-fermentation stage, explaining why teas made from the same raw materials but processed in different places can have different flavors.

Therefore, while sun-drying appears to be a simple step, the process is complex and far-reaching. How do variations in the intensity, duration, and quality of sunlight affect Pu'er tea? Is the fundamental reason for differences in the quality of Yunnan Pu'er tea due to objective differences in the photonic environment of tea mountains and atmospheric microbiota? We do not yet know.

All things grow with the sun, and Pu'er tea sun-drying relies on sunlight. In today's world where infrared drying technology, gamma rays, laser technology, and even blackbody radiation technology have been developed, sun-drying, which also utilizes light, should receive more research and attention.

If after the initial processes of fixation, kneading, and sun-drying, the result is “sun-dried raw tea,” then the next steps of compression and long-term aging, as well as the pile fermentation of ripe tea, mark a new phase—the anaerobic fermentation stage.

However, if the finished sun-dried raw tea is stored as loose material without being completely isolated from oxygen, no matter how long it is stored, it will only undergo aerobic fermentation, lacking the anaerobic fermentation process, resulting in a mass of carbonized waste tea.

This is because for Pu'er tea to achieve the characteristic of becoming more fragrant over time, it must undergo cycles of aerobic and anaerobic fermentation. Aerobic fermentation primarily completes the biological oxidation of substances, while anaerobic fermentation decomposes complex organic compounds into simpler substances that aerobic fermentation cannot accomplish.

In the next article, we will discuss the key differences between sun-dried raw tea and Pu'er tea—compression and anaerobic fermentation.