Pu'er tea is most intriguing due to its transformation, from raw to ripe, from new to aged, constantly evolving and painting a rich Landscape of the Pu'er world.
We often see the results of this transformation easily, such as the change in infusion color to orange-red or the emergence of Plum, fruity, and woody aromas. But where do these flavors come from? What changes are occurring within the Pu'er tea? Let's take a look together.
1
Dry Matter Loss
During storage, there are many reasons for dry matter loss in Pu'er tea. Pu'er tea consumes oxygen and releases carbon dioxide; some proteins and amino acids deaminate and release ammonia; pectic substances decompose and release methanol; certain organic acids are oxidized and release carbon dioxide, among other reactions.
In the storage process, hydrophilic colloids in Pu'er tea undergo decomposition. The Water that was originally bound with the colloidal substances becomes free, leading to a “self-moistening” phenomenon in stored Pu'er tea. As this self-moistening occurs, water-soluble colloidal substances in the tea may undergo hygroscopic dissolution (this is particularly noticeable when storing Pu'er tea with low maturity).
All of these factors contribute to dry matter loss. During storage, the amount of dry matter loss in Pu'er tea is related to the oxidation reactions occurring within the tea.
The more intense and longer the maturation conditions, the greater the gas release and dry matter loss. The higher the content of organic acids in Pu'er tea, the greater the dry matter loss during storage. Dry matter loss is also influenced by storage conditions and the chemical composition of the tea.
The grade of Pu'er tea raw materials, moisture content, Packaging volume weight, and other factors all have different effects on dry matter loss. The lower the grade of Pu'er tea, the greater the dry matter loss. As the moisture content of Pu'er tea increases, so does the dry matter loss. The smaller the packaging volume weight of Pu'er tea, the more the dry matter loss. Additionally, during storage, the temperature of Pu'er tea has a positive correlation with dry matter loss.
Dry matter loss in Pu'er tea leads to a decrease in weight. The extent of weight loss due to dry matter loss can reflect the intensity of chemical reactions during storage, but it does not necessarily indicate the success or failure of the storage method. Weight loss does not always signify successful storage.
2
Changes in Heat
During the storage process, a series of chemical changes occur in Pu'er tea, producing carbon dioxide and other volatile products. These reactions are exothermic processes. Suitable heat is key to the maturation of Pu'er tea, and room temperature is not equal to the center temperature of the tea package.
Two phenomena are commonly observed during the storage of Pu'er tea: high moisture content in raw tea leads to heating of the packaging in warehouses during late spring and early summer as temperatures rise; additionally, in a storage room at 25°C, the center temperature of the tea often exceeds room temperature by 5-7°C. These phenomena suggest that slow oxidation occurs during storage, releasing carbon dioxide and other volatile substances, along with heat.
The greater the heat generation, the more intense the quality changes in Pu'er tea. Based on these principles, we can use the center temperature as a technical indicator for storage process operations to control the changes in stored Pu'er tea. However, it should be noted that the center temperature does not fully represent the size of heat generation.
The size of the tea packaging, tightness, volume of the storage room, speed of heat dissipation, and heat lost due to evaporation of moisture in the tea all affect the center temperature. The center temperature only indicates the excess heat after subtracting heat dissipation from the heat generated by volatilization.
3
Generation and Changes of Other Trace Gases
During storage, the internal components of Pu'er tea undergo complex chemical changes, with some volatile substances being released. For this reason, different odors can be detected during storage. In the initial stage of storage, a strong raw green and miscellaneous odor can be smelled;
In the middle stage, the green and miscellaneous odor significantly decreases, and by the end of the process, it is almost undetectable. When a strong aroma devoid of low-boiling-point green odors is detected, the aroma of the Pu'er tea has reached a level close to purity, eliminating green, miscellaneous, bitter, and fishy flavors. At this point, the tea has entered what is considered a safe period of storage, with relatively stable changes, and the tea's aroma is revealed.
This suggests that the volatile substances emitted by Pu'er tea vary at different stages of artificial storage. According to the objective facts and laws mentioned above, this can be used as a criterion for judging the degree of storage.
4
Changes in pH Value
During the storage process, the changes in pH value follow a certain pattern, although they are not significant. Generally, after storage, the pH value of Pu'er tea shows a decreasing trend.
5
Changes in Tensile Elasticity and Hygroscopic Capacity
Under normal temperature and Humidity conditions, the ability of Pu'er tea to absorb water from the surrounding air (hygroscopic rate) and the mechanical properties of the leaf tissues, such as tensile strength and elasticity, undergo a certain degree of change.
The hygroscopic capacity of Pu'er tea depends on the content of permeable substances and high molecular substances. During storage, the hygroscopic capacity of Pu'er tea decreases. After storage, the hygroscopic capacity of Pu'er tea at different times is reduced by 4.75% to 19.02% compared to before storage. Within a certain moisture content range, the tensile strength of Pu'er tea increases with an increase in moisture content, but further increases in moisture content weaken its strength. Stable hygroscopic capacity promotes benign changes, while exceptionally strong hygroscopic capacity may lead to abnormal changes.
Different storage temperatures have different effects on the tensile strength of Pu'er tea tissues. If Pu'er tea has insufficient moisture content during high-temperature storage, its elasticity significantly decreases, and dry matter loss increases.
6
Changes in Ash Content
After storage, the chemical components within Pu'er tea change, leading to variations in ash content. Upon measurement, there is a slight reduction in chlorine content, indicating a minor alteration in the extractable substances of stored Pu'er tea. Additionally, nitrogen-containing compounds in Pu'er tea, such as ammonia, plant alkaloids, amino acids, and other substances containing “reducing nitrogen,” disappear or transform after storage.
7
Color Changes
After storage, the browning reaction in Pu'er tea is quite evident, and this reaction is considered an important chemical process for the production of volatile aromatic substances. The mechanisms behind the browning reaction during storage are now largely understood, with two main types of mechanisms playing a dominant role in the browning reaction of Pu'er tea.
The hydroxylation reaction (Maillard reaction) is a complex and somewhat difficult-to-solve series of continuous chemical reactions. The aroma compounds produced during the intermediate stages of this reaction play a crucial role in generating the aroma of Pu'er tea;
The second mechanism is caramelization, which involves the heating of sugars leading to a series of reactions and the formation of furan-like aroma compounds, imparting a creamy fragrance. This process reflects a deepening of the leaf color, and after the browning reaction, the colors turn brownish-yellow, brownish-red, and brown, significantly improving the intrinsic quality of the tea.
Varieties of Pu'er tea from different regions have different natures. Under the same storage conditions, varieties with softer leaves, finer tissue