Course:FNH200/Projects/2026/PU-ERH TEA

Introduction

What is Pu-erh tea?

Pu-erh tea is a traditional tea from Yunnan Province, southwestern China, made from Camellia sinensis. Unlike many teas, Pu-erh continues to undergo chemical and microbial changes during post-fermentation and aging, which influence its colour, aroma, flavour, and texture [7]. It is generally divided into raw (sheng) and ripe (shou) Pu-erh. Raw Pu-erh is made from sun-dried leaves and develops gradually through natural aging, while ripe Pu-erh undergoes controlled post-fermentation to accelerate these changes.[9]

History and origin

Pu-erh tea is closely associated with Yunnan, southwestern China, a mountainous region with a long history of tea cultivation. Its history is connected to tea trade routes linking Yunnan with Tibet and other parts of Asia, particularly the Tea Horse Road, where tea was transported by horse and mule caravans across mountainous terrain. Tea was often compressed into cakes to make transportation and storage easier. These trade networks helped establish Pu-erh as an important regional product and contributed to the cultural significance of aged and stored tea.

Raw Material

All tea comes from the same species, Camellia sinensis, but differences in cultivar, growing environment, and processing create different types of tea. Two major varieties are C. sinensis var. sinensis (China type), which has smaller leaves and is adapted to cooler conditions, and C. sinensis var. assamica (Assam type), which has larger leaves and is suited to warmer regions. Pu-erh is traditionally produced from the large-leaf assamica type, particularly in Yunnan. These leaves contain important compounds such as polyphenols, catechins, caffeine, and amino acids. During Pu-erh processing, post-fermentation, and aging, these compounds undergo chemical and microbial transformations that contribute to the tea’s colour, aroma, flavour, and overall quality. [7]

Climate and altitude and soil

Climate strongly affects Pu-erh tea growth and quality through temperature, rainfall, humidity, sunlight, and elevation. In Yunnan, suitable conditions include 15–27°C, 1,200–1,600 mm annual rainfall, high humidity, and frequent fog, which provides moisture. Sunlight and cloud cover influence the plant’s microclimate and chemical compounds. Higher elevations generally have lower temperatures, higher humidity, and more cloud/fog, affecting aromatic compounds. These geographical differences contribute to variations in aroma and flavour among Pu-erh from different mountains. [8]

Pu-erh tea is grown in well-drained, acidic soils rich in organic matter, which are typical of the mountainous tea-growing regions of Yunnan. These soils can contain varying amounts of minerals and nutrients, while their moisture retention and drainage influence tea plant growth and the chemical composition of the leaves. [6]

Processing

Harvesting

During harvesting, the first 4-5 leaves, including the bud, are selectively hand-picked. To prevent immediate oxidation and bruising, the leaves must be processed quickly and handled gently. Piling or compressing the leaves can damage tea quality by accelerating oxidation. Instead, harvested leaves are spread thinly across bamboo mats or mesh trays to prevent bruising and minimize oxidation. They then undergo gradual withering, which reduces moisture and prepares them for further processing.

Kill-green

After withering, the leaves undergo kill-green via stir-roasting in large woks. Repeated tossing reduces moisture and controls enzymatic activity. Precise temperature control is critical as excessive heat destroys essential microorganisms and reduces distinctive characteristics, while insufficient heat leaves excessive enzymes and moisture, producing sour or watery flavours. Proper roasting inactivates most oxidative enzymes, including polyphenol oxidase, while preserving some microbial and enzymatic activity needed for later aging.

Rolling

At this stage, pu-erh leaves remain resistant to water penetration, limiting soluble flavour and aromatic release during brewing. Rolling mechanically breaks down the leaves’ cellular structure to enhance compound release. The intensity and duration must be carefully controlled as insufficient rolling can produce weak, underdeveloped flavours, while excessive rolling may cause leaves to break down too quickly and lose flavour during repeated infusions. Proper rolling ensures gradual, consistent extraction to maintain a balanced flavour across multiple infusions..

Sun-drying

After rolling, the leaves are sun dried for 3-5 hours at temperature above 30°C to reduce the moisture content to approximately 8%. Sun-drying is a defining, essential stage that supports later aging and flavour development as mechanical drying uses excessive heat that can deactivate vital enzymes. In contrast, the heat from sun-drying preserves the enzymatic and microbial activity necessary for the tea to develop its characteristic flavour and aroma over time. Once dry, older and discoloured leaves (huangpian) are removed by hand to ensure consistency and appearance in the final product.

Pressing

Compressing tea leaves into cakes originated during the Tang Dynasty to make tea easier to transport over long distances. Today, compression remains important in pu-erh production, especially for aged teas, as compact cakes provide stability for storage and aging. The process begins by weighing loose tea leaves and steaming them to soften and reduce their volume. The steamed leaves are placed into a cloth bag, tightly wrapped, and compressed using a hydraulic press. Although traditionally round, pu-erh can be shaped into various designs using moulds. After cooling, the cake is removed from the cloth and dried in a heated room for several days before packaging and storage.

Preparation for Transportation

After drying for several days, tea cakes are individually wrapped in protective paper, traditionally made from mulberry pulp but now available in various colours, materials, and thicknesses. Cakes are then grouped into batches of five or seven, depending on their weight, and wrapped in bamboo to form a tong. The tong is briefly and repeatedly heated over fire to reduce moisture and decrease spoilage. The tongs are then kept in a heated room for several days until completely dry, after which they are ready for shipment and consumption.

Fermentation and Aging

The first step of fermentation for sheng (raw) Pu-erh tea occurs naturally through microbial action once the tea leaves have been processed, either loose or pressed into cakes, a form that is historically referred to as mao cha. The compression helps to restrict oxygen exposure and establish a well controlled environment for fermentation. A diverse community of microorganisms that are involved in this process include Aspergillus niger, Aspergillus glaucus, Penicillium, Rhizopus, and various Bacillus species. As these microorganisms inhabit the tea cake, environmental factors during storage, particularly temperature, humidity, and air circulation may control their activity. These conditions can affect both the rate at which fermentation progresses as well as the change in characteristics of colour, flavour, and aroma during the transformation of tea. Young raw Pu’erh can have a broad range of flavour profiles. Some varieties are fresh and floral, while others may be bitter, herbaceous, or astringent. As raw Pu’erh ages, its flavour profile gradually develops, with the bitterness and astringency becoming smoother and less pronounced over time. Ultimately, sheng Pu-erh tea undergoes a gradual and traditional process of fermentation that reaches maturity over an extended period of approximately 10 to 60 years.

For shou (ripe) Pu-erh tea, fermentation is conducted through a controlled wet-piling process where mao cha is arranged into heaps that reach about 70cm in height The tea is then moistened and covered with thermal blankets to help retain the heat generated during microbial activity and allow temperatures within the pile to go up to approximately 50 to 65C. Throughout this fermentation period, the microbial community undergoes a dynamic succession; Fungi tends to predominate during the early stages while bacteria becomes more prominent as fermentation proceeds. Furthermore, selected microorganisms may be added to accelerate the maturation process under controlled fermentation conditions. In this way, the tea can reach maturity within several weeks, which requires a shorter maturation period compared to that of traditionally aged sheng Pu-erh tea. Ripe Pu’erh generally has a mellow, earthy aroma and flavour and is often described as smoother and more soothing to drink.

Differences between raw and ripe Pu'erh

Raw and ripe Pu’erh can also be distinguished by their appearance and liquor colour. Dry raw Pu’erh leaves are typically dark green, whereas ripe Pu’erh leaves have a darker reddish-brown colour. When steeped, raw Pu’erh produces a bright yellow liquor, while ripe Pu’erh produces a much darker red-coloured liquor. Similar to raw Pu’erh, ripe Pu’erh can also be aged to further develop and refine its flavour profile and reduce turbidity in the tea liquor. Pu’erh teas with lighter fermentation are generally considered to have greater potential for further flavour development through aging. In addition, hybrid Pu’erh tea cakes are available, which combine raw and ripe Pu’erh in the same cake. These blends allow drinkers to experience characteristics of both raw and ripe Pu’erh in a single tea.

Storage

Storage, Preservation, and Quality Dynamics

Storage can be considered an extension of the processing of raw Pu-erh tea (RPT), because biological, chemical, and physical transformations continue during storage and actively influence aroma, taste, and overall quality. Heat, moisture, oxygen, and light are among the main environmental factors affecting these changes.

Moisture availability is particularly important. A water activity (aw) range of approximately 0.60–0.70 provided a balance between microbial-driven flavour development and storage safety. Microbial activity increased above aw 0.60, while values above 0.70 were associated with undesirable volatile compounds such as toluene and isophorone, indicating potential quality deterioration. Water activity may therefore provide a more reliable indicator of microbial stability than relative humidity or total moisture content alone.

Temperature and humidity also influence sensory development. RPT stored under wet-hot conditions was associated with a mellower taste and greater formation of aged or woody aroma compounds. In contrast, relatively dry-cold conditions retained more umami, astringency, and fresh floral or fruity characteristics.

Packaging and Food Safety Considerations

Storage packaging must protect Pu-erh while still allowing the oxygen-dependent transformations associated with aging. For teas intended for continued quality conversion, oxygen-permeable materials such as tissue paper, bamboo split, and kraft paper are recommended, although they provide limited protection from moisture and light. Pu-erh should be protected from light because photooxidation can contribute to undesirable changes in tea compounds.

Food safety is another consideration because fungal growth can lead to mycotoxin contamination. In one analysis of 36 Pu-erh samples, no aflatoxins or fumonisins were detected, while ochratoxin A was detected in four samples. Studies of mycotoxins in tea have reported considerable variation, and available evidence remains insufficient for firm conclusions about the need for public-health protection measures.

Overall, there is no one universal temperature or humidity condition for Pu-erh storage. For raw Pu-erh, controlling water activity, moisture, oxygen exposure, light, and packaging is important for balancing desirable flavour development with quality and microbial stability.

Exam Question

What are the factors that affect the extent of nutrient loss due to preservation methods? (Select all that apply)

1. Chemical and physical properties (eg. water/fat soluble)
2. Processing methods and conditions (eg. thermal processing, fermentation)
3. Storage conditions (eg. temperature, light, oxygen)
4. Shape and size of the food (eg. large, long)

Our question should be included on the final exam because it effectively integrates concepts learned throughout the course with information from our research project. The question is particularly relevant to our research project because pu-erh tea undergoes numerous processing steps, during which nutrient retention can be affected by a variety of intrinsic and environmental factors. It also connects directly to the material covered in Lesson 11 while reinforcing concepts introduced earlier in the course. These include Lesson 2, which focuses on the chemical and physical properties of food, as well as Lessons 5–10, which examine various food preservation methods. Including this question on the final exam would provide an opportunity for students to demonstrate their understanding of how these concepts are interconnected.

Correct answers are: 1, 2, and 3