Extract from Wikipedia:

Pu-erh, Pu'er tea, Puer tea or Bolay tea is a type of tea made from a "large leaf" variety of the tea plant Camellia sinensis and named after Pu-erh county near Simao, Yunnan, China.

Pu-erh tea can be purchased as either raw/green (sheng) or ripened/cooked (shou), depending on processing method or aging. Sheng pu-erh can be roughly classified on the tea oxidation scale as a green tea, and the shou or aged-green variants as post-fermented tea. The fact that pu-erh fits in more than one tea type poses some problems for classification. For this reason, the "green tea" aspect of pu-erh is sometimes ignored, and the tea is regarded solely as a post-fermented product. Unlike other teas that should ideally be consumed shortly after production, pu-erh can be drunk immediately or aged for many years; pu-erh teas are often now classified by year and region of production much like wine vintages.

While there are many counterfeit pu-erhs on the market and real aged pu-erh is difficult to find and identify, it is still possible to find pu-erh that is 10 to 50 years old, as well as a few from the late Qing Dynasty. Indeed, tea connoisseurs and speculators are willing to pay high prices for older pu-erh, upwards of thousands of dollars per cake.

Pu-erh tea is available as loose leaf or as cakes of compacted tea.

Introduction and History

Pu-erh tea is traditionally made with leaves from old wild tea trees of a variety known as "broad leaf tea" or Camellia sinensis var. assamica, which is found in southwest China as well as the bordering tropical regions in Burma, Vietnam, Laos, and the very eastern parts of India. Yunnan began to process tea in the Three Kingdoms period (220-360). Allegedly, at this time Zhu Ge Liang from Sichuan, a clever tactician, encouraged the Yunnan people to cultivate tea to improve their lives. Still, Yunnan people call Zhu Ge Liang their tea god. He is still prayed to in the south of Yunnan where he conquered more that 2000 years ago. This is the area of Xichuanbanna and the six famous mountains.  At the heart of Yunnan’s south west is the city of Puer, the base of the province’s ancient tea market, and from where “puer tea” derives its name. (The city Simao in 2007 changed its name to Puer, and should not be confused with the ancient city). From Puer city, tea was distributed to Tibet, and South East Asia on “tea horse trading roads.” The rough terrain of Yunnan demanded efficiently packed tea, so compressed cakes of tea were wrapped and tied into stacks of seven and enclosed into a bamboo shell. For this reason, certain puer cakes are commonly labled “Qi Zi Bing” or literally Seven Piece Cake. Carrying tea though the humid rain forest over the long, hard trading routes may have encouraged natural fermentation.

The shoots and young leaves from this varietal are often covered with fine hairs, with the pekoe (two leaves and a bud) larger than other tea varietals. The leaves are also slightly different in chemical composition, which alter the taste and smell of the brewed tea, as well as its desirability for aging. Due to the scarcity of old wild tea trees, pu-erh made using such trees blended from different tea mountains of Yunnan are highly valued, while more and more connoisseurs are seeking pu-erh with leaves taken from a single tea mountain's wild forests. The history of pu-erh tea can be traced back to the Eastern Han Dynasty.

Pu-erh is well known for the fact that it is a compressed tea and also that it typically ages well to produce a pleasant drink. Through storage, the tea typically takes on a darker colour and mellower flavour characteristics. Often pu-erh leaves are compressed into tea cakes or bricks, and are wrapped in various materials, which when stored away from excessive moisture, heat, and sunlight help to mature the tea. Pressing of pu-erh into cakes and aging the tea cakes possibly originated from the natural aging process that happened in the storerooms of tea drinkers and merchants, as well as on horseback caravans on the Ancient tea route that was used in ancient Yunnan to trade tea to Tibet and more northern parts of China.  Compression of the tea into dense bulky objects likely eased horseback transport and reduced damage to the tea.

Production

All types of pu-erh tea are created from máochá, a mostly unoxidized green tea processed from a "large leaf" variety of Camellia sinensis found in the mountains of southern Yunnan. Maocha can undergo "ripening" for several months prior to being compressed to produce ripened pu-erh (also commonly known as "cooked pu-erh"), or be directly compressed to produce raw pu-erh.

While unaged and unprocessed raw pu-erh is technically a type of green tea, ripened or aged raw pu-erh has occasionally been mistakenly categorised as a subcategory of black tea due to the dark red colour of its leaves and liquor. However, pu-erh in both its ripened or aged forms has undergone secondary oxidization and fermentation caused both by organisms growing in the tea as well as from free-radical oxidation, thus making it a unique type of tea.

In China, where fully-oxidised tea ("black tea") is known as "red tea," pu-erh is indeed classified as a "black tea" (defined as post-fermented), something which is resented by some who argue for a separate category for pu-erh as many other black teas tend to be of lower standard and status.

Raw pu-erh and Máochá

After picking appropriate tender leaves, the first step in making raw or ripened pu-erh is converting the leaf to máochá  literally, "light green rough tea" or "rough tea" respectively. Plucked leaves are handled gingerly to prevent bruising and unwanted oxidation. Weather permitting, the leaves are then spread out in the sun or a ventilated space to wilt and remove some of the water content. On overcast or rainy days, the leaves will be wilted by light heating, a slight difference in processing that will affect the quality of the resulting maocha and pu-erh. The wilting process may be skipped altogether depending on the tea processor.

Relatively young Raw pu-erh. Note the grey and dark green tones.

The leaves are then dry pan-fried using a large wok in a process called "kill green", which arrests enzyme activity in the leaf and prevents further oxidation. With enzymatic oxidation halted, the leaves can then be rolled, rubbed, and shaped through several steps into strands. The shaped leaves are then ideally dried in the sun and then manually picked through to remove bad leaves.  Once dry, máochá can be sent directly to the factory to be pressed into raw pu-erh, or to undergo further processing to make ripened pu-erh.  Sometimes maocha is aged uncompressed and sold at its maturity as aged loose-leaf raw pu-erh.

Raw pu-erh tea, also known as "uncooked pu-erh" or "green pu-erh," is simply máochá tea leaves that have been compressed into its final form without additional processing.

Ripened pu-erh

Ripened pu-erh. Note the orange-brown tone of the lighter leaves due to oxidation/fermentation.

Ripened pu-erh tea is pressed maocha that has been specially processed to imitate aged raw pu-erh. Although it is more commonly known as "cooked pu-erh," the process does not actually employ cooking to imitate the aging process. The term may come about due to inaccurate transliteration due to the dual meaning of "shoú" as both "fully cooked" and "fully ripened" .

The process used to convert máochá into ripened pu-erh is a recent invention that manipulates conditions to approximate the result of the aging process by prolonged bacterial and fungal fermentation in a warm humid environment under controlled conditions, a technique called wòdÅ«i ("wet piling" in English), which involves piling, dampening, and turning the tea leaves in a manner much akin to composting.

The piling, wetting, and mixing of the piled máochá ensures even fermentation. The bacterial and fungal cultures found in the fermenting piles were found to vary widely from factory to factory throughout Yunnan, consisting of multiple strains of Aspergillus spp., Penicillium spp., yeasts, as well as wide range of other microflora. Control over the multiple variables in the ripening process, particularly humidity and the growth of Aspergillus spp., is key in producing ripened pu-erh of high quality. Poor control in fermentation/oxidation process can result in bad ripened pu-erh, characterized by badly decomposed leaves and an aroma and texture reminiscent of compost. The ripening process typically takes anywhere from half a year to one year after it has begun. As such, a ripened pu-erh produced in early 2004 will be pressed in the winter of 2004/2005, and appear on the market between late 2005 or early 2006.

This process was first developed in 1972 by Menghai Tea Factory and Kunming Tea Factory to imitate the flavor and color of aged raw pu-erh. This technique was an adaptation of "wet storage" techniques that were being used by merchants to falsify the age of their teas. Mass production of ripened pu-erh began in 1975. It can be consumed without further aging, though it can also be stored to "air out" some of the less savory flavors and aromas acquired during fermentation. The tea is often compressed but is also common in loose form. Some collectors of pu-erh believe that ripened pu-erh should not be aged for more than a decade.

Video: Breaking apart a Pu-erh Tea Cake - produced by Cloud, author of "First Step to Chinese Puerh Tea