Tea processing refers to the transformation of raw tea leaves into the dried leaves we use for brewing. Tea leaves have a bitter, grassy taste straight off the plant, and processing coaxes out the very different and distinctive characteristics we know in tea. Each type of tea is processed differently, and the steps permit a great deal of variability to account for the wide variety of teas in the world. Please note: pu'erh teas are processed in a completely different manner than white, green, oolong, or black teas.
Many more exhaustive descriptions exist detailing the steps involved in tea processing. Our purpose with these generalized synopses is to give you an overview as it relates to the aromas and tastes you experience when drinking a cup of tea. Hopefully it will allow you to imagine the tea's origins as well as help understand the characteristics you enjoy best in teas.
The simple act of plucking a leaf off a tea plant starts the chain of chemical reactions that leads to the various steps of tea processing. For the most part, the bud and first two leaves are harvested (usually by hand) to make tea. Harvested leaves are quickly brought to a nearby facility where tea processing continues with withering.
Chemically, the youngest leaves and buds contain the highest concentration of nutrients, and these reach their peak in early spring. Additionally, over 600 different flavors are said to exist in the branch tips. Some of these compounds within the tea leaves act as insecticides for the plant in nature by repelling pests. They are the same substances responsible for the flavors and aromas in tea that humans love. Plucking a tea leaf signals to the plant that it is being attacked; the plant's response is to release these compounds. Lucky for us!
Wilted tea leaves from the field are "withered" in the next step of tea processing. During this stage, leaves are spread out on shelves or the floor in a controlled environment (primarily to keep the humidity low), and the moisture in the leaves begins to evaporate.
The purpose of withering is to reduce moisture in the leaves while helping the aromatic compounds emerge. Tea leaves are considerably more fragrant in this step than in their finished form. Aromas develop when the leaf's starches, proteins, and fatty acids break down and become aromatic compounds.
How the wither is done and how long this stage lasts depends upon the type of tea being made. Generally speaking, the longer the withering phase, the more aromatic and darker in color the finished tea will be.
For example, green teas' withering times are intentionally short and is the reason for their green, vegetal taste. For them, artificial heat or steam is used to heat the tea leaves to 160°F (71°C) quickly. Achieving this temperature stops the development of an enzyme called polyphenol oxidase (PPO) that would otherwise make tea leaves brown and give them roasted flavors. Deactivating PPO results in the retention of the leaves' chlorophyll—and ultimately a "greener" color and taste. This step in green tea production is called "fixing."
Oolongs and black teas rely on the continued development of PPO to give them their eventual colors, smells, and aromas. Consequently, they have longer withering times. Because the layers of withering leaves generate heat similar to a compost pile, they are turned during the process to dissipate heat and instead allow further development of the colors, flavors, and aromas. Turning inflicts damage to the leaf structure which is pivotal for the upcoming step of oxidation.
Regardless of the type of tea, by the end of withering the tea leaves have a lower moisture content (between 60–70 percent), but they are still soft, pliable, and ready to be manipulated.
The tea leaves undergo that manipulation in the next step of tea processing called rolling, a task now performed more often by machine than by hand. Rolling accomplishes a couple of things. First, the leaves are gradually shaped into their final form as they're moved to and fro by the tea-maker. Chemically, the leaf structure is increasingly damaged by the manipulation. It sounds bad, but this is a good thing. Damage to the cells causes the release of polyphenols (see Antioxidants in Tea). In making oolong and black teas the polyphenols react with the still-active enzyme PPO and oxygen to start oxidation in earnest.
Taste is influenced by how the rolling is done. Lightly rolled leaves end up as a mellow tea, while those exposed to greater pressure result in a tea more brisk and intense.
Oxidation takes place in the production of oolong and black teas but not in greens since the enzyme PPO was deactivated when fixed. The amount of oxidation a tea undergoes and how quickly/slowly it happens is highly influential in the flavor, aromas, and color of oolong and black teas.
Polyphenols in the leaves continue to react with PPO and oxygen, and the resulting flavors and aromas continue to evolve. Theaflavins are the first compounds to emerge during oxidation. Generally, they make a tea brisk and astringent. Should oxidation continue during a slow, extended period, thearubigins have a chance to develop. These take the edge off the sharpness and round out the flavor of the tea.
The first flavors to emerge in oxidation are herbaceous, floral, and citrus in nature. A longer oxidation brings out stone fruit essences as well as woody and spicy notes; these are followed by additional fruit and nutty flavors.
Oolongs oxidize the most slowly of all teas and, as a result, display the most complexity and mellow characteristics. They are only partially oxidized, and their dominant flavors vary depending upon the level of oxidation they undergo corresponding to the details in the previous paragraph. Black teas are fully oxidized, but they run the gamut in terms of oxidation times. Chinese black teas oxidize slowly enough to have a complexity and smoothness similar to oolongs. On the opposite end of the spectrum, CTC teas (see Whole Leaf or CTC Tea) oxidize so quickly that the bold theaflavins remain dominant.
A fine tea relies upon the expertise of the tea-maker and his/her nose to stop oxidation at just the right moment.
Firing (sometimes also called "drying") halts oxidation in the next stage of tea processing. It also removes additional moisture from the leaves so that the finished tea doesn't mold or rot.
Firing exposes tea leaves to heat from one of a variety of sources (i.e., charcoal, tumble dryers, ovens, sun, steam, wood). Heating concentrates some of the tea's flavors and initiates the last of the chemical reactions to take place including the development of yet additional aromatic compounds.
The most traditional firing methods result in the most distinctive teas. Green teas can be sweetened in this stage while others acquire hints of chocolate or maltiness. Sun-dried teas are known for their fluffiness because of the lack of equipment exerting pressure on the leaves. Charcoal imparts a grassy taste, steaming a vegetal one, and wood adds smokiness.
Firing is a quick process. One of the biggest dangers in over-firing is the destruction of the flavors and aromas developed throughout processing.
By the end of firing, the moisture content in the tea leaves has been reduced to a mere 3–6 percent. The leaves are cooled quickly to stop further chemical reactions and prevent additional moisture loss.
Whew! The tastes and aromas of teas have been safely captured in its leaves. After cooling, the tea is separated by leaf/particle size and graded (depending upon the type), packaged, and transported around the world.
Infusing dry leaves when brewing brings out all the flavors and aromas developed during processing. Something worth remembering when brewing…each of the steps of tea processing involves removing water from the leaves while accentuating the developing aromas and flavors. Infusing the leaves in hot water adds back in the moisture. Brewing with high quality water at the right temperature will give you a cup of tea as the tea-maker intended. Enjoy!