What is Oolong Tea?

Oolong Tea is a category of tea that includes a broad range of tea types. You can generally divide Tea into three main categories: Green, Oolong, and Black. The level of oxidation of the tea is what determines these three categories. Green Tea is unoxidized, Oolong Tea is partially oxidized, and Black Tea is fully oxidized. Almost all tea is made from the leaves of the tea plant Camellia sinensis, though there may be variations in specific variety of plant used.

All About Oxidation

Tumbling oolong tea leaves in a bamboo tumberl

Oxidation is a chemical change in the leaves that turns them from their fresh green color to a reddish brown color. A simple example of oxidation is when you cut an apple and expose it to air, it begins to turn brown. The same effect happens with tea leaves. Tea leaves contain natural enzymes that cause oxidation after the leaves are picked.

In making Green Tea, the leaves are exposed to high temperatures soon after picking. This destroys the enzymes that cause oxidation, which prevents the leaves from being oxidized to any significant degree. In making Black Tea, the leaves are wilted and extensively rolled to expose the sap, allowing the leaves to become fully oxidized. Oolong Tea encompasses a broad spectrum of oxidation levels that range from roughly 10% to 85%. So some Oolong Tea types are very close to Green Tea, while others are similar to Black Tea. And guess what — there are almost endless variations in between! This is what makes the Oolong category so interesting and complex.

Origins of Oolong Tea

It is commonly agreed that Oolong (烏龍) (spelled wu long in the Chinese Pinyin) Tea making originated and was refined into an art form in China. China has many Oolong Tea growing regions, all with their own local tea making traditions. The most famous Oolong Tea in China comes from the Wuyi mountain range in Fujian Province. Other renowned names of Chinese Oolong Teas are Tie Guan Yin, Fo Shou, and Dan Chong. Oolong Tea was exported to Europe and North America as early as the late 1800’s.

During the Qing Dynasty, a significant population of Chinese from Fujian Province emigrated to Taiwan. They brought with them their local traditions, including tea. Taiwan has pioneered modern tea processing in recent decades and has become known for its specialty Oolong Teas.

Taiwan Oolong Tea

Taiwan’s most popular and most produced tea type is High Mountain Tea (高山茶). This is a lightly oxidized tea type that is closer to Green Tea, but is distinguished by its complex aroma and rich, full-bodied texture. Since it is lightly oxidized and unroasted, it is sometimes described as a Green Oolong. The basic qualification for this tea type is that the tea is grown elevations above 1000m. The distinct character of this tea comes from the climate it grows in and its processing methods that were developed only in recent decades. High Mountain Oolong Tea became popular in the 1980’s in response to the sharp increase in consumer demand for specialty tea. The pre-existing local tea industry could not meet this demand that developed along with Taiwan’s economy. So tea production rapidly modernized with the invention of machines that allowed for high volume production and consistent quality. These modern methods are what made Taiwan a world leader in modern Oolong Tea production.

Making Oolong Tea

High Mountain Tea Harvest

While Black Tea and Green Tea can be seen as “all or nothing” in terms of level of oxidation, the broad spectrum of oxidation levels among Oolong Teas is the result of both subtle and complex variations in processing methods. This is where the art of Oolong Tea making lies. Achieving the complexity of aroma and flavor profiles of Oolong Teas requires skill and finesse in partially oxidizing the tea leaves.

Tea making parallels wine making in this respect. Both the growing conditions and the recipe used are key to the quality of tea that results. Both in China and Taiwan, local, and even family traditional recipes have been preserved, mirroring the variables of terroir and traditions throughout different wine growing regions of the world. It is a vast and rich cultural adventure to deepen one’s knowledge and appreciation of the world of tea. 

Basic Oolong Tea Making Steps

The basic steps in making Oolong Tea
  1. Cultivating: Tea plants are grown in rows on tea farms ranging in size from less than a hectare to hundreds of hectares. Tea cultivation in Taiwan has a rich history of migration from mainland China, as well as influences from the Portuguese and Japanese.
  2. Havesting: Tea leaves for Oolong Tea production in Taiwan are typically harvested three to four times a year. Although at lower elevations, it’s possible to harvest year-round, for up to six harvests annually.
  3. Withering: After harvest, enzymes inside the tea leaves immediately start chemical reactions that create the flavor and aroma profile of the final tea. The leaves start to wilt and go limp at this stage as moisture is lost. The tea maker can control how far the reactions go by controlling how much moisture the tea leaves lose in a process is called withering.
  4. Oxidation: The process of oxidation causes more chemical reactions within the tea leaves, adding to the flavor profile of the tea. During this step, the cell walls of the tea leaves are broken to allow exposure to oxygen. In Taiwan, this is typically done by tumbling the leaves in long woven bamboo cylinders.
  5. Kill Green: After the desired level of oxidation is reached, oxidation is halted by high temperature tumble heating the leaves in a process called “Kill Green”, or “fixing”. The name “Kill Green” comes from the Chinese term 殺青 (shaqing), which literally translates as “kill green”. This step deactivates the enzymes responsible for the oxidation process, thereby stopping the chemical transformation.
  6. Rolling & Drying: After the Kill Green process, leaves destined to become Oolong Tea are rolled and dried. During this phase, Oolong Tea leaves that undergo modern machine rolling take on a characteristic ball shape. Semi-rolled Oolong Tea leaves can be less tightly rolled into a “shrimp shaped” dried leaf form using more traditional labor intensive methods.
  7. Roasting: Depending on the type of tea being made, the tea leaves are then roasted. With traditional Oolongs, the more heavily oxidized leaves are roasted repeatedly, which not only removes all water content, but also "toasts" the leaf to varying degrees, transforming the flavor and consistency of the brewed tea.For greener high mountain tea, aka "gaoshancha" (高山茶), the significantly less oxidized leaves are most often left unroasted.
  8. Packaging: When the final processing is done, the tea is packaged

For detailed information, read our post How Oolong Tea is Made.

Caffeine in Oolong Tea

Pure crystallized caffeineMany factors affect how much caffeine goes into a cup of Oolong Tea, including the variety of plant used, where and how it was grown, how it was processed and how it was brewed. Since there is such a variety of Oolongs, the amount of caffeine in a cup of Oolong can vary quite a bit. Read our piece How Much Caffeine is in Oolong Tea to see what Taiwan’s Tea Research and Extension Station (TRES) found when they researched how much caffeine was in Oolong Teas.