When was the first coffee maker invented, and how does tea oxidation work?
Oxidation, long miscalled fermentation, is a reaction between polyphenols and oxygen, not yeast or bacteria. It darkens leaves and drives the aroma, color, and mouthfeel differences between oolong and black tea. Rolling shapes leaves and triggers the reaction by rupturing vacuoles that store polyphenols, so processing degree sets each cup's character.
Lightly rolled tea tends to taste mellow, while harder rolling breaks leaves into smaller pieces for a livelier, more intense brew. The British built a tea rolling machine in the 1880s, trading hand-crafted shapes for labor savings. The same question of when was the first coffee maker invented shows how mechanized processing reshaped both tea and coffee.
Rolling and oxidation are the two key stages that determine the flavor direction of tea. Rolling shapes the appearance of the tea leaves while also influencing the strength and release speed of the taste; oxidation—a process often mistakenly called "fermentation"—is actually just a reaction between polyphenols and oxygen, yet it directly determines the enormous differences in aroma, color, and mouthfeel among oolong, black, and even CTC teas. From the distinctive red edge of oolong leaves, to the apricot-like richness of Chinese black tea, to the strong liveliness of traditional British tea and CTC tea, the degree, speed, and method of oxidation are the core code that shapes the character of every cup of tea. This article will take you deeper into the profound effects of rolling and oxidation on tea flavor.
Rolling After green tea is finished and before black tea turns dark, tea makers roll the leaves, giving them a pleasing shape. In the past, this step was done entirely by hand; today it is mostly handled by machines. The rolling process turns originally flat leaves into twisted, spiral, ball, or needle shapes. Overall, lightly rolled tea has a mellower, smoother taste, while tea rolled under greater pressure breaks into smaller pieces, producing a more lively and intense brew. For centuries, Chinese tea artisans created countless ballet-like techniques, resulting in dozens of tea shapes. In the 1880s, the British invented the tea rolling machine, which saved a great deal of labor but produced tea lacking that uniqueness and refinement.
Oxidation For green tea, rolling only shapes the appearance; but in oolong and black tea, rolling also triggers reactions that darken the leaves. Oolong and black tea are not fixed green teas, but gradually darken through a process called "oxidation." In the world of tea, this process was mistakenly called "fermentation" for centuries, because people once thought yeast or bacteria were involved. In fact, this reaction involves only polyphenols and oxygen.
In healthy tea leaves, polyphenols are stored in "vacuoles"—small compartments within plant cells that are always ready to protect the leaves from insects and other threats. When insects bite the cells, leaves rub against each other, strong winds scratch the leaf surface, or tea makers damage the leaves during rolling, the vacuoles rupture and release polyphenols into the cytoplasm. In the cytoplasm, polyphenols encounter polyphenol oxidase (PPO). In green tea, this enzyme plays no role because when the leaves are heated to 160°F, the enzyme loses its activity. In oolong and black tea, however, polyphenols react with polyphenol oxidase and oxygen to produce brown compounds. In nature, these polyphenols are thought to make insects feel gastrointestinal discomfort, thereby preventing further feeding. Fortunately for tea drinkers, tea polyphenols are not only delicious but also visually pleasing. There are mainly two types of polyphenols in tea, each with its own unique flavor and color. The first to appear are the golden-yellow, lively theaflavins; then thearubigins gradually mature, taking on an attractive copper color. The slower the oxidation, the more thearubigins are produced, and the milder the brew. To learn more about theaflavins and thearubigins, please see this blog post.
Oolong tea oxidizes more slowly than other tea types, and therefore has the most complex, mellow flavor. Some oolong teas begin oxidizing during the withering stage, when tea makers gently slap the leaves spread on tarps in the sunlight. This "shaking" is an extremely light rolling that triggers oxidation along the leaf edges. It is this gentle agitation that gives oolong tea its signature red edge. When oxidation begins, the leaf edges show a deeper red than the rest of the leaf surface. After withering, the leaves gradually curl and enter a complex, intermittent oxidation process lasting six hours or even longer. This gradual oxidation extends the withering time, favoring the formation of more aromatic compounds. The final flavor and aroma depend entirely on the duration of withering and oxidation. Oolong tea with about 25% oxidation contains high concentrations of linalool, methyl jasmonate, and indole, and thus carries aromas of citrus, lemon, jasmine, and gardenia. Teas with an oxidation rate between 40% and 75% develop more apricot and peach flavors, a typical feature of carotenoid degradation, belonging to the later stage in which the pigment carotene is converted into apricot-like compounds such as ionone, damascenone, and damascone.
Chinese black tea has a rich apricot aroma. Although it oxidizes faster than oolong tea because of more forceful rolling, the speed is still slow enough to produce a sweet, mellow tea rich in thearubigins. The way to slow oxidation is not to interrupt the reaction, but to limit the leaves' exposure to oxygen. Traditionally, tea makers begin by rolling the leaves extremely gently, trying to keep them intact so that most polyphenols remain in the vacuoles. Then they pile the rolled leaves into deep, carefully woven bamboo baskets, cover them with cloth, and leave them for several hours, allowing oxygen to seep in only in thin streams. The result is a soft, delicate, full, and rounded brew.
Traditional British tea—that is, the tea the British developed in South Asia—has a liveliness due to rapid oxidation. Perhaps because the British had long preferred teas with a crisp mouthfeel, tea makers in former British colonies rolled the leaves into finer particles. Then they spread these particles in a thin layer, usually using hot air to scatter them, maximizing exposure to oxygen. This type of tea oxidizes for a longer time and more slowly, so its aroma is more complex.
By contrast, traditional British CTC tea is the most astringent of all teas, because it oxidizes almost instantly. CTC stands for "crush, tear, and curl," the three stages of the "rolling" or crushing process: instead of being rolled, the leaves are crushed, broken through fine sieves, torn into tiny fragments, and then curled into fine particles. These particles pass along conveyor belts under powerful blowers; over a distance of 100 feet and several hours, the particles, like chameleons, change from bright green to flat dark brown. This type of tea contains almost no thearubigins; its tannic theaflavins make the brew extremely lively and mouthwatering. These teas pair excellently with milk, because the proteins in milk bind with tannin compounds, making them smoother.
Oxidation turns buds golden and mature leaves black. The source of the dark color is chlorophyll, which degrades during oxidation into dark brown chlorophyll and phosphorus compounds. Buds contain very little chlorophyll, and the small amount they do contain dissolves during withering. Therefore, oxidized buds show only the golden and copper colors produced by polyphenol degradation.
Fermentation Until recently, the oxidation process was still mistaken for fermentation. Before the mid-twentieth century, tea manufacturers still believed that yeast and bacteria converted sugars into alcohol—as in wine and bread fermentation—thereby darkening the tea. In fact, only pu'er tea truly involves fermentation, and the production process of pu'er tea is a closely guarded secret, making it difficult to know exactly how it occurs. But we can speculate: as tea ages, the sugars in the tea convert into monoterpenes, and after years of oxidation and degradation, these monoterpenes transform into sesquiterpenes. Sesquiterpenes have the earthy and camphor-like notes characteristic of pu'er tea.
The above content is compiled by CoffeeHunters, a coffee news website.
