May 29, 2020
Manufacturing Process of Oxidized Tea(Black Tea)

Our daily black tea attracts us with its glamorous fragrance. What do you think lends that fragrance to black tea? The secret is deeply involved with why black tea is called oxidized tea. Let us go through why black tea is called oxidized tea and what gives the fragrance of black tea, focusing on the manufacturing process and the word “oxidation(fermentation).”

Characteristics of manufacturing process of oxidized tea (black tea)

Oxidized tea is a tea produced through full enzymatic oxidation(fermentation). In contrast with unoxidized tea, it is characterized by its use of the activity of the oxidative enzymes in the tea leaf for oxidation(fermentation). “Handmade” (the gongfu method) originates in China about 2,000 years ago, but today, mechanical production is becoming mainstream. Major methods are the orthodox manufacturing method and the unorthodox manufacturing method. There are also mixed methods of these two. This article introduces the traditional “orthodox method.”

Fermentation or oxidation?

In the world of tea, fermentation refers to oxidation by oxidative enzymes in the tea leaf, not microbial or bacterial fermentation as in miso and yogurt. Oxidation refers to the reaction of enzymes with oxygen to change the original ingredients. Some teas such as fermented tea utilize microbes for fermentation, but as a rule, fermentation in the tea industry means oxidation.

From plucking fresh leaves to shipping them

Tea leaves are grown in a tea plantation. The tea leaves plucked in the picking season become Aracha (crude tea) after kneading and drying. Then, through “finishing,” the tea becomes a product to be shipped.

How to make Aracha

The plucked fresh leaves undergo the processes of withering, rolling, ball breaking/sieving, oxidation, and drying in this order to become Aracha. Then, after the finishing process, the tea is shipped across the country.

1. Withering

Withering is the process to wither the fresh leaves to evenly remove the moisture in them. Shade-drying used to be mainstream, but “artificial withering,” which uses a withering tank and withers the leaves with hot wind, is common now.

2. Rolling

This process is to break down the tea leaf cells and activate the oxidative enzymes in the leaf to shape it. The oxidative enzymes become active in contact with oxygen to cause the oxidative fermentation of catechins, pectines, and chlorophyll. It is this oxidative fermentation that is a key element to make black tea’s aroma, flavor, richness, and liquid color and is a fundamental difference between black tea and green tea. This process requires 45– 90 minutes for oxidation. To prevent a rapid oxidative oxidation, the leaves are put in the ball breaker. After cooling down, the rolling work is repeated.

3. Ball breaking/sieving

The rolling process makes balls of tea leaves. This process is to break the balls so that the leaves evenly contact the air, which promotes further oxidative fermentation. The leaves are put in the ball breaker every 20 to 30 minutes. They are sieved by the machine’s mesh. “Minus sieve” refers to the leaves which fell through the mesh to proceed to the next step. “Plus sieve” refers to the larger leaves left on the mesh to go back to the rolling step.

4. Oxidation(Fermentation)

In the oxidation chamber kept at 25–26 degrees C and 90% humidity, the minus sieve is spread evenly to the 4–5 cm thickness and left for 2–3 hours. This step changes the leaf color from green to bright copper and lends a characteristic aroma to the black tea.

However, excessive oxidation spoils the original aroma of black tea and blackens the liquid. It is important to know the right timing to stop oxidation.

5. Drying

The tea leaves still have moisture when the oxidation process is finished. To prevent further oxidation by themselves, the leaves are put in the dryer that blows hot air of about 100 degrees C. Drying deactivates oxidative enzymes and reduces the moisture to less than 5%.

Finishing

The tea after drying is already called “Aracha,” but it cannot be shipped as a product yet. In the finishing phase, it undergoes sorting/shaping and blending in this order to be ready to ship as a product.

6. Sorting (Grading)

Aracha goes through a sifter several times to be sorted by the size and shape. Sorting classifies the tea leaves into grades. This grade is called “leaf grade.”

7. Blending

The final step is blending the leaves. Over 20 types of tea leaves are used, but the purpose is not to mix various kinds. It is to stabilize the quality by choosing teas from the same production area. Blending influences the price of each black tea, and what matters is how to suit consumer tastes by blending.