Tea can be classified into “unoxidized tea,” “semi-oxidized tea,” “oxidized tea,” and “fermented tea” according to the fermentation/oxidation degree. This article guides you through “semi-oxidized tea,” which is least-talked-about among them.

What is semi-oxidized tea?

Semi-oxidized tea is a tea that has been oxidized halfway. “Oolong tea” is representative of it. You can consider it is just in the middle between black tea (oxidized tea) and green tea (unoxidized tea). Oolong tea used to be the only semi-oxidized tea in Japan. However, an unusual semi-oxidized Hōjicha (roasted green tea) produced by some farms is becoming a hot topic these days. In China, the home of tea, semi-oxidized tea is further classified into three types according to the oxidation degree.

Features of semi-oxidized tea’s flavor, aroma, color

Just think about oolong tea, and you will easily understand that semi-oxidized tea has a pleasant scent that is more aromatic than green tea. It is slightly bitter, but has a deep, savory flavor. The refreshing tea is often preferred after a greasy meal with a strong taste, such as meat and Chinese food.

Features of semi-oxidized tea’s ingredients

Tea leaves contain various ingredients. The process of oxidation yields aromatic components to make aromas and pigment components to make colors. Catechin is a component that influences the color of tea. As oxidation turns the liquid color (the color of brewed tea) redder, black tea, an oxidized tea, has a red liquid color. In the case of semi-oxidized tea (oolong tea), which is oxidized only half, the liquid color stays brownish, halfway from green to red.

Features of semi-oxidized tea process

Semi-oxidized tea’s fresh leaves are sun-dried and then spread and roasted in the room for some time. The roasted tea leaves are put in a cloth bag for a while. After kneading and drying, the tea is ready. While oxidized tea is placed in a humid room to be fully oxidized, semi-oxidized tea undergoes roasting to halt oxidization.

Types of semi-oxidized tea

It is uncommon in Japan, but in China the name of semi-oxidized tea varies by oxidation degree.

White tea (bai cha)

White tea is the least oxidized semi-oxidized tea and also called “weakly oxidized tea.” Its manufacturing process does not include “kneading.”

Blue tea (qing cha/oolong tea)

What we know as oolong tea is a member of blue tea. Its oxidation degree varies a lot by type, but blue tea is the most common semi-fermented tea.

Yello tea (huang cha)

Yellow tea has undergone a special heating treatment. After ripening the half-oxidized leaves, the liquid color becomes yellow, as its name suggests.

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How to Brew Nice Oolong Tea(Chinese Tea)

Manufacturing Process of Semi-Oxidized Tea(Oolong Tea)

What Kinds of Ingredients/Nutrition are In Oolong Tea(Semi-Oxidized Tea)?

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Do you know what kind of ingredients oolong tea contains?

Of course, it is not necessary to be familiar with the ingredients, but if you know that, you can see what kind of effects you can get from black teas.

Let’s see what’s contained in your cup of black tea!

Oolong tea Ingredients

Because oolong tea is created by fermentation(oxidation), its ingredients are similar with black tea. Most unique characteristics of black tea ingredients are catechins and aroma component.

Catechin

Catechin is a one of polyphenols and also called as tannin. It is a typical ingredients of tea, the astringency taste is coming from catechin.

However, in the case of oolong tea, catechin turned into theaflavin & tearbidine through the oxidation.

Theaflavin / Thearbigin

The brown color of the oolong tea consist mostly of theaflavin & thearbigin, both formed by oxidation of catechin.

Catechin doesn’t have any color, but theaflavin has an orange, and thearbigin has a red color. That’s why the color of tea leaves changed from green to dark brown.

Oolong tea had less oxidation compared to black tea, means the amounts of theaflavin & thearbigin is less than black tea. Therefore, the color of oolong tea isn’t like black teas.

Caffeine

Caffeine consist the bitterness taste of the tea.  It doesn’t change during the processing of oolong tea leaves, therefore, the amount of caffeine is same before and after processing.

Actually, caffeine is contained about 3% of the weight of the tea leaves, this number is 2~3 times larger than the same amount of coffee beans. However, the amount of caffeine contained in a cup of tea is much smaller than coffee, because the amount of tea leaves(or coffee beans) per cup is different and caffeine is much easier to be extracted from crushing coffee beans.

Caffeine doesn’t extract with 50~60℃ water, but extract well with hot water(higher than 80℃). As usual, 95℃ water is used for oolong tea, the amount of caffeine extract is larger than green tea.

Saponin

Saponin is also an ingredients that consist the bitterness of the tea, but the amount is quite small. Because saponin has a property as a surfactant, it causes tea to foam.

The amount is too small to expect its health benefit, but it is a constitution of bitter taste of the tea.

Amino acid

Amino acids are the ingredients that consist umami of the tea.

There are some kinds such as theanine, glutamic acid, aspartic acid, arginine, serine etc… in particular, theanine occupies about 50% of total amino acid, and is typical ingredients of tea.

Because amino acids don’t change through oxidation, they are contained in oolong tea leaves. However, there is no covering process and usage of nitrogen fertilizer in the cultivation process of oolong tea leaves, the final amount of amino acid remains in the leaves is smaller than green teas.

In addition to this, caffeine and catechin are extracted largely with high temperature, means bitterness and astringency will be strong enough to cover up the umami of amino acids.

Since oolong teas are usually brewed with high temperature, the taste coming from amino acids are rarely stand out.

Aroma component

Actually, there are few aroma component contains in tea leaves before processing.

Through the crumpling process(one of the processing steps of tea leaves), the cell walls of tea leaves are destroyed, and the oxidation is intensified, flower-like or fruit-like aroma component is formed.(Green tea doesn’t contain many aroma components because there are no oxidation process)

About 200 types of components exist in green tea, and more than 600 types exist in black tea and oolong tea.

Basically, the more fermentation progresses, the more aroma components are created. Therefore, the total amount of aroma component in oolong tea is less than black tea.

In the case of oolong tea, Linalool(lemon-like scent), Geraniol & Indole(flower-like scent), Nerolidol(jasmine-like scent), Jasmine lactone(peach, apricot-like scent) are the main component of its aroma.

[Tadakatsu Takeo(1983), Variations in the Aroma Compound Content of Semi-Fermented and Black Tea]

https://www.jstage.jst.go.jp/article/nogeikagaku1924/57/5/57_5_457/_pdf/-char/ja

Vitamin

Fresh tea leaves contain a lot of vitaminA,C,E,B and so on. However, they are oxidized and changed to other ingredients during the processing of tea leaves, almost no vitamins remain in the oolong tea leaves.

Chlorophyll

Chlorophyll is contained in fresh tea leaves, but they are oxidized and changed to other ingredients during the processing of tea leaves, almost no chlorophyll remain in the oolong tea leaves.

Many ingredients are contained in black tea leaves

Like this, the taste, aroma and color is consisted from many ingredients. The type and quantity differ depends on the types and the processing method, it causes the difference of each oolong tea.