When it comes to Tea, people often talk about “fully fermented,” “semi-fermented,” and “lightly fermented.” Is this the same kind of fermentation as in common “fermented foods” like yogurt, wine, and vinegar? What are their differences?
Today, let's explore the mysteries of tea “fermentation” together!
| What is Fermentation
The term “fermentation” generally refers to a certain decomposition process carried out by organisms. The phenomenon of fermentation has long been recognized by people, but understanding its essence is a matter of only the past 200 years. According to microbial physiology, “fermentation” is strictly defined as the process by which organic substances are oxidized and degraded into oxidation products, releasing energy, and this is collectively referred to as biological oxidation.
In industrial production, “fermentation” – “industrial fermentation” – is broadly defined as all industrial production that relies on the life activities of microorganisms. This includes processes like beer brewing and monosodium glutamate (MSG) production. “Fermentation” in food refers to a class of food products that are processed using beneficial microorganisms, possessing unique flavors. Examples include yogurt, cheese, sweet rice wine, pickled vegetables, soy sauce, rice vinegar, fermented soybeans, yellow rice wine, beer, and wine.
| “Fermentation” of Tea – Biological Oxidation
People often say that Chinese teas are classified into six categories based on different degrees of “fermentation” and manufacturing methods. However, the term “fermentation” used in the context of Chinese tea is entirely different from microbial fermentation. In tea, the same green leaf is processed into Green Tea, Black Tea, Oolong Tea, etc., through controlled biological oxidation, a process mistakenly referred to as “fermentation.” This process is more akin to a series of enzymatic reactions and should perhaps be called “biological oxidation.” The “biological oxidation” of tea involves the oxidation of catechins catalyzed by oxidases present in the cell walls after they are damaged.
In the cells of tea leaves, catechins are found in the cytoplasm, while oxidases are primarily located in the cell walls rather than in microorganisms, hence the need for cell wall damage. This naturally explains why “fermented” teas require kneading. Depending on the degree of oxidation of polyphenols, we can distinguish between “fully fermented,” “semi-fermented,” and “lightly fermented” teas. In black tea, where the degree of polyphenol oxidation is high, it is referred to as “fully fermented”; in Oolong tea, where the oxidation level is about half, it is called “semi-fermented.”
For example, during black tea processing, the purpose of fermentation is to oxidize the catechins in the leaves. The leaf color changes from green to copper red, generating the distinctive color of black tea. After the cell membranes of the tea leaves are damaged, the polyphenols, amino acids, and other substances within the vacuoles are gradually oxidized. Meanwhile, due to the oxidation of catechins, some of the leaf components undergo chemical reactions, producing the characteristic color, aroma, and taste of black tea.
The above is the basic meaning of “fermentation” commonly referred to in the context of Chinese tea. However, due to the vast variety of Chinese teas and the rich diversity of processing techniques, the criteria for quality formation vary. In some tea-making processes and the formation of quality characteristics, apart from the enzymatic reactions of biological oxidation, there may also be participation by microorganisms. For example, in the heap fermentation process of ripe Pu'er tea, apart from enzymatic action, microorganisms are also involved. Through isolation studies, the main microorganisms include Aspergillus niger, Aspergillus oryzae, Aspergillus clavatus, Aspergillus glaucus, Rhizopus, lactic acid bacteria, and yeasts. Nevertheless, it is important to clearly distinguish between “fermentation” involving microorganisms and “fermentation” in the sense of biological oxidation – otherwise, confusion in concepts can easily lead to misunderstandings about the mechanisms underlying the formation of tea quality.