When discussing tea, terms like "fully fermented," "semi-fermented," and "lightly fermented" are often used. But is this the same fermentation as in common fermented foods like yogurt, wine, or vinegar? What are the differences?

Today, we explore the mystery of tea "fermentation"!

| What is Fermentation?

Fermentation generally refers to the breakdown of organic matter by living organisms. While fermentation has been observed for centuries, its true nature was only understood in the last 200 years. In microbiology, "fermentation" is strictly defined as the process by which organic matter is oxidized and degraded by organisms, releasing energy.

Industrial fermentation: In industrial production, any process relying on microbial activity is called "fermentation," such as beer brewing or monosodium glutamate production. Food fermentation: Fermented foods are those produced using beneficial microorganisms, offering unique flavors, like yogurt, cheese, wine, pickles, soy sauce, vinegar, and more.

| Tea "Fermentation"—Biological Oxidation

In Chinese tea classification, teas are divided into six major types based on "fermentation" level and processing methods. However, in tea terminology, "fermentation" differs from microbial fermentation. In tea, the same green leaf can become green tea, black tea, or oolong tea through controlled biological oxidation—a process more accurately described as enzymatic reactions. This "biological oxidation" occurs when cell walls break, allowing oxidative enzymes to catalyze the oxidation of catechins.

In tea leaves, catechins are in the cell fluid, while oxidative enzymes are mainly in cell walls, not microbes. This explains why "fermented" teas require rolling. The oxidation level of polyphenols determines terms like "fully fermented," "semi-fermented," or "lightly fermented." For example, black tea has high polyphenol oxidation, making it "fully fermented," while oolong tea, with about half oxidation, is "semi-fermented."

In black tea processing, fermentation oxidizes catechins, turning leaves from green to copper-red, creating black tea's unique color. When cell membranes are damaged, polyphenols and amino acids oxidize, forming black tea's distinct flavor and aroma.

This is the basic meaning of "fermentation" in Chinese tea. However, due to the diversity of teas and processing methods, some teas involve microbial activity alongside enzymatic oxidation. For example, Pu-erh tea's pile fermentation includes microbes like Aspergillus niger, Rhizopus, and yeast. Still, it's crucial to distinguish microbial fermentation from enzymatic oxidation to avoid misunderstanding tea quality formation.