Traditional style tea brewing in a clear cup

Fermentation vs Oxidation of Tea: Let's Learn the Difference

Sep 02, 2022

Johnathan Milton

Tea is a ubiquitous, relaxing, and delectable beverage that helps us stay fit and healthy. It may be the first drink we anticipate after a restful night's sleep or when we feel agitated and fatigued at work or elsewhere. Tea comes in diverse flavors and types, and we appreciate them all. Tea leaves begin to wither with the tea plucking, necessitating great care and skill. Any minor modification, from harvesting them at the right time to treating them properly, can substantially influence the overall outcome of flavor, aroma, and nutritional content. Two such conditions are oxidation and fermentation. Many seem to think these two processes are the same, so we will clarify this in detail for you. What exactly is the difference between fermentation vs oxidation? Let’s find out. 

Tea is a popular brew in many nations worldwide for its many flavors and health advantages. Tea making is a meticulous process that has evolved over hundreds of years of trial and error. Scientific studies have now helped us clarify the chemical processes that occur during the various steps of manufacturing these flavorful teas. Still, the long history and closely guarded crafting traditions have resulted in some common misunderstandings and muddled terminology. You may have overheard tea experts discussing the effects of tea oxidation and tea fermentation on the overall flavor of the tea. These are among the most important steps of tea making, so it is essential to learn them correctly. 

What is Oxidation?

Oxidation is a biological and enzymatic reaction. If you slice an apple and leave the cut section out in the open for a while, you will see that the natural color of the apple turns brown and then black. In a short amount of time, the fruit or vegetable becomes mushy. This is the process of oxidation occurring naturally around us. It occurs when the host material begins to absorb oxygen, resulting in a chemical and enzymatic alteration. Oxidation has several distinct properties and attributes that we must learn as beginners.

Now, let's understand how oxidation occurs in processed tea leaves. During tea making, there’s a step called “rolling”. For this process, the tea leaves go through various rolling machines, whereupon the cell walls of the leaf get damaged and exposed to the air. Tea leaves contain a special enzyme called “polyphenol oxidase”. When a tea leaf is damaged, this enzyme reacts with atmospheric oxygen to convert polyphenols in the tea into color and flavor-developing compounds. In other words, the color of the leaves darkens as a specific polyphenol reacts with oxygen. This process also changes the flavor, chemical content, and strength of the tea. The oxidation process was not very clear in the early days of tea production. People believed that this process resulted due to activities of microbial organisms, so many started to use the term fermentation to identify this reaction. And, this is where all the misunderstandings began! 

What is Fermentation?

Fermentation, a biological procedure incorporating microorganisms (bacteria), has long been employed in food and beverage preparation to produce various health and flavor benefits. The fermentation process in food and beverages uses microscopic organisms like yeasts and other helpful bacteria to convert one component into another under specific conditions that vary depending on the intended outcome. Here’s what happens in this process. Microbes invade foods to obtain energy from chemical reactions. During these chemical reactions, microbes produce acids (acids such as lactic acid) and alcohols while helping better food preservation. This process has different effects. Few include enhancing nutrient bioavailability and nutritional density, changing the flavor profile of a meal or beverage, and lowering antinutrient levels.

In tea processing, some tea types go through a fermentation process to develop a specific flavor and aroma. During this process, tea makers first produce black tea, and then this tea undergoes a specific post-fermentation process called “curing”. The most popular fermented tea type is the “Pu-erh” tea, which originates from the Yunnan province of China. Tea leaf fermentation necessitates the presence of bacteria that consume some chemicals to produce others. This gives the finished product a distinct flavor. 

Effects of Oxidation and Fermentation

The oxidation process can be a regulated process or an unregulated one. This process may result in both positive and negative effects. A basic illustration of spontaneous negative oxidation is an apple turning brown and finally black. On the other hand, if apple slices are dried and treated in a controlled atmosphere, positive oxidation could occur. This will be further discussed in the section about fermentation and oxidation in tea. 

Fermentation is a historical method of food preservation. It produces energy without using oxygen, which may be used to manufacture food and some beverages like bread or beer. Moreover, fermentation is an anaerobic process. This means it can happen even if there isn't enough oxygen in the cell.

Both of these processes do not change the caffeine content of the tea. Follow this comprehensive guide to discover caffeine levels in tea

Differences between Oxidation and Fermentation

When comparing fermentation vs oxidation, first you must know that these are entirely different processes occurring around us. The key differences between these two processes can are as follows, 

Requirement of Oxygen: Fermentation vs Oxidation

This is the main difference when we compare these two processes. The oxidation process uses enzymes and molecular oxygen for its reactions. However, the fermentation process does not use molecular oxygen for its reactions & it only uses enzymes.

Starting Compound

The starting compound of the oxidation process can vary depending on the type of reaction. However, for fermentation, the starting compound is known as sugar moiety, a specific molecular arrangement of sugar. 

End Products 

During oxidation, the end product can vary depending on the compound that the reaction occurs in. But for the fermentation process, the end products are consistent, as it produces lactic acid and ethanol. 

Tea Fermentation vs Oxidation

When considering tea, we need to grasp several basic characteristics that distinguish fermentation from Oxidation.

Whether we are mindful of it or not, soon after the harvesting of tea leaves, unregulated and rapid oxidation begins. This happens due to leaf damage that could occur during the handling of the fresh tea leaves. However, depending on the tea type produced, tea makers either allow or prevent oxidation. For instance, teas like green or white tea do not require any tea oxidation. Therefore, white tea masters take utmost care not to damage the tea leaf during harvesting and handling. Green tea producers are also careful during the harvesting process. They immediately apply the “firing” process through pan frying or steaming the tea to prevent further unregulated and negative oxidation to occur. The important factor to note is that heat works to inhibit the enzymes from performing their oxidation function. This results in little or no oxidation in the tea leaves, thus no color development or darkening. This explains why green tea retains its color even after its intense preceding steps. 

While all pure teas come from the same plant, the amount of oxidation influences how dark they become. This process can further determine the individual attributes of each tea type. Teas like oolong tea, black tea, and pu’erh tea go through tea oxidation under controlled conditions. These oxidized teas have variations among each other due to the different degrees of oxidation it goes through during production. 

In contrast, fermentation is the process of extracting energy from carbohydrates in the absence of oxygen. Most teas do not follow the fermentation process. However, fermenting techniques are applied to some fermented black tea, which creates another classification of tea called pu’erh . During the production of dark teas such as pu’erh  tea, the processed black tea leaves are compacted under humid conditions to allow microbial reactions. These microbial activities create changes in the tea leaf's chemical structure, changing its flavor and aroma. When you come across the term black tea fermentation, remember it refers to this stage of producing pu’erh tea. 

Final Thoughts 

Though they appear to be the same, oxidation and fermentation in tea are not equivalent. There are many differences in fermentation vs oxidation, and as a tea learner, this is a critical point that you should remember. Both fermentation and oxidation are responsible for many of tea's health benefits as well as its flavors and aromas. Now you have something new to keep in mind while enjoying your tea!

Leave a Comment

Please note, comments must be approved before they are published

Join Our CommuniTea