In addition to the characteristics of the variety, flavor, and aroma of tea are determined by different oxidation levels, therefore, almost every part of the tea production processes focus on controlling the oxidation level of tea. In Chinese tea, there are 6 type of tea. Green tea is non-oxidation, black tea comes with full oxidation, and there are also tea with different oxidation levels between these two that belong to semi oxidaiton tea like white tea and oolong tea. 

The aims of steps include Withering, Rolling and Rubbing are controling the level of oxidation. Therefore, the oxidation of tea is the most crucial step of the tea production process.

Catechins in tea

The oxidation of tea keep being misunderstood as the fermentation of tea, the only type of tea that has the process of fermentation is dark tea, including the pu erh tea. When it comes to oxidation in tea, it means oxidation of Catechin in tea. Take oolong tea as example, the aroma will change from herbaceous(grassy and cereals) to floral, and then fruity also woody.

In general, green tea is considered to have the highest content of catechins, making it the most beneficial type of tea for the human body, however, this is not entirely accurate. During the oxidation process of tea, catechins can transform into theaflavins and thearubigins. Green tea, being a non-oxidized tea, tends to have a relatively high catechin content.

In fact, all of catechins, theaflavins, and thearubigins are types of polyphenols. Polyphenols are often referred to as antioxidants, and the content of polyphenol in all types of tea is quite similar. Therefore, the impact of each type of tea on the body is also similar.