For a few times, people asked me, why are many tea workers' hands as dark as ink-stained when they work on fresh tea leaves? It's actually a simple chemical phenomenon - oxidation. As we know, tea contains de-oxidants. That's partially why tea is considered healthy. What a de-oxidant does is reacting with any oxidant molecules (which is a major cause of cell aging and cell damage) very fast. Therefore many oxidant molecules are removed from the cells by such reactions, and the cells live healthier and longer.
Oxidation often causes color change, which can be observed in a lot of fruits and vegetables. For example, if you cut open a potato or apple and leave it on the countertop for a few hours, browning happens to it. The browning is because of oxidation of catechol, which is a biomolecule found in potato, apple and many other plant tissues. Catechol, when oxidized, is turned into benzoquinone. The reaction requires oxygen and an enzyme in the plant tissues. In healthy cells, catechol is not exposed to oxygen or the plants' own enzyme, therefore browning doesn't happen until the cells age or are damaged.
In tea leaf tissues, one of the major tea polyphenols is catechin (there are a group of molecules that are called catechin), which is similar to catechol but a even stronger de-oxidant. Catechin can be oxidized into molecules (such as thearubugins, dark color molecules in red/black tea) that cause dark color. Oxidation of catechin requires an enzyme in fresh tea leaves. When the enzyme is "killed" (which happens in the "kill green" step of green tea processing), the oxidation cannot happen and therefore the tea leaves will be maintained green for a long time. However, any strong de-oxidant can be easily oxidized. That's why green tea has a relatively short shelf life. Eventually oxidation can happen, only more slowly, even without the enzyme from the tea leaves. If the enzyme in tea leaves is active, then when the tea leaf cells are damaged (in harvesting and rolling step of red tea processing), then catechin in the tea leaves will be oxidized, which causes the color of red tea and darker oolong. The mechanisms of catechin oxidation is described by Wan et al. in Oxidation Mechanism in Tea Catechins, in Natural Products Research and Development (2006, Vol. 18, pp171-181. But it's not new discovery and was documented earlier in Tea Biochemistry (a university textbook) written by the same first author.
When tea workers handle fresh tea leaves, the leaf "juice", containing both catechin and enzyme from leaf cells, gets all over their hands. Exposed to the air, oxidation of catechin happens very fast. In a harvest season, a tea worker's hands remain black for weeks.
The above photo tells us two things:
1. Tea is indeed a powerful de-oxidant source.
2. It takes a lot of hard work of tea workers to make the tea we drink.
3. If someone says tea workers' hands are "dirty" or "contaminated", you know they are not.
4 weeks ago