Have you ever noticed such phenomena? The mung beans become black after they were cooked in iron containers. The surfaces of apples and pears will be black if they are cut by iron knives. The reason of such phenomena is that there is a substance called in the mung beans, apples, pears and many other types of fruit. The tannic acid can combine with iron to compose a black substance when they are exposed to oxygen. 

Have you ever tried the taste of tannic acid? Oh, it is very dry. The dry tastes of many kinds of fruit are due to the tannic acid. For example, there is a large amount of tannic acid in the cells of persimmons. Once you have broken the cell membrane, the tannic acid comes into your mouth and makes your tongue dry. The pure tannic acid is yellowish in color and rather soluble. The persimmons on the market are usually dealt with limewater or dry lime. It is because that lime can make the tannic acid insoluble. After such dealing, the persimmons will not taste dry again. ing has the same effect. So some people like to heat the persimmons to drive away the dry taste as well as the bacteria. Sometime the pears and apples would also become black even if you do not use a iron knife. It is again the oxidation of tannic acid. The dry taste of wine is also caused by this substance. No one would like the bad taste of the tannic acid itself. But many people enjoy the dryness of wine. We know that dry red wine is often more expensive than sweet wine.

Tannic acid is everywhere in nature. It mainly exists in barks. In Sichuan and Yunnan, China, a tree called gall tree grow fruits containing rich tannic acid.

There is much tannic acid in tea leaves as well. It is the tannic acid that makes strong tea do harms to our health. Tea can refresh our mind and help digest. But strong tea is not good for health, especially for the elderly. Once you have drunk too much strong tea, the following symptoms would appear. First of all, your gastric juice is so little that you cannot digest in normal way. A healthy adult needs 1.5 to 2.5 liters of gastric juice a day, which can normally deal with the proper ingestion of food. Once there is not enough gastric juice, you may feel bellyache and dyspepsia. Secondly, it keeps the human body from ingesting iron. The tannic acid in tea would combine with iron in the human body and even lead to anemia. Thirdly, the tannic acid can also combine with the proteins in food and would sometimes cause coprostasis.