Anyone who has ever tasted an unripe persimmon knows: it is not only “sour”, but also “appealing”: a strange sensation, as if the mouth were suddenly dry, or the tongue turned to paper. That sensation is a physical, measurable effect, due to precise molecules: i tannins. Inside the pulp of the fruit, when it is still unripe, large quantities of soluble tannins accumulate, usually in specific cells of the pulp. Persimmon is one of the fruits in which tannins reach their most extreme form: enormous, chemically sophisticated chains, designed by the plant to defend itself and discourage animals when the seed is not yet ready. When we eat an unripe persimmon, these molecules bind to proteins in the saliva and cause them to precipitate, i.e stop working as a lubricant. This is why the mouth “tightens”. Based on the type and quantity of tannins, persimmon it can be more or less astringent. When the fruit ripens, those same molecules change state, become insoluble, and the taste transforms from aggressive to sweet.
What tannins are in persimmons and why do they tan
In persimmon, tannins are not simple molecules. That’s almost everyone condensed tanninsalso called proanthocyanidins. This name indicates polymers, that is, chains made of many smaller units linked together. The basic units are molecules of the dei family flavan-3-olshow catechin, epicatechin, gallocatechin and epigallocatechin. Some of these have a small chemical “ornament” called Gallate groupwhich makes the molecule even more reactive.
In kaki these units are not scattered randomly: they form very large and complex structures. A very detailed chemical study has shown that persimmon tannins are huge chainswith molecular weights of thousands of atomic units, and with bonds of two different types, which is rather rare in vegetable tannins. Basically, the tannins of persimmon is a kind of “super-tannin”larger and more elaborate than that of grapes or tea, for example.
These tannins accumulate in the vacuoles (a sort of sacs filled with water and/or other substances) of specialized cells in the pulp, called Tannin Cells. As long as the fruit is unripe, most of these tannins are soluble in water. And it is precisely this solubility that makes them astringent. When we eat an unripe persimmon, the tannins soluble they bind to proteins in saliva. Proteins normally keep the mouth moist and flowing: when they are “captured” by tannins, they precipitate and stop working. The result is that sensation of dryness and a “wrinkling” mouth. It is therefore not a subjective sensation, but a real one chemical phenomenon of interaction between molecules and a strategy to prevent animals from eating the fruit before it is ripe by protecting the seed.
Ripening reduces the budding of the fruits: the less astringent types
From a botanical point of view there are four large types, which are distinguished by how and when they lose astringency: some become non-astringent on their own as they mature, others only if pollinated, others remain astringent until treatments or full maturation. The difference lies above all in what type of tannin they contain and in what proportion. In persimmons that remain astringent longer, one key molecule appears to dominate:epigallocatechin-3-O-gallate (EGCG)one of the main units of tannins in persimmon. The more EGCG there is in the polymers, the more aggressive the fruit tends to be on the tongue.
The magic of maturation does not consist in “making” tannins disappear. It consists of transform them. During ripening, or when the fruit is treated (for example with alcohol, CO2 or freezing), tannins they become insoluble. Chemically, their chains clump together or with other plant components and lock together. When this happens, they are no longer able to dissolve in saliva, and therefore are no longer able to bind to its proteins. A study freezing persimmons at −20°C and −80°C clearly showed this step: soluble tannins decrease, while insoluble tannins increase, and astringency almost disappears.
The potential properties of tannins
Persimmon proanthocyanidins aren’t just annoying molecules. Precisely because they are large, rich in gallate groups and very reactive, they also have a strong biological activity. A study in vitro on astringent persimmons he showed that tissues rich in tannin (such as the peduncles, the parts that connect the fruit to the branch) inhibit the production of nitric oxide (NO) in immune cells, a key signal of inflammation. The more tannin there is, the stronger theanti-inflammatory effect. This would explain why, in traditional Asian medicines, parts of the persimmon rich in tannins have been used for centuries for disorders related to inflammation, but solid clinical studies are lacking to confirm this property also on humans.
