“The sensorial and naturalistic exploration of a family of overlooked molecules: tannins, omnipresent in the colors, tastes, smells and objects of our everyday life.” This sentence is written on the back cover and describes very well the content of the book titled Les goûts et les couleurs du monde (Actes Sud, 2019), a popular essay entirely devoted to tannins. The author is Marc André Selosse, a renowned biologist and mycologist who collaborates with prestigious European universities and is currently a professor at the National Museum of Natural History in Paris, as well as the vice-president of the Botanical Society of France. 

But why devote an essay to tannins, nowadays? And why not only for a small circle of scholars, but for all nature lovers? We reached the professor Selosse in videocall to ask him directly.

Why tannins? What’s so fascinating about them?

It wasn’t a sudden revelation. I didn’t learn a lot about tannins during my studies. The encounter took place years later, when I was already teaching, thanks to the pressing questions of my students. They asked me: “Why does this fruit have an astringent taste?”, “Why is wood so hard?”, “Why is the soil so dark?”. Questions only apparently trivial, but I felt I had no answer to them. So, I went deeper and I discovered a family of polyphenolic substances that we encounter many times in our everyday life, which we can rarely see or recognize: tannins.

What makes them unique, scientifically speaking?

As a scientist, what fascinates me most about tannins is their ability to bind to several  chemical compounds. They bind to almost anything, especially proteins. Astringency is a consequence of this property, since  tannins link to the lubricating proteins of our saliva. They can even “stick” to metals, such as aluminum (and this allows, for example, the blue coloring of some flowers). We could say that they like to “socialize” with many other molecules!

How are tannins essential in nature?

Nature is based on a dynamic balance, a permanent multilateral exchange that follows different rules and in which interactions between plants and animals, or plants and microbes, play a key role. Tannins redefine these interactions, and thus make an essential contribution to the dynamics of nature. 

How, concretely?

For example, tannins provide the berries and flowers with a characteristic color that allows them to be recognized by animals. Pollinating insects and birds are in fact attracted by certain colors, such as, respectively, blue and red. Birds and mammals can also pick the fruit to feed on, leading to more effective seed dispersal once digested.

Furthermore tannins are responsible for the beautiful colors of the autumn leaves. That’s because tannins do not permit leaves to decompose easily: microorganisms have a hard job ahead! This is good, because the leaves will slowly decompose and release their nutrients into the soil, which will then become a “reserve” of nitrogen and phosphorus ready for the plants to use it during the next season. Those who have a garden would do well not to sweep away all the leaves when autumn comes! Otherwise, you would have to compensate for the loss of nutrients through fertilization. It is fascinating to see how nothing is wasted in nature.

Tannins have also proved to be decisive for many of man’s activities…

The history of tannin is somehow also the history of mankind. Everything, for us humans, is a tool to use! This includes tannins, which have been used in the most different ways. Notably, they have been decisive in leather tanning. We know for a fact that the Celts already used them, and they were probably known even before. The word “tannin”, not surprisingly, comes from the old Celtic tongue and means “oak”, a wood used for tanning. Our ancestors commonly used tannin in various processes, but they were not always aware of what it really was: they knew the vegetable sources from which colors or medications could be made, but they had no clear idea of what made them so effective. Today we know very well these molecules, but we still find it hard to see tannins as a single family that brings beneficial effects in different fields of our daily life.

Let’s think about food and beverages: tannins occur naturally in spices, fruits and in many drinks, such as coffee, wine, beer and tea. They are antioxidants and modulate the development of microorganisms. This is why the presence of tannins in beverages makes them safer. They can also be used in veterinary medicine to fight intestinal parasites and reduce the emission of gasses, particularly methane, in ruminants. Furthermore, their capacity to improve the animal welfare and the gut motility leads several animals, such as cats, to instinctively eat grass.

Tannins are hidden in every corner of our daily lives. And from every perspective they tell us a unique and ancient story, just waiting to be rediscovered.

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