Tannins come from plants. They’re a large family of naturally occurring compounds found in the bark, leaves, fruits, seeds, and roots of thousands of plant species. Plants produce tannins primarily as a chemical defense system, protecting themselves against leaf-eating animals, bacteria, and fungi. If you’ve ever bitten into an unripe banana or sipped a bold red wine and felt that dry, puckering sensation in your mouth, you’ve encountered tannins firsthand.
Why Plants Make Tannins
Tannins serve as a built-in security system. When an insect or grazing animal chews on a leaf, the tannins bind to proteins in the animal’s saliva and digestive tract, making the plant taste bitter and harder to digest. This discourages the animal from coming back for more. Tree bark is especially rich in tannins for exactly this reason: they form a chemical barrier against bacteria and fungi that might otherwise infect the wood.
The defense role goes beyond just deterring hungry animals. Many woody plants pack tannins into their bud scales, the small protective coverings that shield new leaf growth. The first leaves that sprout from a germinating seed also tend to be high in tannins, protecting the plant during its most vulnerable stage. Unripe fruits are loaded with tannins too, which is nature’s way of discouraging animals from eating fruit before the seeds inside are ready to be dispersed. As fruit ripens, tannin levels drop, and the fruit becomes sweeter and more appealing.
Where Tannins Concentrate in a Plant
Tannins show up in virtually every part of a plant, but the concentrations vary enormously depending on the tissue. Bark, seeds, and fruit skins tend to hold the highest levels. In grapes, for example, both the skins and seeds are major tannin reservoirs. The skin tannins peak before the grape begins to change color (a stage called veraison) and then steadily decline as the fruit ripens. Seed tannins follow a slightly different pattern, rising initially after fruit set and then tapering off toward harvest. This is why winemakers pay close attention to how long grape skins and seeds stay in contact with the juice during fermentation: longer contact means more tannin extraction.
Roots, stems, buds, and even plant galls (abnormal growths triggered by insects or disease) also contain tannins. The word “tanning” itself comes from the medieval Latin word tannum, meaning oak bark, because oak bark was one of the original sources used to preserve animal hides into leather.
Two Main Types
Chemically, tannins fall into two broad categories. Condensed tannins are chains of smaller molecules linked together in a structure that resists being broken apart by water. These are the dominant type in most fruits, wine, tea, and chocolate. Hydrolyzable tannins have a sugar molecule at their core with other compounds attached by bonds that water can break. Oak bark and persimmons are notable sources of this type. Both types bind to proteins, but they do so through slightly different mechanisms, which affects their taste and biological activity.
Common Dietary Sources
Tea is one of the richest everyday sources of tannins. Polyphenols (the chemical family tannins belong to) can account for up to 35% of green tea’s dry weight, with the majority being a specific group of tannin-related compounds. Black tea undergoes more processing, so its monomeric polyphenol content drops to 3 to 10% of solids, but it develops higher concentrations of larger, more complex polymers (23 to 25% of solids). This is why black tea tastes more astringent than green tea when steeped for the same amount of time.
Red wine typically contains 1.1 to 3.4 grams of total tannins per liter. The condensed tannin fraction alone ranges from 500 to 1,500 milligrams per liter, depending on the wine’s age. White wine, by contrast, has far less: only about 10 to 50 milligrams per liter. While grape skins and seeds are the primary source of wine tannins, aging in oak barrels introduces additional hydrolyzable tannins from the wood, contributing anywhere from 0.4 to 50 milligrams per liter.
Fruit and fruit juices vary widely. Pear juice is surprisingly tannin-rich, with concentrations from 1,450 to 4,060 milligrams per liter depending on variety and ripeness. Apple juice ranges from nearly undetectable levels up to 690 milligrams per liter. Cranberry juice sits around 200 to 210 milligrams per liter. Among whole fruits, pomegranates stand out with condensed tannin levels of 62 to 140 milligrams per gram of dry weight, while grape seeds can reach up to 241 milligrams per gram.
Nuts contribute tannins too. Acorns contain 2.4 to 5.2% tannins by dry weight, which is why they taste so bitter without extensive processing. Almonds eaten with their skins have roughly five times the total phenolic content of blanched almonds, because the skin is where the tannins concentrate.
What Tannins Do in Your Mouth and Gut
The dry, puckering sensation you feel when drinking strong tea or young red wine is astringency, and it’s a direct result of tannins binding to proteins in your saliva. Your saliva contains proline-rich proteins that act as a first line of defense, essentially intercepting tannins before they reach your digestive system. When tannins latch onto these salivary proteins, they form clumps that reduce the lubricating effect of saliva. The result is that sandpapery, drying feeling across your tongue and cheeks.
This protein-binding ability extends to the digestive tract. Tannins can bind to dietary proteins and digestive enzymes, which is why they’ve historically been called “antinutrients.” At high concentrations, tannins reduce the activity of digestive enzymes by physically altering their shape. But the picture is more nuanced than the label suggests. At low concentrations, tannins can actually increase the structural organization of certain enzymes, boosting their catalytic activity. Even when tannins do form complexes with enzymes, those complexes retain some residual function. The effect depends on dose, the type of tannin, the specific protein involved, and the pH of the environment.
Industrial and Historical Uses
Long before anyone understood the chemistry, people recognized that certain plant materials could preserve animal hides. Vegetable tanning, the oldest method of making leather, relies entirely on tannins extracted from bark and leaves. The primary bark sources used commercially include chestnut, oak, hemlock, quebracho, mangrove, and wattle (a type of acacia). These barks were ground in bark mills and soaked with raw hides, where the tannins cross-linked with collagen proteins in the skin to create durable, flexible leather. The process gave us both the word “tanning” and, eventually, the chemical name “tannin” itself.
Beyond leather, tannin-rich plant extracts have been used in wood stains, inks (iron gall ink powered much of European writing for centuries), and traditional medicine across cultures. Today, tannin extracts from tree bark are also used as natural adhesives in wood products and as flocculants in water treatment.