Tannin

Tannins are a special group of compounds found in many plants. They are called astringent, which means they can make things feel dry and tight, like when you bite into an unripe fruit or sip some red wine or tea. These compounds help protect plants from being eaten by animals and also play a role in how plants grow.

The name tannin comes from old ways of turning animal hides into leather, using oak bark that is rich in these compounds. Tannins can bind to proteins and other important molecules in the body, which is why they cause that puckery feeling in your mouth.

Tannins come in different sizes, with some being quite large molecules. They are found in many types of plants and are important for both the plants and for things we use and eat, like tea and wine.

Structure and classes of tannins

There are three main types of tannins. These are special plant compounds that help bind to proteins. To work well, tannin molecules usually need many parts called hydroxyl groups and phenyl groups.

Oligostilbenoids are another small group of tannins made from stilbenoids.

Pseudo-tannins are smaller compounds that come with other plant materials. They do not change color during certain tests and cannot be used to turn animal hides into leather.

History

In 1831, a chemist named Henri Braconnot discovered ellagic acid, gallic acid, and pyrogallic acid. Later, Julius Löwe made ellagic acid by heating gallic acid with arsenic acid or silver oxide.

Maximilian Nierenstein studied natural substances called phenols and tannins in plants. In 1905, with Arthur George Perkin, he made ellagic acid from algarobilla and some other fruits. He used a special enzyme called Tannase to create a substance called m-digallic acid from gallotannins. In 1931, he found catechin in cocoa beans. In 1945, he showed that luteic acid, found in a tannin from the fruit of Terminalia chebula, helps make ellagic acid.

Back then, scientists used combustion analysis to understand these molecules. In 1943, Martin and Synge discovered paper chromatography, a new way to study plant substances. This led to more research in this area, especially by Edgar Charles Bate-Smith and Tony Swain at Cambridge University.

In 1966, Edwin Haslam created a clear definition for plant polyphenols, building on earlier ideas from Bate-Smith, Swain, and Theodore White. This definition is known as the White–Bate-Smith–Swain–Haslam definition.

Occurrence

Tannins are found in many plants. They are common in both types of seed plants, called gymnosperms and angiosperms. Scientists have studied tannins in many plant families. Some families, like Aceraceae and Actinidiaceae, always have tannins in their species. Others, like Solanaceae and Asteraceae, often have very little tannin.

Most tannins are called condensed tannins. They are found in almost all plant families and can make up to half of the dry weight of leaves.

Tannins are made inside special parts of plant cells called tannosomes. They are stored in areas of the cell called vacuoles or in the surface wax of plants. This helps protect the plant from animals that might eat it.

Tannins also affect soil and water. They can slow down the breakdown of plant material and affect how nitrogen moves through the environment. In water, tannins can make the water look dark, like in blackwater rivers or streams flowing from bogs. This can affect animals like waterfowl, giving their feathers a reddish-brown color. In aquariums, tannins from wood can change the water color and lower its pH, but this can be managed by boiling the wood or adjusting the water’s pH.

Extraction

There isn’t just one way to get tannins from plants. The methods used can be very different. Sometimes, using a liquid called acetone in the process can help get more tannins by stopping them from sticking to proteins or breaking the bonds that hold them together.

Tests for tannins

There are three ways to check for tannins: by seeing if they stick to proteins or special plant compounds, by watching how they react with certain chemicals, and by breaking them apart.

One way is to use special plant compounds like caffeine. These can help show if tannins are present. Another test uses a piece of skin soaked in a special liquid. If tannins are there, the skin turns a dark blue color after adding another chemical.

There are also other methods, like using powdered leather or certain mixtures of chemicals, to measure how much tannin is in a sample. These tests help scientists study tannins in plants and other materials.

Food items with tannins

Pomegranates

Main article: Pomegranate ellagitannin

Accessory fruits

Strawberries have special kinds of natural substances called tannins.

Berries

Many berries, like cranberries and blueberries, also contain these tannins.

Nuts

Nuts have different amounts of tannins. Some acorns from oak trees have a lot, but you can wash them to remove the tannins and make them safe to eat. Other nuts like hazelnuts, walnuts, pecans, and almonds have smaller amounts.

Herbs and spices

Cloves, tarragon, cumin, thyme, vanilla, and cinnamon all have tannins.

Legumes

Most legumes have tannins. Red beans have the most, while white beans have the least. Peanuts without shells have very little. Chickpeas also have smaller amounts.

Chocolate

Chocolate liquor has about 6% tannins.

Drinks with tannins

We get a lot of tannins from drinks like tea and coffee. Wines aged in charred oak barrels pick up tannins from the wood, which helps give wine its special taste. Soils with lots of clay can also add tannins to wine grapes.

Coffee pulp has only small amounts of tannins.

Fruit juices

Apple, grape, and berry juices have lots of tannins. Sometimes people add more tannins to juices to give them a special taste.

Beer

In beer, besides the bitter parts from hops, there are also tannins. In some beers, this bitty taste is okay or even wanted, like in a Flanders red ale.

In some lighter beers, tannins can mix with certain proteins and make the beer look cloudy when it gets cold. People can prevent this by taking out some tannins or proteins using special materials like PVPP or silica.

Properties for animal nutrition

Tannins have been seen as something that can make it harder for animals to get the nutrients they need, depending on how they are made and how much there is.

Some studies show that tannins found in chestnuts can help make better feed for animals, especially in certain types of stored feed. They can also help animals break down proteins from plants more easily. Tannins can also be useful in helping animals stay healthy by fighting tiny pests in their stomachs. This can be important because some medicines that fight these pests are becoming less effective.

Tannin uses and market

Tannins have been used for a long time to help make leather from animal hides and to protect iron objects, like Japanese teapots.

During the 19th century, people started making tannins in factories because they needed more leather. Before this, they used plants, but it took a very long time—up to six months. In the 1950s and 1960s, many small tannin factories closed when synthetic tannins were created. These were made because there wasn’t enough plant tannins during World War II.

The price of tannins depends on how they are made, especially if certain chemicals are used. For large amounts, the cheapest way is to use hot water. Food-grade tannic acid is used to clear and add flavor to drinks.

Uses

Tannins are very important for making leather. Tanbark from oak, mimosa, chestnut, and quebracho tree has been the main source of tannins for leather factories, though other materials are also used today and make up most of the world’s leather production.

Tannins can change color when mixed with ferric chloride, creating blue, blue-black, or green colors. They are used to make iron gall ink by mixing with iron(II) sulfate.

Tannins are also helpful for dyeing natural fabrics like cotton. They are part of some special glues used in making wood products and can help protect rusted steel by turning rust into a safer material.