Tannin

Tannins are special compounds found in many plants. They are called astringent, which means they can make things feel dry and tight. This is like when you bite into an unripe fruit or sip red wine or tea.

These compounds help protect plants from being eaten by animals and help plants grow.

The name tannin comes from old ways of turning animal hides into leather using oak bark. Tannins can bind to proteins and other important molecules, which causes that puckery feeling in your mouth.

Tannins come in different sizes and are found in many plants. They are important for 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. Tannin molecules usually need many parts called hydroxyl groups and phenyl groups to work well.

Oligostilbenoids are a 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 from gallotannins. In 1931, he found catechin in cocoa beans. In 1945, he showed that luteic acid, found in a tannin, helps make ellagic acid.

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

In 1966, Edwin Haslam created a clear definition for plant polyphenols. This definition is known as the White–Bate-Smith–Swain–Haslam definition.

Occurrence

Tannins are found in many plants. They are common in plants like Aceraceae and Actinidiaceae. Other plants, like Solanaceae and Asteraceae, often have less tannin.

Most tannins are called condensed tannins. They are found in almost all plant families and can make up a lot 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 change how plant material breaks down and affect how nitrogen moves. In water, tannins can make the water look dark, like in blackwater rivers. This can affect animals like waterfowl. In aquariums, tannins from wood can change the water color and lower its pH, but this can be managed.

Extraction

There are many ways to get tannins from plants. Sometimes, a liquid called acetone is used. This 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. You can see if they stick to proteins or special plant compounds. You can also watch how they react with certain chemicals. Another way is to break 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 natural substances called tannins.

Berries

Many berries, like cranberries and blueberries, contain tannins.

Nuts

Nuts have different amounts of tannins. Some acorns from oak trees have a lot, but washing them can remove the tannins. 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 tannins from drinks like tea and coffee. Wines aged in charred oak barrels pick up tannins from the wood. 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 for a special taste.

Beer

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

In some lighter beers, tannins can mix with 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 can sometimes make it harder for animals to get the nutrients they need. But they can also help. For example, tannins in chestnuts can improve animal feed, especially in stored feed. They help animals break down plant proteins more easily. Tannins can also help animals stay healthy by fighting tiny pests in their stomachs. This is important because some medicines that fight these pests are not working as well anymore.

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 1800s, people began 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. 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.

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.