TANNINS



OUTLINE
Reading
CHAPTER 15 IN THE TEXT, 374 ff.

Introduction
The conversion of raw animal hides into leather has traditionally been carried out with plant derived tannins. Many different cultures have developed the process of tanning. The compounds that bind to the plant proteins are called (by definition) tannins.
Leather sandals are found in Egyptian excavations from 3,300 years ago. By at least 1500 B.C., records that indicate that tanning was carried out in the Mediterannean region are found. Clearly tanning was being done before that time.
Some of the most commonly used plants for tanning are listed on page 376.
Tannins are found in most plants, especially most woody plants. The quantities vary. Often 1-5% is encountered. There are two major types of tannins: condensed and hydrolyzable. Both have been used for tanning. The tannins of the most important commercial tannins are condensed tannins.

Oak gall

Plants used in tanning
In Europe the most common tannin sources were sumac (Rhus species, Anacardiaceae) and oak (Quercus species, Fagaceae). Later in European history, spruce (Picea, Pinaceae) and pomegranate (Punica granatum, Punicaceae) were used. In England most tanning was done with oak bark. In North America, American Indians used many native plants to make leather.
The colonists learned how to use many of these same plants. They especially favored hemlock (Tsuga canadensis, Pinaceae) and stripped the bark from this plant and almost extincted it in the northeastern U.S. As hemlock became depleted, emphasis shifted onto chestnut (Castanea dentata, Fagaceae). As the chestnut blight destroyed the chestnut forests, the logs of the trees became available and over 100,000 tons of tannins from dead trees alone became available in the 1930's, especially in Pennsylvania.
Formerly hides were sent from South America to New York and New England and then hemlock was used to tan them. The leather was sent to Europe. This continued until the hemlock was almost all gone.
In the tropics, mangroves are often used to make tannins. Several Rhizophora species (Rhizophoraceae) are especially important among these. Although these would seem to provide an almost limitless source of tannins, mangroves represent an unstable ecological community and their destruction has proven to be costly in terms of seafood.
Although the trees had been utilized earlier, quebracho (Schinopsis balansae and S. lorentzii, Anacardiaceae) and wattle (Acacia mearnsii, Fabaceae) became important tannin sources about a century ago.
As other sources of tannins became depleted and, because of other economic factors, these trees now provide about 90% of all commercial tannins and almost all of those used in the U.S.

Quebracho
Quebracho (Schinopsis balansae or S. lorentzii, Anacardiaceae) is probably the best quality tannin material for many purposes. The wood of this tree from Argentina, Paraguay, and Brazil (the Chaco) is usually about 20% (up to 40%) tannins.
The tannins are extracted in water and then spray dried. Quebracho is wild harvested at present.

Quebracho

Wattle
Wattle, Acacia mearnsii, (but a general name for many acacias in Australia) is native to Australia. The plant was introduced into South Africa about 150 years ago and is widely cultivated there today. The bark of the tree as 30-40% tannins.
The tree is mostly cultivated in South Africa, Sri Lanka, Australia, Brazil and a few other southern African countries.

Chestnut tree

Other tannins
Several other plants are still used for tannins. Among these are Canaigre from the S.W. U.S. and Mexico. This plant (Rumex hymenocephalus, Polygonaceae) grows in sandy soils and has potential for being a row crop. The root is up to 35% tannin.
However, the amount of starch present causes problems in tanning. European chestnut (Castanea sativa, Fagaceae) still accounts for much of the 10% of the market attributable to other tannins. Most of this comes from Italy, Spain and Portugal.
Sumac (mostly hydrolyzable tannins) is used for certain types of tanning, but the color properties are not always desirable. The tannin content is high (20-35%).
The cups of acorns have been widely used in the Middle East to make tannins. This is the source of commercial tannic acid. Oak galls are also a source of tanning materials.

How tanning works
Animal skins are made up of protein called collagen (among other things). This protein is readily degraded by bacteria and fungi. When tannins bond to the collagen, the crosslinked fibers are no longer susceptible to attack.
The tannin must effectively crosslink the protein, but must also have desirable color properties and meet many other requirements.

Tanning of hides
Hides are usually salted to prevent decomposition. The hides are first soaked in lime (or enzymes) to remove hair (depilatories). The proper concentration of tannin solution must be used because if it is too concentrated, it seals the outside of the hide and the inside portions don't get tanned.
To avoid this problem, the hides are usually first soaked in a solution of "spent" tanning liquid. After tanning for an appropriate period, the hides are washed, dried and then treated with oil or grease for softness. The leather is finished and coated with a layer of gum, wax, or resin.

Prospects of tannin use
Tannins are still widely used. Commercially produced quebracho and wattle have replaced local tannins in many countries. In some, however (such as India), locally produced products are still widely used. In the U.S. and Mexico, quebracho and wattle make up more than 90% of the tannins used in the leather tanning industry. Several billion pounds of hides are tanned annually. After tanning, about 30% of the weight of the leather is tannins.
About 15% of all tanning in the U.S. is initially carried out with vegetable tanning. Almost all thick leather products are still vegetable tanned. Shoe soles, brief cases, luggage, and belts are made in this manner. On the other hand, shoe uppers, are tanned with chrome alum. However, most of these inorganically tanned products are later retanned with vegetable tannins.

Other uses of tannins
Other uses of tannins account for about 15% of the total market. In the past tannins and iron salts were used to make ink. Gums were also added. Tannins are sometimes used medicinally and are used in oil field drilling muds.




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Revised April 2005

© David S. Seigler, Integrative Biology 363, Plants and Their Uses, Department of Plant Biology, 265 Morrill Hall, 505 S. Goodwin Ave., University of Illinois, Urbana, Illinois 61801, USA. 217-333-7577. seigler@life.uiuc.edu.