Value-Added Industrial Applications of Tannins

Tannins are natural products found in most higher plants. The name "tannin" derives from the use of this class of compounds in the leather tanning process. Tannins generally come in a variety of forms, from white to off-white amorphous powders, from shiny, almost colorless pasty substances to reddish-brown powders when produced by drying and spraying. They are known for their astringent taste.

Tannins are produced in almost every part of the plant - seeds, roots, bark, wood and leaves - because of their fundamental role in defense against insects, food-borne infections, fungi or bacteria. The defense mechanism relies on tannins' ability to irreversibly complex proteins. They are also considered to be one of the components that can effectively reduce cardiovascular disease, and even alleviate certain forms of cancer as part of fruit and vegetable-rich diets. In addition to their effects on human health, tannins are also important for the well-being of ruminants: protein-rich forage such as alfalfa can trigger the production of trapped methane in the form of protein foam, which can lead to the death of the animal. Tannins prevent weathering by interacting with the ruminant's food proteins, saliva and microbial cells, altering their digestive process and preventing the plant's soluble proteins from interacting to the same degree in the rumen.

Having reached a certain technological maturity for new industrial applications, and no longer only for the traditional applications for which they have been used for centuries, tannins, which are highly reactive products, offer great flexibility for a wide variety of applications in very different fields, from resins to plastics to pharmaceuticals. The problem with their use lies not in their applicability, but in their supply. Although millions of tonnes could potentially be extracted from the bark of relatively common trees, and from other sources such as grape waste after wine pressing, the number of existing plants remains relatively limited, and current world production is around 220,000 tonnes/year. Thus, the bottleneck is the construction of new extraction plants so that tannins can be extensively exploited in the many industrial applications already in existence or under development. This is of interest, however, as in recent years a small number of new extraction plants have been built, as in Sumatra, or are under construction, as in Chile. After a review of the origin and properties of tannins, as well as the various extraction techniques, the article sets out to relate them to potential applications, taking into account their added value. An overview is then given of the candidate material.