Natural Resins

Besides the essential oils, considerably larger amounts of non-volatile substances are produced in the plant cells as a result of their metabolic routes. They are generally known as

“resins” and their solutions in essential oils as “balsams”. Nevertheless, it is difficult to classify the different types and find a common identification for resins.

Since resins and essential oils are mostly composed of terpenoids they would be classified by plant physiologists as secretions, as the end-products of plant metabolism which are collected in secretory depots and become immobile although research results proved that they may undergo chemical rearrangements².

A. Tschirch was the first scientist who tried to classify them. He thought that common features among resins were not their properties but their origin. He also indicated that resins represent plant excretes, that is substances, which should be eliminated from the plant at the end of the metabolic pathways, in contrast to secretions actually serving the plant metabolism.

He divided resins into two groups: “physiological” for the usual metabolic end products and

“pathological” where the production of these substances was observed first when the plant was wounded. Although the chemical route for resin formation was not very clear, for their accumulation in the plant body he assumed that cellulose in many cases played an important role. He called “lysigenous resin formation” when the cell walls in the plants were dissolved and instead of a cell a cavity was formed which was then filled with resin.

A second type of formation was the development of the resin in the cell and its diffusion through the cell wall. The plants which had this kind of cell formation had usually bigger intercellular gaps where the resin was collected. These gaps were usually small at the beginning but grew with time either equally in all directions or only in one direction which was mostly parallel to the stem to form the “resin channels”. This kind of formation of resin channels through restructuring of the cell gaps and accumulation of the resin in these gaps was called “schizogenous resin formation”.

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first the schizogenous formation occurred then the cell wall disappeared with a chemical regeneration, and the resin channels were enlarged^{7, 8}. His terminology in resin chemistry is still in use although some of his hypothesis was proved to be incorrect² such as the disappearance of the cell walls. They only became unrecognizable because of the enlarged excrete channels.

Such channels are typically observed in many plant families; Pinaceae, Anacardiaceae, Hypericaceae, Guttiferae, Dipterocarpaceae, Burseraceae. These channels are surrounded with epithel, thin walled and very tight placed cells, like the excrete pockets. This kind of cellular organization has a double function for the plant; first it isolates excrete in a separate space and secondly it closes the excrete passage hermetically from the intercellular system of the body. These resin channels are generally connected to each other through horizontal gaps to the interior. By this way, the plant succeeds to impede secretions to flow outside.

It is possible to make different classifications for the resins, for example, according to their appereance or their chemical composition. In many cases three classes of resin types were recognized: “balsam”, viscous but fluent plant products; “resin”, when this fluent product becomes solid after a short time; “gumresin”, contains the resin constituents together with the plant gum⁸.

Chemical classification of resins is based on their constituents. It is common to classify them into three groups: “terpenresins”, “benzresins” and “gumresins”.

Terpenresins contain mainly diterpenoic and triterpenoic acids or triterpenoic alcohols.

Colophonium, dammar, mastics belong to this class of resins.

On the other hand, benzresins are composed of mostly phenylpropane derivatives. Besides cinnamic acid and coniferylalcohol they contain lignanes, xanthone and highly condensed coumarines. In most cases they are found in their esterified forms. Peru balsam and guajak resin can be considered as examples of this class.

Gumresins flow as a yellow or white latex from the incisions of the plant and hardens there into amorphous, tear shaped products with an aromatic scent. They are composed of 30-60 % resin, 5-10 % essential oil, which is soluble in organic solvents, and the rest is made up of polysaccharides, which are soluble in water. The darkening of the color is a result of autoxidation, polymerization and enzymatic reactions. The most well-known examples are myrrh and olibanum⁶⁰.

Still, the significance of the resin for a plant is not precisely known. But in contrast to Tschirch who believed that they do only serve for the healing of wounded parts in the plant organism since some of them exhibit no obvious physiological activities, they, nevertheless, have been used in folk medicine since very old times, as antiinflammatory and antiseptic materials or to cover wounds like the plasters of today. In daily rituals and religious ceremonies, resins have been burned for their aromatic scents. Olibanum and myrrh are the oldest scents mentioned in history since 5000 B.C.⁶¹. Today the utilisation of resins is still

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common in pharmaceutical, perfume and cosmetic, aroma, laquer and varnish industries in addition to their cultural and folkloric uses.

Methods Used for The Analysis of Plant Extracts