©: Kat Morgenstern, December 2006
When we think of plant products we think of wood, of medicinal herbs, of vegetables and maybe even essential oils, fibres and dyes. But rarely will we think of gums and resins - the sticky stuff that many plants usually exude when their protective membrane or bark has been injured. (Some also excrete resin freely, without any prior injury). Resins, gums and latex are almost ubiquitous in the plant kingdom and many of them continue to play an important role in our daily lives.
Most famously, Frankincense and Myrrh, two oriental species of Burseraceae trees also know as the family of 'balsam trees' have engraved themselves on our consciousness ever since the three holy men presented the baby messiah with this sacred gift. Few who have not heard the names - but even fewer who know what they are, where they come from, smell like or what they do.
Numerous plants produce some kind of resin, latex or gum, but only a few are commercially important today, even though their uses and applications are truly manifold. They have been used as adhesives, emulsifiers, thickening agents, they are added to varnishes, paints and ink; they lend their aromas to perfumes and cosmetics and even play a role in pharmacy and medicine.
The ancients regarded the fragrant resins and oils as sacred and considered them the life blood of the plant, the carrier of its soul, and burnt them ritually as a sacrifice to the Gods. It was believed that the Gods lavished themselves on scent, rather than substance. Scent is the subtle (or not so subtle) body of a being, the medium by which messages are transmitted chemically, below the threshold of consciousness. We still use this method of communication, though our aim is usually to get the attention of the opposite sex, rather than to win favour with the Gods.
Before we move on to a detailed examination of specific plant gums and resins, let's first of all examine the different chemotypes of these exudates.
This term has long been used as a rather vague description for any soothing resinous fragrant plant exudates. These days chemists use the term to describe a rather distinct class of resinous substance, which contains high amounts of cinnamic and benzoic acids and their esters as well as essential oils, which lends them their characteristic balsamic scent. Typical examples of balsams are Balsam of Peru, Tolu Balsam, Balm of Gilead, and the Balsams of Copaiba. Physically these substances can vary greatly - they can be a clear, viscous substance or a dark sticky mass, but upon exposure to air they all sooner or later tend to solidify. Traditionally balsams are used medicinally, often for skin complaints or respiratory afflictions. They are widely used as aromatic agents for skin care products or in perfumery. However, some people are highly allergic to benzoic acid an can react severely to even small amounts.
Gums are substances that are either water soluble or can absorb water - they are not soluble in oil. Chemically they are complex polysaccharides (Carbohydrates).
Gums either derive from the a resinous sap or from the endosperm of certain seeds, e.g. like Guar Gum, which is derived from the seeds of Cyamopsis tetragonolubus, an African herbaceous plant of the pea family. Gums are widely used as emulsifying and thickening agents in the food industry, but they also find uses in other industries, from pharmacy to cosmetics and skin care products, to the manufacture of inks, paper, watercolours and even the adhesive on the back of stamps. Water soluble gums also play an important role in a healthy diet as they are able to bind endotoxins and help to excrete them by adding bulk to the stool. Psyllium seed - a well known dietary supplement often used for minor constipation is a prime example of this action. Some seaweeds also yield gums - e.g. agar agar is a well known and widely used in cooking as a thickener.
These most commonly found types of plant exudates are chemically completely different to gums. Resins are terpene-based compounds. Terpenes constitute one of the largest groups of plant chemicals, and they can be very complex. They are not water soluble, but can be either oil soluble or spirit soluble, depending on their specific chemical composition.
Most resins and latexes are obtained from the tree trunk by making incisions, and 'bleeding' the tree - a process that is known as 'tapping'. While it is possible to harvest resins sustainably, it must be remembered that deliberately injury puts a considerable strain on any tree and there must be strict limits to the number of incisions and period of productivity. Recent research has shown that the carbohydrates present in the exudates are important energy reservoirs for the trees concerned and that excessive tapping reduces the numbers of flowers and the size and viability of the seeds. Such guidelines are particularly important to observe where the gum is collected from wild populations where regeneration is left to nature.
Resins used to be much more commonly utilized in industry, e.g. in the production of oil paints and varnishes or to waterproof ships. These days their industrial uses have largely been diminished in favour or synthetic substitutes. Some resins have powerful medicinal properties that have long been utilized in herbal medicine, but others can be toxic. For example resins derived from certain plants of the Artemisia family or from the Cypress contain thujone, which is a known neurotoxin.
These resins are chemically terpene compounds with a high volatile oil content. Because of the rich oil content they are softer and more pliable than resins. The volatile oil content is often extracted by solvent extraction to create various essential oils that are used in perfumery or as scenting agents for numerous household products. The term 'Gum-Resin' is also sometimes erroneously applied, but it is a confusing oxymoron and should not be used.
As the term implies, latex has a milky appearance: it is usually found as a white, thin, slightly sticky substance, which coagulates by boiling. The degree of elasticity of the resulting coagulant depends on the nature of its polyisoprenes. Cis-polyisoprenes confer a greater degree of elasticity. The best known and most important plant latex is derived from the rubber tree. A whole boom and bust economy flourished in the Amazon at one time - until by an act of biopiracy some rubber tree seeds where stolen and planted in a plantation in India, thus breaking the monopoly dependence on South-American supplies. But it wasn't long before natural rubber was replaced by synthetic substitutes and plastics. Latex has found many applications from sealant paints to rubber tires, to insulating sheathing for electrical wires to rubber gloves, boots and other kinds of eclectic apparel.
In the next issue we will examine specific gums, resins and latexes and their uses.
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