The diversity of natural chemical compounds in plants is remarkable. For many years, scientists thought they could be broken down into simple primary and secondary categories: the primary compounds being those that contribute directly to growth and development, and the secondary ones being byproducts that don’t directly contribute to those primary functions. Most of the aromatic oils, gums, and resins that humans have adored for thousands of years fell into this wide category of secondary natural compounds. Why do plants produce them? Understanding the roles these compounds play in plants helps us understand how to use them most effectively. We went to our friend, fourth-generation botanist and plant physiologist Karen Hall, to get a better understanding of natural gums, resins, and gum-resins in plants.
We now know that the world of natural chemical compounds in plants is much more complex than scientists first assumed. It turns out that many of these compounds are vital to a plant’s healing and protection against herbivores and microbial infection, and/or they act as attractants through their pigments or scents for pollinators, seed-dispersing animals, and other helpful species.
Gums are a group of natural plant compounds that form specialized ducts in many plants. They include seed gums, exudate gums, seaweed gums, mucilage gums, and more. Agar is a gum from seaweed, neem is a gum from tree seeds, and guar gum is from legumes. However, the gums we tend to recognize are the “exudate” gums: the ones that come out of the plant and accumulate on the exterior. When the plant is injured, cut, or stressed, cells break down and a sticky substance (gum) exudes out, absorbs water, and swells to seal off the injury and help heal the damage. This process is called gummosis. Drought, viruses, herbivores, extreme temperatures, or fungi can kick off the process of gummosis. Upon contact with air, the exudate gum dries and hardens into translucent, amorphous shapes or flakes that can be easily harvested. Some exudate gums (but not all) are safe for human consumption and have a long history of use in food applications for emulsification, thickening and stabilization. Tree gum exudates are also used in non-food applications, such as pharmaceuticals, cosmetics, textiles, lithography, etc.
Gum Arabic or acacia gum, harvested from Acacia trees (Acacia senegal and related species), is the oldest known exudate gum, having been in human use for 5,000 years. Ancient Egyptians used it as an adhesive, (acacia gum adhesive glued the wrappings of mummies). However, its unique properties also make it an excellent addition to food. Gum arabic can add gloss (bakers use it to make marzipan shine), emulsify liquids (binding sweeteners and flavorings), and thicken mixtures like icing or other confections. Dyers using natural dyes also use gum Arabic to create areas on cloth that resist dye or, alternatively, to prepare an area for dye.
In North America, larch gum is extracted from the wood chips of western larch trees (Larix occidentalis). Although we have known about the gum producing prowess of this conifer for nearly a century, it is only recently that people have started to look to it commercially for gum production.
Because gums can be colored and are found on the exterior of plants, people often confuse them with resins. And to make identification more challenging, some plants produce gums and resins at the same time in the same place, making a gum resin category that we’ll talk about in a moment.
Although resins occur in a variety of flowering resinous plants, they mostly occur in Gymnosperms (a large group of cone-producing plants that include conifers, cycads, Ginkgo, etc.), especially of the Pinaceae family (such as pine, fir, and cedar trees). Resin secretion occurs in specialized cavities called resin passages and, like exudate gums, exudes or oozes out from the bark of the trees, the flowers of an herb, or the buds of a shrub, slowly hardening as it is exposed to air. As with gums, plants secrete resins in response to injury or environmental stress; resins protect plants from insects and pathogens and, thanks to the scents associated with them, are particularly good at confusing or capturing a variety of potentially harmful herbivores and insects. At the same time, phenolic compounds in resins can attract allies like parasitoids and other helpful predators.
The primary difference between gums and resins is that resins are not water soluble, while gums are. This is what makes resins so valuable in furniture coatings like varnishes and rosins, etc. Resins used in these products are transparent or semi-transparent and have a characteristic luster.
Humans used resins for thousands of years before anyone wanted to polish furniture. We have historical evidence of resin use from the Bronze Age, and the trade of amber—fossilized plant resin—can be traced back to the Stone Age. Amber is found in deposits around the world, usually in river deltas or in sediment where ancient waters washed plants downstream, and then the plant, along with its resin, fossilized. The best known and utilized amber today comes from a variety of conifers that have long been extinct.
Other uses for resin are vast and varied; resins are used as incense, waterproofing in the ship building industry, for maintaining the bows on stringed instruments and keeping skateboards in shape. Resin is a key ingredient in turpentine, solvents, cleaners, fragrances, dry cleaning, cosmetics, and insecticides. It is also used in artificial flavors such as lemon, peppermint, and nutmeg. And the natural resin in hops is what gives beer its aroma and bitter taste.
As the name indicates, gum-resins are a mixture of both gums and resins, and they possess the properties of both groups. They can also contain essential oils. Typically, the best gum-resin trees grow in dry, arid regions. These include the infamous Burseraceae family—also known as the torchwood or incense tree family—which includes frankincense and myrrh. There is evidence of frankincense in human use and trade for over 6,000 years. Gum-resins are used in incense, perfume, medicine, flavoring, cosmetics, and dentistry.
Many plants contain gums, resins, and gum resins. The concentration of these compounds depends on the plant’s environment, ecological factors, the species, the part harvested, the method of preparation for the market, and also how they were extracted and utilized.
If you are considering wildharvesting resins or gums, it's very important to do it properly. The tree is producing these to protect itself and seal off any injuries from insect infestation or microbial invasion. Thus, in order to avoid further harm to a tree, we need to be careful to harvest resin where it has dripped down the body of the tree or fallen on the ground, rather than harvesting directly from the wound. For more information, see our blog "The Forest in Winter."
These aromatic resins, gums, and gum-resins may be burned and compounded for incense, ritual, and perfumery. Some are appropriate for cosmetic applications, to incorporate into body care recipes, and for use in tincture infusion. They are said to attract good and eliminate negative energies and to stimulate dreams.
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