Guayule: Hypoallergenic Latex and Biofuel From a Desert Plant
A Potentially Important Shrub
Guayule (pronounced why-OO-lee) is a desert shrub that is on the verge of becoming very important to humans. It produces a latex that can be turned into rubber. Unlike the latex from the Para rubber tree, which provides most of the world’s natural rubber supply at the moment, guayule latex doesn't contain allergenic proteins. In some people rubber tree latex can cause allergy symptoms that are life-threatening.
The guayule plant grows naturally in dry areas of the southwestern United States and northern Mexico. In addition to providing a safer latex, guayule may also be helpful in alleviating the growing shortage of natural rubber in the world. In addition, it contains resins and waxes that may prove useful to humans. It may also become a very useful biofuel. Farmers, commercial companies and researchers are growing guayule plants with an eager eye on the future.
The Guayule Plant and its Uses
A Useful Member of the Aster Family
Guayule is a flowering plant in the aster family and has the scientific name of Parthenium argentatum. It grows as a low, woody shrub that is generally less than three feet tall. The plant has numerous narrow, grey-green leaves with hair-like structures called trichomes on their surfaces. Small, pale yellow flowers are produced at the tips of long stems that grow from the top of the plant.
Guayule is adapted for survival in a dry habitat. The leaves are densely haired and covered with a white wax to help prevent them from drying out. The plant has an extensive root system to absorb as much water as possible. There is one taproot extending downwards and fibrous lateral roots extending from the taproot to the side. In some plants the taproot is longer than the lateral roots, while in other plants the opposite is true.
Guayule is a perennial plant. It has the ability to lose most of its leaves and become dormant for as long as several years when the environment is extremely dry. When a new supply of water is available the plant can quickly produce fresh leaves.
Most of the Guayule's latex is located in cells in its bark, unlike latex from the rubber tree (Hevea brasiliensis), which is located in tubes called lactifers. At one time it was difficult and expensive to extract guayule latex, but improved technology has made the extraction a commercially viable process.
Latex and Natural Rubber
The words "latex" and "rubber" are often used interchangeably, but technically they mean different things.
Latex is a milky liquid produced by plants. It contains water, proteins, sugar, rubber particles and other chemicals. Latex can be concentrated so that the rubber particles coagulate to form solid rubber, which is also known as natural rubber. In everyday life, the word latex is often used as a synonym for rubber, however, as in the term "latex gloves".
Guayule plants were used to produce rubber during the second world war when the Para rubber supply to the United States was cut off. The Russian dandelion, another plant that contains latex, was used for the same purpose. After the war ended, the supply of Para rubber resumed and the alternate production of guayule rubber, which was more expensive than the cost of Para rubber, ended. Now interest in guayule rubber is increasing again.
Despite the fact that synthetic rubber can be made, natural rubber is still essential. It has useful properties that synthetic rubber lacks. For example, natural rubber offers better resilience and flexibility than synthetic rubber.
The Para rubber tree population in South America has been decimated by a fungal infection, so most of the natural rubber supply now comes from Southeast Asian Para rubber tree plantations. However, the production of rubber by these plantations can't keep up with world's demand.
Guayule Rubber in Tires
Rubber Production From Guayule Plants
In the United States guayule latex extraction is carried out by the Yulex corporation, which has been licensed by the U.S. Department of Agriculture to use extraction technology developed by the Agricultural Research Service (ARS), part of the Department of Agriculture. The name "Yulex" was created by a contraction of the words "guayule" and "latex".
The Para rubber tree has lactifers - latex-transporting tubes - in its trunk. The tubes can be tapped, allowing the latex to drip out, but this process doesn't work in guayule plants. Most of the rubber particles are located inside bark cells known as parenchyma cells. These parenchyma cells must be opened up to release the cell liquid and its rubber particles.
The ARS says that their latex extraction process uses water instead of the potentially dangerous solvents used in other extraction methods. The guayule shrubs are ground up in water, which breaks open the cells and releases the liquid containing the rubber particles in the form of a suspension. The suspension is then centrifuged, which causes lighter particles (including the rubber particles) to collect at the top of the liquid and heavier particles to fall to the bottom of the liquid. The upper part of the liquid is removed and purified to create a thick, white latex. The latex has a very low concentration of proteins. Furthermore, these proteins are different from the Para rubber proteins that cause allergies. The coagulation of the guayule latex produces a high-quality rubber.
Rubber gloves made from guayule latex were approved for public use by the FDA (Food and Drug Administration) in the United States in 2008. The FDA acknowledges that laboratory tests show that people with allergies to Para rubber latex do not react to guayule latex, and carefully says that guayule gloves "may" be a safer alternative for "some" people with a sensitivity to traditional latex. However, it won't allow the term "hypoallergenic" to be used on any types of medical gloves. Guayule may be most valuable when used to produce medical devices that are used internally, such as catheters.
Some scientists feel that more tests need to be done to prove that guayule latex is completely hypoallergenic. They may never be able to be prove that guayule rubber - or any material - is hypoallergenic for absolutely everybody.
Latex Allergy Symptoms
One reason why guayule may be very useful is that a severe allergy to Para rubber latex can be deadly. Latex gloves can cause symptoms in someone with a latex allergy, but so can other objects made of latex, including some balloons, elastic bands, some types of first aid tape and Halloween masks.
A mild latex allergy can cause itching, skin redness and hives where the latex product touched the skin. More severe responses include typical allergy symptoms such as sneezing, a runny nose, itchy eyes, a sore throat, wheezing and coughing.
A very severe reaction to latex can cause someone to experience anaphylactic shock. This is a body-wide response that can be very dangerous. Symptoms include breathing problems, abdominal pain, angioedema (swelling under the skin), a very serious drop in blood pressure, a rapid heartbeat and loss of consciousness. The condition is a medical emergency. Luckily, a severe latex allergy is rarer than a mild or moderate one.
Guayule as a Biofuel
Stems and branches of guayule are ground up in the latex extraction process. The remaining material after the latex has been removed is called bagasse and isn't a waste product, since it can be used to produce biofuel. Bagasse is a brown, granular substance.
Cellulose is the main component of the cell walls of plant cells. The cellulose in the guayule bagasse is fermented to make ethanol. The hemicellulose and lignin in the cell walls are useful in industry.
Bagasse has been used experimentally to make a bio-oil by heating it without the presence of air. It has also been used to make a gas known as a "syngas", or a synthesis gas, which contains a mixture of carbon dioxide, carbon monoxide and hydrogen and has about fifty percent of the energy-producing ability of natural gas. Syngas can be used as a fuel to produce electricity.
Researchers say that guayule is available for harvesting at any time of year and can regrow rapidly if it's cut just above the ground. They are investigating the possibility of inserting genes into the guayule plants to enable them to make more latex.
With all its potential benefits, guayule is a plant that we may hear a lot about in the near future. The potential to create a hypoallergenic latex and a productive biofuel from guayule plants is intriguing and could also be extremely useful.
© 2012 Linda Crampton