Essential Oil Basics: Chemistry

In the first post of this series, I told you about the basics, what essential oils are, some thoughts on getting started, and why you want to be selective when choosing an essential oil distributor.  In the second post, we surveyed different general techniques for essential oil use and application as well as some specific applications I’ve used in my home.  Now, is the moment we’ve all been waiting for, today I’ll go into the science behind what essential oils are, where they come from, and how they work.  First, though, I want to give credit where credit is due.  My inspiration for this post came from the Dr. David Stewart’s book Chemistry of Essential Oils Made Simple: God’s Love Manifest in Molecules and I’ll cite it by linking to it when a point or idea references something Dr. Stewart talks about therein.  Dr. Stewart also writes a quarterly newsletter on various topics on the science and usage of essential oils, which are archived here.  In any case, Chemistry of Essential Oils Made Simple is a hearty textbook with way more information that I could ever go into in a blog post, so if you’re yearning for more information after you read this post, I highly recommend reading this book.  I was not disappointed in its breadth and depth of insight, chemically speaking.

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I defined essential oils in the first post of this series, but I believe it makes sense to mention it again here.  Essential oils come from plants.  They are “essential” because they sustain the life of a mature plant, by directing vital processes in plants such as metabolism, nutrient absorption, regulation of plant functions, protection, healing, repair, and reproduction.  And, they are “oils” because they are made up, primarily of hydrocarbons (atoms of hydrogen and carbon), which a simple definition of an oil.

There are actually two types of oils found in plants: fatty oils (aka vegetable, carrier, neutral, or base oils) and essential oils.  Fatty oils are large, found only in plant seeds, and feed the seed before it germinates.  For that reason, fatty oils are only found in plant seeds and do not circulate in plant tissue.  Essential oils are light, found in all plant matter, and sustain the life of the mature plant via the processes mentioned above.  For those reasons, they circulate through all plant tissue and are able to pass through cell walls and membranes.


Since essential oils come from plants, the plants need to be grown, cultivated and harvested under the right conditions at the right times.  I’m not going to go into all the details of that process here except to say that the same things that concerns me about the growing conditions of my food, concerns me about the growing conditions of my essential oils since they’re also be absorbed into my body.  Concerns like whether the seeds used are heirloom, hybrid, or genetically engineered.  Concerns like whether or not the growing conditions are organic or conventional.  And, concerns like whether the plants are grown indigenously, in the parts of the world where they naturally grow best, which will most likely yield an essential oil with the most therapeutic value.


Most therapeutic grade essential oils are made by a process called steam distillation.  Citrus oils are also considered essential oils, but are not collected by steam distillation.  Rather, they are collected by a process referred to as cold expression of the pressed peel where the essential oil is collected by using mechanical pressure to squeeze the oil out of the citrus peel.  But, since most essential oils are made by steam distillation, I’m going to focus on that method here.  I believe the below image designed by Rain Shadow Labs illustrates the process of steam distillation the most simply and clearly.

1.  Water is heated to create steam.

2.  The steam is passed through plant material; this could be any part of the plant, roots, stalks, stems, leaves, flowers, etc. because essential oils circulate through all plant tissue.

3.  The steam pulls the volatile essential oil out of the plant material; the oil rises with the steam.

4.  The steam/volatile oil mixture are then cooled in a condenser, which takes them from gases (steam and volatile oil) to liquids (water and essential oil).

5.  The liquid essential oil is then separated from the floral water (aka hydrosol).

6.  The floral water/hydrosol can be used in beauty products.

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When steam distillation is done at low pressure and minimum temperatures, the resulting oil will be closest to that in a living plant with the most therapeutic benefits.  If the steam distillation process is too harsh (at higher temperatures and pressures), the resulting oil will not be as beneficial therapeutically, if at all, but more oil can be obtained in doing so.  This is one of the reasons I encouraged you know your distributor in the first post of this series.  Some interesting facts, it requires about 250 pounds of lavender to yield 1 pound of lavender essential oil; 2,000 pounds of cypress for 1 pound of cypress essential oil and 6,000 pounds of melissa for 1 pound of melissa essential oil.  So, if you’ve ever wondered why melissa is more expensive than lavender, this is in addition to the varied cost of optimal growth requirements are contributing factors why.

Another note, most of the time, the steam distillation is only done once because the plant matter only has so much volatile oil to offer, so harvesters usually optimize the amount of time needed to run the steam distiller accordingly.  However, in the case of a few plants, like peppermint and ylang-ylang, longer distillations (increasing temperatures and pressures midway) can be done to yield more oil, more cost effectively; however, it’s most likely at the expense of the therapeutic value of the oil.  But, again, bear in mind that this is only the case for very few oils.

One last note about steam distillation, the process of steam distillation oil only removes the volatile oil from the plant matter, meaning only components 500-1000 amu (atomic mass units) or less are going to come through the steam distillation process (500 amu would be a conservative estimate; 1000 amu would be a more liberal estimate).  Bear in mind also that there’s healing in the large molecules, which can be accessed only by eating fresh and dried fruits, vegetables, and herbs and not by using essential oils by definition.  I believe this is one of the reasons why we can’t expect essential oils to fix all our problems.  We also need to eat a good diet and be other wise healthy physically, emotionally, and spiritually by exercising, resting, reducing stress, etc.


After the essential oil is collect by steam distillation, it’s usually tested using gas chromatography and mass spectrometry (GC/MS).  While testing is an important part of the process of determining the composition and purity of essential oils, Adrienne, a fellow writer, researcher and health nut at a Whole New Mom candidly discusses some valid concerns with relying solely on GC/MS for determining purity here.  She discerningly points out several great points to consider when selecting a therapeutic grade essential oil:

Growing Conditions:  Is the plant organic and/or wild-crafted with natural fertilizers and without the use of synthetic pesticides/herbicides/etc. and far enough away from farms that choose to use these products?

Steam Distillation:  Is steam distillation conducted without the use of solvents or adultering of any other kind?  Is it conducted using minimum pressures and temperatures?

Testing and Final Product:  Is the resulting essential oil pure and free of artificial oils?  Does the resulting oil fall within GC/MS guidelines?


Once obtained, the essential oil of any plant species is a complex mixture of tens to hundreds of small organic molecules (again, here, small is defined as less than 1000 amu and organic is defined as containing carbon atoms).  To date, no essential oil has been completely analyzed for all it’s constituents.  According to Dr. Stewart, the small molecular structure of essential oils means that 1 drop of essential oil can contain up to 40 million trillion molecules.  Recently, Bianconi et al. estimated that the human body contains around 40 trillion cells.  So, if these estimates are correct, mathematically, each drop of essential oil contains enough molecules to cover each cell with 100,000 molecules.  Dr. Stewart writes here that it only takes one molecule of the right kind to open a receptor site and communicate with the DNA to alter cellular function; however, too many oil molecules can saturate receptor sites, rendering them ineffective and unresponsive.  Think along these lines, I applied a lot of essential oil (either accidentally or intentionally).  If the receptors of my cells fill up with essential oil molecules of the wrong kind, the molecules I my body needs can remain outside the cells searching for an open receptor.  That’s a scenario, but I write all that to say and as Dr. Stewart points out, in terms of essential oil usage, a can go a long way.

Living organisms, plants, animals, and microbes, produce thousands of small organic molecules and little is known about them.  However, we do know that the isoprene unit is one of the most frequently found and fundamental structural patterns in small organic molecules in plants, animals, and microbes.  Compounds containing isoprene units are referred to as isoprenes or terpenes.  Gershenzon from the Max Planck Institute and Dudareva from Purdue University cite that around 25,000 terpene structures have been reported and discuss some of the major roles of terpenes in nature in this review article.


There are three main classes of terpenes found in essential oils:  phenylpropanoids (aka hemiterpenes), sequesterpenes, and monoterpenes.  Each class containing hundreds or thousands of different compounds.  Phenylpropanoids help resist bacteria and viruses as well as clean cellular receptors.  Sequesterpenes are the largest class of natural products, with structural varieties numbering around 10,000.  According to Gershenzon and Dudareva, sequesterpenes have potent antibacterial and antifungal activity, a tremendous benefit for protection, healing, and repair; sequesterpenes have also been reported to act as toxins, growth inhibitors and deterrents in plants, demonstrating their ability to regulate plant functions, including metabolism.  Dr. Stewart states here sequesterpenes can also play roles in cellular oxygenation as well as correcting miswritten codes in DNA.  Monoterpenes have been implicated in plant defense and protection as well as synergistically enhancing the effect of sequesterpenes.


Gershenzon and Dudareva speculate that the structural diversity and vast number of terpenes within organisms may allow cellular messages to be very specific with high information content.  Terpene variety may also result in a synergistic effect whereby the actions of individual terpenes are enhanced in the presence of other terpenes.  Many of the ancient essential oils (like those mentioned in the Bible) as well as some modern essential oil blends contain all three types of terpenes.  Dr. Stewart additionally points out here that if an essential oil contains all three types of terpenes, the phenylpropanoids may clean the receptor sites, allowing the sequesterpenes access to the cell through cellular receptors to regulate cellular function and the monoterpenes to restore, or even reprogram, cellular function.

While generalizations may be easier for us to understand, Gershenzon and Dudareva wisely point out that making generalizations about the activity of classes of small organic molecules as varied as the terpenes, can be dangerous.  Remember that each class of terpenes contains hundreds or thousands of different compounds.  Even the smallest of changes in structure, as in the case of (R)– and (S)- carvone, which have the exact same chemical formula but are structurally sightly different (being “right” and “left” handed), result in noticeable differences – we know (R)-carvone as spearmint and (S)-carvone as caraway.


Similarly, the (+)– and (-)- gossypol found in cottonseeds also have the exact same chemical formulas and are altered just the slightest structurally, but enough so that (-)-gossypol is toxic to humans while (+)-gossypol is not.  So, we can see how we need to be careful.

Scientific research on the composition and effects of essential oils is limited, but I believe there are enough good resources on essential oil composition and usage to support safe, reliable, and responsible use.  So, it’s important to do your research and trust your instincts.  The chemistry of an essential oil determines what is possible and what is not.  Dr. Stewart aptly points out in the Chemistry of Essential Oils Made Simple that a given essential oil is only capable of the possibilities within its constituents.  And, that our prayerful, informed, and discerning application and usage determines which possibilities actually happen.




FDA Disclaimer: The products and statements made about specific products in this presentation have not been evaluated by the United States Food and Drug Administration, and are not intended to diagnose, treat cure, or prevent disease. All information provided during this presentation, or any information contained on or in any product label or packaging, is for informational purposes only and is not intended as a substitute for advice from your physician or other health care professional. Any testimonials presented are based on individual results.


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