Factors that Cause Loss of Quality in Essential Oils: Part I – Headspace

Does Your Oil Spoil?

Do you ever find that once you have used your essential oils a few times the liquid that remains in the bottle begins to “go bad” a great deal faster? You begin to notice, that suddenly, when you use the same bottle of essential oil, with your usual henna powder, following your usual henna recipe and mixing process, the depth and tones of the color of the stain results achieved by your henna body art is significantly reduced?

It seemed like the very same bottle
of essential oil was fine
just a few days ago;
what’s happened?


What in Essence are Essential Oils?

Essential oils, found in plants, are plant metabolites comprised of a multitude of different yet often structurally similar compounds that are categorized according to a variety of different chemical classes[1]. A single essential oil can be comprised of only a few or up to more than 100 different compounds[1][3] characterized by several key qualities:

  1. Volatility: heir ability to vaporize or evaporate far more readily than other substances at the same temperature and pressure;
  2. Lipophilicity: their tendency to dissolve in fats and oils and opposed to water-based or aqueous solutions;
  3. Molecular weight: generally does not exceed 300

Their molecular properties allow essential to be easily be separated from the plant components and tissues in which they naturally are found. While the actual role of essential oils in nature is still the subject of research and discussion, their medicinal and therapeutic properties as well as their high degree of benefit in the pharmaceutical, food, and cosmetics industries as a natural alternative to synthetic chemicals has been clearly demonstrated within the scientific community[1].


Natural Henna Body Art and the Role of Essential Oils

The henna plant, botanically known as Lawsonia inermis L., is used for body art due to its capacity to stain skin cells. When then leaves of the henna plant are harvested, dried, and crushed into a fine powder, they can be made into an paste using an acidic aqueous solution[2]. When the resulting paste is applied to the skin, the henna plant’s natural dye molecule, lawsone, can gradually migrate from the paste into the outer layers of the skin. It binds to keratin, the key proteins comprising the skin, by way of the Michael addition, creating a covalent bond between the keratin and the dye pigment[2].

As every natural henna artist knows, ‘terping’ henna paste, is an entirely natural way to achieve darker and deeper tones in the final stain that remains on the skin. ‘Terping’ refers to monoterpenes or monoterpene alcohols, a key compound comprising many essential oils. Essential oils with particularly high levels of monoterpene alcohols, such as tea tree, cajeput, eucalyptus or lavender, can be added directly into henna paste to facilitate darker stain results[2].


What is Essential Oil Quality Loss?

Quality loss, respective to essential oils, specifically refers to the chemical change of the components that cause a given essential oil to possess its beneficial properties.

The primary causes of quality loss in essential oils is their susceptibility to conversion and degradation reactions. Numerous factors exist that can bring about these two types of reactions and thus affect the stability of essential oils. The foremost of these factors being[1]:

  1. Light: UV exposure
  2. Temperature: exposure to heat
  3. Oxidation: exposure to oxygen
  4. Chemical Reaction: contact with substances that the compounds in the oil can chemically react with

The topic of this post will be the discussion the effect of oxygen, specifically headspace oxygen, on the degradation of essential oils.

canning-jar-head-space

 

Headspace is the area within a sealed container that lies above the liquid phase[1].  Headspace measurements and calculations are important for understanding and implementing accurate methodologies for the preservation of the contents of the container. Biochemical engineers Across numerous industries such as food, cosmetics, pharmaceuticals and others, use extensive mathematical calculations and knowledge of chemistry, in order to establish the exact molecular composition of this seemingly small area, and from those calculation can predict changes in composition at any given temperature and pressure.

Identifying the Signs of Quality Loss in Essential Oils

There are several qualitative indicators that can reveal that an essential oil has undergone loss of quality[1]:

  • a shift in the scent or flavor of the oil to those that are pungent and often unpleasant; 
  • change in color of the oil, often a yellowing;
  • changes in consistency of the oil such as thickening or resinification which is the formation of thick dry films that are often found on the inner cap or the neck of the bottle;

In addition to changes in physical characteristics of essential oils, some essential oils that have undergone quality loss due to oxidation have revealed skin sensitizing capability. Skin sensitization is the onset of allergic contact dermatitis or a similar irritating dermal reaction to contact with a specific substance.


The Relationship Between Size of Headspace and Rate of Quality Loss

When essential oils are used, the bottle’s cap is removed, some of the oil is poured out and the cap then replaced. A direct relationship between the size of the headspace and the rate of quality loss of the essential oil has been determined[1].

This means that the less essential oil there is remaining in the bottle, the faster it will tend to spoil and thus be unable to produce the same results when used in henna paste. NOTE: This is true irrespective of the volume of the container, i.e., it is as true for a large vat as it is for a vial with similar physical proportions.

Solution & Best Practice:

Keep a collection of different sized bottles with capacities to store different volumes of liquids in order to significantly extend the life of your essential oils and their stain-producing capabilities. When the oil contained in a given bottle begins to drop below 75% full, transfer into a smaller bottle should be made to minimize the headspace.

These UV-proof essential oil storage bottles from Anandaapothecary.com are as good as it gets. The are made of Swiss UV-proof glass and can come with a whole bunch of useful accessions that you can read about on their website: https://www.anandaapothecary.com/ananda-apothecary-news/state-of-the-art-storage-bottles-for-your-essential-oils

uv-proof-bottles-300x216

They are pretty amazingly reasonable priced for being just about the best storage bottles that I have come across, and If anyone, say, wants to get me a gift for no particular reason… umm these bottles are just about tops on my list.

A second best alternative within an affordable price range are a collection of the standard metal essential oil storage bottles, used mostly for storage of large quantities of essential oils ranging from 250 mL to more than 1L.

pack2

These are outstanding for long-term storage. Be sure, however, to get the bottles with the airtight plug on top as some companies just sell bottles with a screw-on cap. The airtight plug makes a significant difference in the preservation of the oil.

The downside to these bottles, however, is the potential for the essential oil to interact with some of the metallic compounds that make up the bottle. I hope to make this issue the topic of a future blog post.

The ideal bottle type for storing essential oils:

  • is made from glass 

  • has an air tight (or close to it) cap

  • is dark in color, protecting the oil from light essential oil storage box

 

Best Practices for Storing Essential Oils Include:

Keep stored bottles in a cool, dry, dark place for best results.

My favorite storage container: the wooden slotted essential oil storage box with a clasped lid and a carrying handle–have oil will travel.

 


So, there it is: the problem, the cause, and the solutionall wrapped up in a few paragraphs.

If, however, you are a henna nerd and possibly possess henna OCD ( not yet in the DSM V but I am still sure that I can be diagnosed with it) feel like you absolutely must understand the specifics of the science behind just why this happens, the rest of this post is for you. 


1.0 Liquid-Vapor Equilibrium 

Picture7

When your essential oils are contained within a tightly closed jar, they can be said to form what is known to biochemical engineers as a closed liquid-vapor system[4].

A closed liquid-vapor system, respective to essential oils, is simply an air-tight vial or jar that contains some essential oil and some air at the top, i.e., headspace.

 

A closed liquid-vapor system, such as a glass bottle containing an essential oil reaches and settles in a state of liquid-vapor equilibrium. Liquid-vapor equilibrium is simply the preferred state of the matter contained within the system, and it is a function of temperature and the internal pressure of the container[4].

So, at any given temperature and internal pressure the distribution of species (i.e., different chemical components) within the liquid and vapor phases in the closed system reaches a constant value that is measured in moles of species in each phase [4].

Rosemary-Essential-Oil.jpg

The total pressure of the vapor phase of the system i.e., the headspace is the sum of the partial pressures of each chemical species within the system. Knowing that essential oils are comprised of different combinations of several compounds that possess the characteristic of volatility (high tendency to vaporize) the total pressure in the headspace of the essential oil bottle is then the sum of the pressures exuded by each species within the oil. This will be important later in understanding essential oil spoilage.


2.0 Changes in the System 

Picture9.jpg

When the essential oil bottle is opened and some liquid poured out–say, to be added to a henna paste mixture–and the cap resealed , the closed system (i.e., the bottle) will reach a new state of vapor-liquid equilibrium because the volume of liquid as well as the size of the headspace have both changed.

Picture3.jpg

The system will now tend towards a new state new equilibrium that will be a function of how many moles of substance are left in the liquid phase and vapor phases. When the lid is replaced and the system re-sealed:

  • some of the volatile essential oil molecules, due to their tendency to vaporize, will vaporise or evaporate and leave the liquid phase passing into the vapor phase and occupying the bottle’s headspace and increasing the vapor pressure of the headspace;
  • as the pressure in the vapor phase or the headspace increases, some of the oxygen in the headspace will be “pushed out” of the vapour phase and into the liquid phase, i.e., the essential oil,

Now, with more oxygen dissolved in the liquid phase of the essential oil, oxidative damage is occurring at a faster rate. It can be seen from this example that the greater the headspace the faster the rate of oxidation in the liquid, i.e., essential oil spoilage.

 


Picture11.jpgTo Summarize:

After some of the essential oil has been poured out and the bottle resealed:

  1. less liquid remains in the bottle;
  2. there is an increase in headspace;
  3. at the precise point in time of sealing the bottle, the headspace or vapor phase is comprised of the air that entered the bottle when the cap was removed and liquid was poured out;
  4. due to the tendency of components that comprise essential oils to vaporize (volatility), the species from the liquid phase will begin to evaporate and enter the vapor phase;
  5. as species enter the vapor phase, the pressure of the vapor phase increases;
  6. as the pressure of the vapor phase increases due to vaporization of the volatile components oxygen from the air in the vapor phase is “pushed out” by the pressure and dissolves into the liquid phase (i.e., the essential oil)
  7. when the system reaches equilibrium at a constant temperature, the total pressure of the vapor phase, at any given time is equivalent to the partial pressures of each of the species within the system;
  8. with a greater amount of oxygen dissolved into the essential oil, the rate of oxidation or loss of quality of the essential oil due to oxidative damage is ingreased;

 


3.0 Effects of Temperature

The specifics of temperature as a cause of essential oil spoilage as well as the mechanism by which oxidization causes loss of quality in essential oils will be saved for the topic of future blog post, however, given the information we have now it can be understood why an increase in temperature affects an essential oil causing it to loose quality at a faster rate:

  1. the higher the temperature the greater the kinetic energy possessed by the molecules in the system;
  2. the greater the kinetic energy possessed the greater the tendency for the volatile compounds in the liquid phase to vaporize, leaving the liquid phase and passing into the vapor phase
  3. As more molecules enter the vapor phase the pressure within the headspace increases
  4. The increase in headspace pressure forces some of the less volatile compounds such as oxygen molecules within the air to dissolve into the liquid phase
  5. Once the liquid phase contains more oxygen the rate of oxidative damage is greater;

So a higher temperature causes a higher rate of vaporization of the liquid phase, which in turn causes an increase in pressure of the vapor phase and results in more oxygen dissolving into the essential oil.


4.0 Raoult’s Law & Henry’s Law 

The rates of quality loss of essential oils during the manufacturing process is one of the topics of concern of biochemical engineers functioning in quality assurance roles in industry. Rates can be is can specifically calculated given knowledge of the specifications of the manufacturing process as well as the precise composition of the essential oil, usually obtained through high performance liquid chromatography (HPLC)

Calculations are concerning the amounts of species in both the liquid and the vapor phases can be made using Raoult’s Law or Henry’s Law. Raoult refers to François-Marie Raoult who discovered the thermodynamic relationship between gases and liquids in a closed system in 1887.  Henry, of Henry’s Law also refers to similar but distinct thermodynamic gas law formulated by British scientist William Henry in 1803[4].

Both laws state that in a closed system (e.g., an airtight bottle) that the distribution of substances between gaseous and liquid phases are determined by specific relationships that exists between the partial pressure of a volatile liquid–that is, the one that can evaporate–and the molar composition of the component in the mixture. Both gas laws are the subject of vapor-liquid equilibrium.


 

5.0 Conclusion

In conclusion, of the best practices for preservation of essential oils and prevention of quality loss is keeping a set of numerous containers varying in size. When the liquid levels begin to decrease, storage in a smaller container prevents an increase in the rate of quality loss due to oxidation. 


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References:

[1] Turek, Claudia, and Florian C. Stintzing. “Stability of Essential Oils: A Review.” Comprehensive Reviews in Food Science and Food Safety, vol. 12, no. 1, 2013, pp. 40–53., doi:10.1111/1541-4337.12006.

[2] Gallo, Francesca R., et al. “Henna through the centuries: a quick HPTLC analysis proposal to check henna identity.” Revista Brasileira de Farmacognosia, vol. 24, no. 2, 2014, pp. 133–140., doi:10.1016/j.bjp.2014.03.008.

[3] Handbuch des Arznei- und Gewürzpflanzenbaus. Band 1 Grundlagen des Arznei- und Gewürzpflanzenbaus I, Blitzke T., 2009

[4] Felder, Richard M., et al. Elementary principles of chemical processes. Wiley, 2017.

[5] Hüsnü K., Başer C., Demirci F. (2007) Chemistry of Essential Oils. In: Berger R.G. (eds) Flavours and Fragrances. Springer, Berlin, Heidelberg

 

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