Factors affecting Degradation of Essential Oil

Essential oil is composed of many chemical compounds and any deviation for their original composition may have a strong impact on the flavor and aroma value.

The components are known to easily convert into each other by oxidations, isomerization, cyclization, or dehydrogenation reactions triggered either enzymatically or chemically. Thus, plant volatiles are subject to fluctuation in their composition. They are especially prone to oxidative damage, chemical transformations, or polymerization reactions.

Degradation can occur during processing and storage of plant material, during distillation, as well as the subsequent handling of the oil itself. (Schweiggert U, Carle R, Schieber A. 2007. Conventional and alternative processes for spice production – a review. Trends Food Sci Technol 18:260–8.)

Harvest time, plant health, soil fertility, growth stage, climate, etc. affect the chemical composition of the oil. The moisture content of plant material when distilled also plays a role. Additional chemical changes such as oxidation, hydrolysis, and exposure to light and heat can occur during distillation and storage causing even more changes. (Grosch W. 2007. Key odorants of food identified by aroma analysis. In: Ziegler H, editor. Flavourings. 2nd ed. Weinheim, Germany: Wiley-VCH.p 704–43.)

 

Methods of degradation of essential oil

degradation of Essential Oil Components

degradation of essential oil

Figure 1 Stability of Essential Oils: A review. Turek, et al. 2013

  • Isomerization – chemical change; same formula, different arrangement
  • Oxidation – compound gains oxygen, loses hydrogen, or loses electrons
  • Dehydrogenation – chemical reaction that involves the removal of hydrogen
  • Hydrolysis – decomposition where water breaks chemical bonds
  • Polymerization – combining molecules into larger molecules – includes hydrolysis
  • Thermal rearrangement –

Degradation of essential oil during storage

After distillation and during storage the most common factors causing degradation of lavender essential oil are:

  • Hydrolysis
  • Oxidation
  • Light
  • Temperature,
  • Reflux

Hydrolysis – occurs when water is not removed

  1. “Hydrolysis is a chemical reaction during which polymers (large molecules) are broken down into monomers (small molecules). During hydrolysis, covalent bonds between monomers break, which allows for the breaking of polymers. Covalent bonds are broken with the use of water.” (https://www.studysmarter.de/en/explanations/biology/biological-molecules/hydrolysis-reaction/)
  2. linalyl acetate is rapidly hydrolyzed (t1/2<5 min) to linalool changing the chemical consistency of the essential oil (https://pubchem.ncbi.nlm.nih.gov/compound/Linalyl-acetate)
  3. In addition, hydrolysis of components can result in:
    • Unpleasant odor
    • Unbonded water in suspension ruining oil’s keeping quality
  4. To avoid hydrolysis:
    • Remove water from essential oil as soon as possible after distillation. Water will appear as bubbles at bottom of collection container.
    • Options to remove water:
      • Allow the oil to breath and the water to evaporate off the oil
        • This results in loss of the most volatile organic compounds which give the oil its top notes
      • Freezing is often used to remove water.
        • This is not totally effective as ice crystals can be suspended in the oil leaving water in the oil resulting in hydrolysis continuing to degrade the oil
        • The essential oil is placed in the freezer and the oil poured of once the water freezes.
        • The drop in temperature of the soil increases solubility of any oxygen contact with the oil resulting in increased oxidation – see Henry’s Law in Temperature Discussion below
      • Use dehydrated sodium sulfate
        • Shake into oil
        • Shake container until sodium sulfate absorbs all water
        • Filter oil through unbleached coffee filter

Oxidation – loss of electrons due to oxygen in the essential oil

Oxidation results in skin sensitizing compounds resulting in allergic contact dermatitis. https://pubmed.ncbi.nlm.nih.gov/30779160/

    • Oxidation can increase production of hydroperoxides.
    • The resulting free radicals “steal” electrons from the lipids in cell membranes, resulting in cell damage, increasing pain, negatively affecting mood, and damage health. Free radicals can increase cancer and various sicknesses and diseases.
    • Pure compounds are not allergenic, but their oxidation products can cause contact allergy
    • Radicals possess an unpaired electron, which makes them highly reactive and thereby able to damage all macromolecules, including lipids, proteins and nucleic acids.” More
  1. To avoid oxidation of essential oils,
    • Recommendation: Remove the oxygen by using food grade Nitrogen gas during the distillation, oil water separating, and storage procedures. This is heavier than oxygen.
      • Add to separator
      • Add to essential oil collective container
      • After removing water, top off the storage container with Nitrogen gas
  1. The allergenic potential of essential oil is mainly attributed to terpenoid hydroperoxides due to autooxidation. This depends on the molecular structure of the components, the oxygen concentration in the essential oil and the energy input. Without oxygen this chemical change cannot occur. Choe E, Min DB. 2006. Mechanisms and factors for edible oil oxidation. (Compr Rev Food Sci Food Saf 5:169–86. Neuenschwander U, Hermans I. 2010. Autoxidation of α-pinene at high oxygen pressure. Phys Chem Chem Phys 12:10542–9.)
  2. As the temperature drops in the essential oil, the solubility of oxygen in the oil increases. See Henry’s Law in the discussion under temperature

Light – UV and Visible

  1. Ultraviolet and visible light are the energy source necessary to accelerate autooxidation which increases alkyl radicals.
    • Breakdown of essential oil components
    • Transformation of compounds
  2. Monoterpenes are rapidly degraded under light energy
  3. Store essential oils in the dark

Temperature

  1. Chemical reactions accelerate with increasing temperature (Arrhenius equation).
    • An increase of 10o C (18o) results in a doubling of chemical reaction.
  2. Lavender peroxides increase at 5o C (41o) as opposed to room temperature
    • Peroxides form when oxygen is available
    • Peroxides have unpleasant odors
  3. Oxygen in the oil increases as the temperature drops due to:
    • Henry’s Law
      • Lower temperature results in more oxidation solubility
    • Half-filled containers with oxygen in the free space has more oxidation potential than completely filled containers.
      • oxidation occurs unless oxygen is completely removed.
  4. Hydrolysis increases as temperatures increase
    • Must have water in the essential oil for this to occur
  5. Recommendation: Store essential oils at room temperature

Metal contaminants Impurities of metals can be released into essential oils.

  1. Lavender is considered reactive with metals
    • The degradation due to metal contamination can be equal to light and heat
      • copper and ferrous ion
      • Promote auto-oxidation, especially if hydro-peroxides are present.
  2. How to Prevent:
    • Use a Food-Grade Stainless steel essential oil still.
    • Do not store in copper vessels

Reflux

  1. Results in loss of oil and reduction of quality.
    1. Will cause chemical change of oil due to hydrolysis of oil in water
  2. Oil drips into the boiling water at the bottom of the pot instead of moving with steam out through the condenser.
  3. Up to 50% of potential oil can be lost due to reflux. This can result when:
    • Inadequate steam is produced to move the essential oil up the charge
    • To prevent:
      • Have a rolling boil before placing basket/charge in still
      • Ensure first drop of condensate drips from the condenser within 5 minutes of placing the basket in the still.
      • Increase heat if necessary to enhance steam
  4. The distance between the water in the pot and the charge is not adequate and splashing of water into the charge occurs.
      • It is critical the space between the water and herb should have a depth equal to one-fifth of the still’s diameter.
      • May need a baffle system to prevent splashing.
  5. How do you know reflux has happened?
      • Determined by dark change in color of water in pot.
      • If water drips from basket of plant material when it is removed from the still
  6. How can it be prevented?
    1. Ensure water in pot does not exceed the critical space mentioned above.

Additional Reference on Degradation of Essential Oils

 

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