Notes on the Oxidation
of Oils in Aromatherapy.
Massage Oils and Free Radicals.
1 a (usually slow) initiation phase
The initiation phase involves the formation of a free radical from a triglyceride molecule in the fat – this may be promoted by the presence of heavy metals in the oil, or by heat or light. The next stage is the reaction of the triglyceride free radical with oxygen to produce a peroxide free radical, which can react with another triglyceride to produce a hydroperoxide and another triglyceride free radical. Steps 2 and 3 can repeat in a chain reaction until two peroxy free radicals collide and neutralize each other. In symbols and words this is as follows (after Talbot 2004):
Autoxidation is assisted by higher ambient temperatures (the rate doubling for every ten degrees Centigrade rise) and by the presence of heavy metal ions, especially copper. The degree of unsaturation of the oil is also relevant to shelf-life – oils with a high linolenic fatty acid content (3 double bonds) being more prone that those with a higher saturated fatty acid content. Autoxidation can be minimized by the presence of anti-oxidants, which can act as free-radical inhibitors. Massage oils should therefore be stored in a cool place away from heat and light, and should only come into contact with inert (glass of stainless steel) containers which will not leach heavy metals. Blanketing under nitrogen is ideal for storage and a few aromatherapy oil suppliers are using this technique to maintain the quality of their oils. In any case, the instructions for the storing oils should be very closely followed.
Butylated hydroxyanisole (BHA)
As a yard-stick, synthetic anti-oxidants for food use are typically used at 0.02% maximum and have to show a lack of toxicity, and commercially are typically sold in carriers such as propylene glycol, or pure glycerides like glyceryl mono-oleate. Such carriers may not be suitable for adding to essential oils, causing insolubility or oil appearance problems (lack of clarity).
The following anti-oxidants are examples that aromatherapists might be a little more sympathetic to; the first three items are universally accepted in terms of legislation!
Ascorbic acid (vitamin C)
Diterpene-rich extracts of rosemary, sage, thyme and turmeric (often produced by CO2 extraction technology) are finding anti-oxidant & preservative applications in cosmetics and other non-food products, and many can be made odourless, colourless and tasteless. Rosemary extracts owe their anti-oxidant activity to the presence of diterpene constituents like carnosic acid, carnosol, rosmanol and rosmarinic acid, which will even protect vegetable oils from oxidation during the frying of food. The higher the phenolic diterpene content of the extract, the greater the anti-oxidant activity, and supercritical CO2 extraction has been used to produce the very active, deodorised and tasteless rosemary products described above.
High Linalol Containing Essential Oils.
The logic behind this recommendation for high linalol containing oils is based on the fact that during storage, linalol undergoes autoxidation, building up products including hydroperoxides such as 7-hydroperoxy-3,7-dimethyl-octa-1,5-diene-3-ol (Skold M. et al. 2002), which has been identified as apparently causing allergic reactions on exposed skin. The authors unfortunately and distastefully cite animal testing data, which reportedly found that with guinea pigs, ten week old samples of linalol sensitized the animals’ skin, but highly purified linalol produces no reaction. Auto-oxidation was therefore identified by the authors as necessary for the sensitizing process.
There has been some resistance to the implementation of this Standard (which anyway doesn’t come into effect until 2005) from certain essential oil sellers, on the basis that they do not feel happy adding anything to their ‘pure essential oils’. But not adding anti-oxidants will predictably cause potential problems for the clients of therapists or perfume manufacturers, as the applied oils build up allergens on aging. Perhaps the best situation would be where the therapist has the choice between those essential oil suppliers who will, and those who will not, adhere to GMP.
Does this phenomena extent to other oils containing moderate to high amounts of linalol such as Rosalina oil Melaleuca ericifolia, Basil oil linalool types (Ocimum basilicum & other Ocimum spp.), linalol rich oils of Mentha citrata, lavender & lavandin oils from Lavandula spp. etc. etc.? It is not known for sure, since peroxide formation might be capable of being reversed by certain procedures, and certain oils might naturally contain components which might function as natural anti-oxidants to a certain extent, such as benzyl benzoate.
Oxidation Degradation of Essential Oils.
Anti-oxidant activities of Natural Materials.
We already know that various terpenoids, flavonols, phenylpropanoids, coumarins and xanthones demonstrably show anti-oxidant properties in vitro. An example of this might be familiar to therapists already – where the keeping properties of Bergamot oil furano-coumarin free (usually prepared via rectification of expressed Bergamot oil) appear to be inferior to those of the photo-toxic expressed oil.
Much research has been centered around Thyme (Thymus vulgaris) and Rosemary (Rosmarinus officianalis) terpenoid extracts which appear to inhibit lipid peroxide formation and the scavenging effects of superoxide anions. It would seem that a certain configuration of terpenoids (ortho-dihydroxy groups in a catechol-like part of the molecule) seem to trap free radicals effectively preventing them from doing further damage. However as essential oils typically have very low concentrations of diterpenes (due to extremely low steam volatility) so perhaps this area of research is more correctly labelled as phytochemistry.
Houghton (2004) examined the anti-oxidant properties of an ethanolic-, water (flavonoid rich)-, and (essential oil containing) chloroform extracts- of Spanish sage Salvia lavandulifolia, finding the anti-activity of the latter extracts similar to that of propyl gallate, causing the author to speculate that the extract might prevent brain cells from damage by reactive oxygen species.
GMP – Good Manufacturing Practice
Houghton P.J. (2004) “Activity & Constituents of Sage relevant to the potential treatment of symptoms of Alzheimer’s Disease” Herbalgram 61, 49 et seq.
Skold M., Borje A., Matura M., & Karlberg A.T. (2002) “Studies on the autoxidation and sensitizing capacity of the fragrance chemical linalool, identifying a linalool hydroperoxide.” Contact Dermatitis 46(5), 267-272.
Skold M., Borje A., Matura M, & Karlberg A.T. (2002) “Sensitization studies on the fragrance chemical linalool, with respect to auto-oxidation” Contact Dermatitis 46 (Suppl. 4), 20.
Talbot G. (2004) “When Good Taste
Goes Bad” International Food Ingredients 3, (2004) 23-25.