from an article in Aromatherapy Today 1(62), 9-10 by Tony Burfield.
Copyright © Tony Burfield August – October 2004.
undergo undesirable deterioration reactions in the presence of oxygen from the
air, familiar to us as the phenomena of rancidity, with accompanying
off-flavours and smells. Essential oils are attacked by oxygen over time as
well, resinifying and developing artefacts that detract from original odour
profile. As we will see, some of these reactions may also cause allergenic
constituents to arise in essential oils.
Massage Oils and Free Radicals.
The mechanism of autoxidation of vegetable massage
oils is classically regarded as following a number of stages:
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 neutralise each other. In symbols and words this is as follows (after Talbot 2004):
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 minimised 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
Anti-oxidants commonly added to edible fats and oils include the synthetics:
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!
Tocopherol isomers naturally occur in vegetable oils –
for example soya bean oil is rich in gamma-tocopherol, and sunflower oil is rich
in alpha-tocopherol. Palm oil contains moderate amounts of tocotrienols, but the
anti-oxidant properties of tocotrienols are not well established.
We also have evidence that alpha-tocopherol and vitamin C together have
synergistic effects – this is exemplified in the use of ascobyl palmitate
above which only has a weak action by itself, but is synergistic in its action
with natural tocopherols. Finally, natural tocopherols in vegetable oils may be
destroyed by relatively small amounts of heavy metals like iron, and the
presence 0.01% citric acid can be enough to stop this. You might find that your
processed massage oil has been treated in this way to increase its shelf life.
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
High Linalol Containing Essential Oils.
A bit of excitement has been generated lately via
the 38th Amendment to the IFRA Standard, which suggests that for high
linalol containing oils, such as coriander (Coriandrum sativum), rosewood
(Aniba spp.) and ho oils (Cinnamomum
camphora var. linaloolifera
& Cinnamomum camphora var. glavescens
), Good Manufacturing Practice (GMP) should involve the addition of 0.1%
of an anti-oxidant (BHA or tocopherol) at the point of production, to keep
peroxides at the lowest practical level. IFRA further suggests that the level of
peroxides should not exceed 20 mmoles/l. You might recall that anti-oxidants are
already recommended to be added to citrus oils and pine oils from the Pinaceae,
under the existing IFRA Standards. This is based on evidence that
hydroperoxides formed from the limonene and delta-3-carene contents of these
oils can cause skin sensitivity problems. It should also be said that essential
oils used for flavouring purposes, often have anti-oxidants such as BHA and BHT
added as a matter of course during manufacture.
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
sensitised the animals’ skin, but highly purified linalol produces no
reaction. Auto-oxidation was therefore identified by the authors as necessary
for the sensitising 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
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
Oxidation Degradation of Essential Oils.
As well as the hydroperoxides produced by linalol, limonene and
delta-3-carene mentioned above, other oxidation and resinification effects
progressively causes other fairly major changes in essential oil quality over
time. It is interesting to note for example that the criteria for essential oil
testing in the British Pharmacopoeia 2003 includes passing a resinification
detection test for many essential oils and an eugenol dimerisation test (eugenol
being the major constituent of Clove Oil (Syzygium
The topic is too large to discuss here, but is currently the subject of a
larger article in preparation by the author.
Anti-oxidant activities of Natural Materials.
This is an interesting area of research which
might provide us with raw materials which ultimately help prevent or reduce the
effects of aging. Cell oxidative damage from reactive oxygen species is known to
occur in living systems, membrane lipid peroxidation being a major area of
susceptibility; mitochondrial damage from lipid oxidation is known to cause a
number of age-related diseases. Altzeheimers disease in particular appears to be
associated with damage caused by reactive oxygen species (Houghton 2004).
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
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.
It would be a pity if aromatherapists & perfumers
do not eventually use commercial extracts of these herbs with such useful
activities, to preserve essential oils from deterioration in storage, especially
since we are beginning to understand the importance of degradation artefacts in
relation to allergenicity (as outlined above for high linalol-containing oils).
GMP – Good Manufacturing Practice
GRAS – Generally Recognised As Safe
Houghton P.J. (2004) “Activity & Constituents of Sage relevant to the
potential treatment of symptoms of Altzeimer’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),
Skold M., Borje A., Matura M, & Karlberg A.T.
(2002) “Sensitisation 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.
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