Benoit Roger, Ph.D.

When we talk about essential oil quality, the first analysis to be discussed beside organoleptic evaluation (odor, color, clarity, etc.) is gas chromatography (GC-MS or GC-FID, which you can discover through our GC analysis blog series). This is entirely understandable and even justified as it brings a good overall view of essential oils composition and purity. Actually, if we had to choose one and just one analysis to perform on an essential oil, this would probably be the GC.

However, and this is a generality in analytical sciences, no analysis brings all information at once. The classic GC analysis does not tell us if the oil contains some traces of pesticides or heavy metals, nor does it tell us if it contains non-volatile products. Other missing data include physical characteristics such as density, refractive index and optical rotation, and even if we can see some signs of oxidation on GC, we will generally not observe the oxidation level of the oil. Here we won’t discuss all these aspects, just the last one, the ‘’oxidation level’’, that can be evaluated by the determination of the peroxide value.

The vast majority of EO easily tends to oxidize in time. Sensitive compounds slowly ‘’absorb’’ oxygen and turn into other compounds such as peroxides, known to be much reactive and sensitizing (easily give their oxidation state and induce hypersensitivity by repeated exposure) [1,2]. Of course, protecting oils from oxygen (air), light and elevated temperatures usually slows down the oxidation process, but in practice, it cannot be completely prevented.

So, knowing that the ‘’oxidation level’’ is an important aspect of oil quality, how do we estimate it? Of course, we can partly rely on the date of production, but an oil stored in optimal conditions can be less oxidized than a much more recent one that was poorly stored. Moreover, depending on their composition, different oils can have strikingly different sensitivities to oxidation. Could we use the GC? As we saw above, we can catch some signs of oxidation on GC (for example, conversion of geranial to geranic acid in lemongrass). However, hydroperoxides, which are common oxidation products, are generally not detected in classical GC analyses due to their thermal instability [3].

There are several good old methods of analytical chemistry that still require a burette and a colorimetric indicator. The systematic and now widespread application of GC analyses has in part overshadowed these classics, but they can still provide valuable information. One of these methods – the determination of peroxide value – has been developed a long time ago for the evaluation of unsaturated oils and fats oxidation and in the vast majority of cases, it can also be used on essential oils (an ISO norm adapted in 2015 even exists for this purpose [4]). But what exactly is a peroxide, how is the peroxide value determined and what information does it give?

Peroxides

The term peroxide defines the chemical function R-O-O-R’ and if R’ is an hydrogen (R-O-O-H), it’s called an hydroperoxide. When a molecule features these functions, we can use the term peroxide to define it, in the same way we talk about ketones or esters in essential oils. In the presence of oxygen, hydroperoxides are spontaneously formed from some unsaturated (featuring a least one double-bond) molecules such as limonene or α-pinene, which are almost ubiquitously encountered in essential oils. For the chemists, peroxides are mostly formed on tertiary carbons (carbons bonded to three other carbons) and in alpha of (the carbon next to) an alkene [5]. Here is what can be spontaneously formed when limonene meets oxygen:

Figure 1. Some of the peroxides arising from the oxidation of limonene.

Determining the Peroxide Value

As we mentioned above, these oxidation products are highly reactive and sensitizing but their concentration in the essential oil can be estimated using the determination of peroxide value.

For this test, we put a known mass of EO in an Erlenmeyer with an acid solution (chloroform and acetic acid) then we add an excess of potassium iodide (KI). The iodide (I^–) reacts with the peroxide function, and cleaves it by giving it two electrons:

ROOH + 2I^– + 2H⁺ → ROH + H₂O + I₂

This reaction can be ‘’seen’’ because we usually add some starch that has the property to bind with I₂ (actually I₂ + I^–) formed during the reaction and giving a dark blue/purple complex (the fun side of chemistry !).

Then I₂ is titrated with a solution of sodium thiosulfate (Na₂S₂O₃):

I₂ + 2 S₂O₃^2- → 2I^– + S₄O₆^2-

When the dark blue/violet color disappears, that means that there is no I₂ anymore in the solution, and if we know how much sodium thiosulfate was added, we know how much I₂ was formed and we know how much peroxide the oil contained. Here is how it looks in practice:

The peroxide value can be expressed using several units, among which mEq O₂/kg. There is no official maximum limit in peroxide value for essential oils but you can remember that vegetable oils (theoretically consumed in much larger quantities) must have a peroxide value under 10-15 mEq O₂/kg [6]. It should also be noted that sensitives essential oil stored in small vials regularly opened and not protected from light and warm temperatures can easily reach 2-300 mEq O₂/kg… In all cases, the lowest is the best.

Conclusion: determination of the peroxide value is not the ultimate tool in essential oils quality control. As any method, it has its own limits including the fact that it does not evolve linearly with essential oil oxidative degradation. Still, in the vast majority of cases, it provides a good information regarding the oxidation level of essential oils. Information we don’t have with only GC profiles.

References

[1] Tisserand, R., Young, R. (2014) Essential oil safety, 2nd edition, Elsevier, p. 73.

[2] Christensson, J. B., Johansson, S., Hagvall, L., Jonsson, C., Börj,e A., Karlberg, A. T. (2008) Limonene hydroperoxide analogues differ in allergenic activity, Contact Dermatitis, 59(6), 344-352, doi: 10.1111/j.1600-0536.2008.01442.x

[3] Turnispeed, S. B., Allentoff, A. J., Thmpson, J. A. (1993) Analysis of trimethylsilylperoxy derivatives of thermally labile hydroperoxides by gas chromatography-mass spectrometry, Analytical biochemistry, 213(2), 218-225, doi: 10.1006/abio.1993.1412

[4] ISO 18321, Essential oil – Determination of peroxide value, 2015

[5] Kaloustian, J., Hadji-Minaglou, F. (2013) La connaissance des huiles essentielles: qualitologie et aromathérapie – Entre science et tradition pour une application médicale raisonnée, Springer, p. 34.

[6] Food and Agriculture Organization of the United Nations. (1999) Codex general standard for fats and oils, [On line], (page consulted on May 18, 2017), URL: http://www.fao.org/docrep/004/y2774e/y2774e03.htm