Part IV. The chemistry of deterioration

An essential oil does not stand still once it is bottled. Oxygen, acid, light and heat transform constituents that carry double bonds and aldehyde groups. The consequences are threefold: the odour drifts, the original certificate of analysis no longer holds, and, most importantly, an old oil is a stronger skin allergen than a fresh one. This part presents the mechanisms only at a conceptual level, together with their practical consequences.

4.1 Autoxidation: from non-allergen to allergen

Atmospheric oxygen attacks the position adjacent to a double bond, forming hydroperoxides (–OOH group) as primary products; the hydroperoxides go on to decompose into alcohols, epoxides, ketones and aldehydes. For linalool, a theoretical computational study (DFT) describes the mechanism of hydroperoxide formation (Bäcktorp et al. 2006, doi:10.1021/jp0603278; abstract only read); the main hydroperoxide is 7-hydroperoxy-3,7-dimethylocta-1,5-dien-3-ol.

The two figures of 15% and 0.6% hydroperoxides do not contradict each other. They reflect two different conditions: forced oxidation in the laboratory and commercial products stored for at least 2 years Inference. Clinically, patch testing with oxidised linalool in 3,418 dermatitis patients gave positive rates that rose with concentration; the authors proposed a standard test concentration of 6% Measured (Christensson et al. 2010, doi:10.1111/j.1600-0536.2009.01657.x; abstract).

In tea tree, the cyclic terpenes with two double bonds (α- and γ-terpinene) are dehydrogenated to p-cymene, a stable benzene ring. A comparison of one fresh sample with one sample stored for about 25 years showed p-cymene roughly doubled, while α-terpinene, γ-terpinene and terpinolene decreased Measured (Shimadzu GC×GC application note, n = 1 versus 1, not peer-reviewed). Consequence: a genuine tea tree oil that initially met the standard may, after many years, exceed the 8% p-cymene ceiling and fall below the 14% γ-terpinene floor of ISO 4730 Inference.

4.2 Citral in acidic media: loss of lemon odour, formation of off-odours

Schieberle et al. 1988 (doi:10.1007/BF01454320; abstract only read) held citral in citric acid solution at pH 1.8 and 40 °C for 4 days and then analysed it by gas chromatography–olfactometry Measured. Citral cyclises to a p-menthane skeleton and is then aromatised into compounds with very low thresholds:

Because the degradation products are far more potent than the amount of citral lost, conversion of only a small fraction of citral is enough to create a medicinal off-odour and to strip away the lemon character. At alkaline pH, citral is cleaved by a different mechanism into methyl heptenone and acetaldehyde (Guo et al. 2022). Application Inference: lemongrass, litsea and lemon oils are unsuitable for low-pH products unless stabilisation measures are taken.

4.3 Isomerisation and polymerisation

• Anethole E → Z. Because the cis form is much more toxic, the JECFA specification limits cis-anethole to ≤ 1% Standard. The rate of conversion under storage conditions has no data in the sources read (NR).
• Acid-catalysed cyclisation of linalool, geraniol and nerol to α-terpineol: the reaction is an industry description Industry; according to the threshold comparison of the two pure compounds in Table 3, α-terpineol is about 50 times weaker than linalool.
• Polymerisation. Temperature, light and oxygen availability are the three main factors that degrade essential oils through oxidation and polymerisation (Turek & Stintzing 2013, doi:10.1111/1541-4337.12006; abstract). Oils rich in polyunsaturated terpenes (citrus, tea tree) thicken and darken with long storage Industry; no quantitative data are available yet.

4.4 Ageing markers measurable by GC

A monograph compiled from 718 sourced facts collected under a common protocol and cross-checked independently. Items labelled Inference are the compilers’ own judgements. Diagrams are newly drawn from published data. Scientific reference material — not legal or medical advice, and no substitute for the text of the standards.