What Is Oxidation, and Which Fragrance Oils Are at Risk?
The bottle of fragrance oil sits on the shelf, nobody touches it, yet the molecules inside slowly react with the oxygen in the air. This is called oxidation, and its consequences can be read from the odour: citrus turns to turpentine, aldehydes go sour, pine notes become pencil-sharpener-like. Oxidation is not spoilage, it is chemistry; but if left uncontrolled, it spoils the product.
Not every molecule carries equal risk. The most sensitive groups are terpenes (limonene, pinene, terpinene: the main constituents of citrus, conifer and herbaceous oils), aldehydes (citral, aliphatic aldehydes) and molecules containing unsaturated bonds (linalool, geraniol, some lactones). Musks, woody molecules and most esters are relatively robust.
Oxidation has two consequences: the odour deteriorates, and some oxidation products sensitise the skin. Limonene and linalool are weak allergens on their own; their oxidised forms (hydroperoxides) are much stronger sensitisers. So an oxidised fragrance oil does not just smell bad, it also changes its safety profile.
Signs: Nose, Eye, Measurement
Oxidation is detected through three channels; the first is available in every workshop, the third requires a laboratory.
| Channel | Sign | Most noticeable in | Reliability |
|---|---|---|---|
| Odour | Turpentine, pencil-sharpener, sour, metallic, "stale oil" notes; dulling of the top note | Citrus, pine, lavender, aldehydic, green | Early and sensitive; reliable when compared with a retain sample |
| Appearance | Colour darkening, yellowing, increased viscosity, resinous sediment at the bottom | Citrus (darkens), some natural oils (thicken) | Late sign; colour change also occurs for reasons other than oxidation |
| Peroxide value (PV) | Iodometric measurement of hydroperoxide content (meq O₂/kg) | Terpene-rich oils | Objective and numerical; requires a laboratory or test kit |
| Refractive index / density | Deviation from the reference lot | All fragrance oils | Indirect; shows advanced oxidation, misses early |
| GC-MS | Appearance of oxidation products (limonene oxide, carvone etc.) | All fragrance oils | Definitive but expensive; for suspect lots and supplier disputes |
In practice the most powerful tool is smelling side by side with the retain sample. On its own, an oxidised fragrance oil smells "a bit different"; next to the retain, the difference becomes obvious. Same dilution, same blotter, same timings.
Lot Comparison: Step by Step
Suspicion of oxidation arises in two situations: the stock on the shelf smells as if it has aged, or the newly arrived lot differs from the previous one. The method is the same in both cases.
- Prepare three samples
The suspect sample, the approved retain and, if available, a fresh lot. Dilute all of them at the same time, in the same solvent (10% in ethanol).
- Compare appearance
Three tubes side by side, white background, daylight. Photograph the colour difference. If the suspect sample is noticeably darker, note it.
- Measure density and refractive index
If you can. Deviation from the retain can have causes other than oxidation (dilution, wrong product), but the direction of the deviation is a clue: in an oxidised terpenic oil, density and refractive index usually rise.
- Smell: 10 min, 1 hour, 4 hours
Oxidation shows most in the top note: the bright opening of fresh bergamot is dull and turpentine-like in an oxidised one. If the difference closes by hour 4, the problem is superficial; if it does not, it is deep.
- Run a triangle test
Two retains, one suspect (or the reverse), three people. If four out of five trials pick out the odd one, the difference is real. This is the only sensory evidence you can show a supplier.
- Peroxide value or GC-MS if needed
If there is a dispute or a large stock at stake, send it to a laboratory. For peroxide value, ask for a comparative result against the reference lot.
- Record and decide
The result goes into the lot record. The decision: use, use with restrictions (in products where the odour difference is insignificant), do not use.
Prevention: Six Habits That Slow Oxidation
Oxidation is accelerated by three things: oxygen, heat and light. Prevention means reducing all three.
Leave no headspace. Half a bottle means half air. Transfer dwindling fragrance oil to a smaller bottle; do not leave it half-full in a large one. For sensitive oils, flushing the headspace with nitrogen or argon is standard in industry; in the workshop, transferring to a smaller bottle does the same job.
Keep it cool. Every 10°C roughly doubles the rate of oxidation. Citrus and conifer oils last longer in a cool cabinet (but not frozen). Windowsills, the top of radiators and car boots are the worst places for fragrance oil.
Store in the dark. Light, especially UV, triggers radical formation. Amber glass or aluminium bottles; clear glass only for short-term working stock.
Antioxidant. BHT or tocopherol at a low level in terpenic oils (0.05–0.1 per cent for BHT) slows oxidation markedly. Find out from the supplier whether the fragrance oil already contains an antioxidant; adding it again is unnecessary.
First in, first out. A new lot is not placed in front of the old one. Shelf order follows lot date; the oldest lot is used up first.
Close the cap immediately. A bottle left open during weighing takes in air every minute. Plan the weighing, open the bottle, weigh, close.
Frequently Asked Questions
The most common questions about the oxidation of fragrance oils.
My fragrance oil smells like turpentine; can I use it?
Will an oxidised fragrance oil recover if I dilute it?
Can I measure peroxide value in the workshop?
Which fragrance oils oxidise fastest?
Does adding an antioxidant affect IFRA or the label?
Is it right to store in the fridge?
The new lot smells different from the retain but not oxidised; what could it be?
Is a fragrance oil's shelf life determined by oxidation?
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