In short
Fragrance materials degrade in a few recurring ways. Unsaturated terpenes react slowly with air to form hydroperoxides. Aldehydes oxidize to acids. Esters hydrolyze when water and acid or base are present. Some molecules rearrange under acid. Knowing these pathways lets you predict which lines are at risk, and design storage and tests around them.
What you will learn
- Describe the main degradation pathways
- Predict which lines in a formula are at risk
- Design a simple aging comparison
Before this lesson: Flash Point in Context: Study Guide
The main pathways
| Route | Typical targets | Result | Helped by |
|---|---|---|---|
| Autoxidation | Limonene, linalool, other terpenes | Hydroperoxides, then many products; flat, sharp or turpentine notes | Air, light, heat, metal traces |
| Aldehyde oxidation | Citral, decanal | Carboxylic acids: fatty, sour notes | Air, heat |
| Hydrolysis | Esters, including linalyl acetate | Acid and alcohol; perhaps a vinegary note | Water with acid or base |
| Acid rearrangement | Citral, linalool | Other terpenes, often off-notes | Acidic conditions |
| Polymerization or resinification | Some oxidized oils | Thickening, darkening | Age, air |
| Condensation | Aldehydes with amines or with alcohols | Schiff bases, acetals | Time, concentration |
Autoxidation in outline
Autoxidation is a chain reaction with oxygen. A radical removes a hydrogen, often from a position next to a double bond. The resulting radical picks up oxygen, then takes a hydrogen from another molecule to form a hydroperoxide, and passes on the chain. That is why:
- Traces of starter can accelerate it, including light and metals.
- Reducing oxygen helps: full containers, or an inert gas blanket in commercial settings.
- Antioxidants that stop the chain can help, although they have their own rules. See the guide on antioxidants and UV absorbers.
The safety relevance came up in Level 3: oxidized terpene products are recognized as contact allergens in the literature. IFRA's guidance also treats oxidation and peroxide values as something you should get from suppliers or measure. Peroxide value is a lab measurement that needs reagents and training, so most independent learners depend on supplier data and on good handling, and retire old stock.
Reading a formula for risk
Run through your formula with these questions:
- Which lines contain terpenes with double bonds? How old are they?
- Which lines are aldehydes, and might they oxidize, or meet an amine?
- Is any water present, and does the product have a high or low pH? Esters may suffer.
- Will the product see light or air? What about the packaging, such as clear glass or a part-empty bottle?
Level 8 converts these questions into a stability test. The failed-test lesson shows how to work backward from a change.
What you can test without instruments
Your nose and eyes can detect some of this, but only some. Sharpness, flatness, color changes and thickening are signs that something has happened. Their absence is not evidence that it has not. Hydroperoxides can be present without an obvious change in smell, so treat smell as a quality check rather than a safety check.
Practical exercise: Age one oil on purpose
This is a slow experiment, using a cheap oil such as sweet orange oil. Treat it as a learning demonstration only.
- Split a fresh oil into two small labeled glass vials. Fill A nearly to the top, sealed, in a cool dark place. Fill B one-third full, loosely capped, in indirect light, away from heat, children and pets.
- Keep a small sealed reference in the refrigerator or coolest place, if safe to do so. Label everything.
- At 2, 4 and 8 weeks, compare the three on strips at the same dilution, blind if you can. Note color, thickness and smell.
- Write what changed and when. Dispose of the aged oil according to the SDS. Do not use it in anything.
You have done this well if your conditions are stated and you describe what changed without concluding anything about safety from smell alone.
Common mistakes
- Trusting your nose for safety. Use dates and supplier data as well.
- Keeping old citrus because it still smells fine. Fine is not fresh.
- Adding antioxidant as an afterthought. It has limits and its own regulatory status.
Safety
Treat aged terpene samples as more hazardous than fresh ones. Do not put them on skin. Store them clearly labeled, away from heat, and dispose of them properly.
Key terms
- Hydroperoxide
- A reactive oxidation product containing an O-O-H group, formed for instance when terpenes react with air.
- Peroxide value
- A laboratory measure of peroxides in a material, used as an indicator of oxidation.
Knowledge check
Written and reviewed by the RUŌOD Lab team. This is general education, not legal, regulatory, medical or safety advice. Example dilutions and exercises are for learning, are not validated products, and have not been safety-assessed. Completing a lesson confers no qualification. How we check these lessons.