Question (by Helen Bairstow): I’d like to know if tea tree starts to oxidize is it still safe? And how can you tell?
Answer: The more that Tea Tree oil (TTO) oxidizes, the less safe it is in terms of adverse skin reactions. Since oxidation is a very gradual process, it’s difficult to set a simple and absolute guideline. Having said that, TTO should always be stored refrigerated, and used within 1-2 years of purchase. Also note that when there is more air than essential oil in a bottle, oxidation speeds up somewhat. The way an essential oil is stored makes a huge difference to oxidation.
How you can tell – the smell of TTO does change. It changes from fresh/green/clean/penetrating to a duller, more “funky”, hard, nasty medicinal odor. Part of what you will smell in an oxidized oil is ascaridole, a powerful odorant that only forms on oxidation of the oil. A lab report will also show oxidation, as the para-cymene % increases on oxidation. If it’s around 6% or more, you may have an oxidized oil (2-4% is typical in a fresh oil).
In a very oxidized TTO, the oil in and around the cap becomes sticky. This is the beginning of polymerization – some of the monoterpenes form chains, becoming more like a resin. After a few decades, the oil becomes virtually solid, and smells really foul – something like gasoline or varnish, but worse.