Essential oils contain no water, so bacteria can hardly live in them. So why do they still go off?
Because of a completely different mechanism: oxidation.
What happens in the bottle
Every time you open the cap, air gets in. Oxygen reacts with the molecules in the oil, especially those with double bonds.
Limonene is the typical example. It oxidises into peroxides and heavier alcohols, and the oxidation products smell of damp paper instead of fresh peel.
This is exactly what happens to a piece of pomelo peel left overnight — only many times more slowly, because a closed bottle holds less oxygen.
Four groups, four speeds
Citrus — fastest. Rich in limonene, and cold-pressed so it still contains non-volatile compounds. One to two years is reasonable.
Ester-rich oils such as lavender — medium. Esters gradually hydrolyse into alcohols and acids, drying the smell and losing its soft sweetness. Two to three years.
Phenol-rich oils such as clove and cassia leaf — slow. Eugenol is fairly stable. Three to four years.
Woods, roots and resins — slowest, sometimes improving. Sandalwood and vetiver can become rounder after a few years, like wine.
Three signs it has gone off
The smell has a note of damp paper or of paint. This is the clearest sign and usually appears first in the citrus group.
The liquid thickens or turns darker than it was.
On an analysis, the share of caryophyllene oxide rises unusually compared with a fresh lot — it is the oxidation product of beta-caryophyllene, so the ratio between the two works as a kind of clock.
Four ways to store it
Dark glass bottles, not plastic, because some components attack plastic.
Close it tightly straight after use. How long the cap stays open matters more than how many times it is opened.
A cool, dark place. The fridge works for the citrus group, but let the bottle come back to room temperature before opening it, to keep condensation from getting inside.
Move it to a smaller bottle once more than half has been used. A ten-millilitre bottle with two millilitres left holds a lot of air above it, and that air is what decides how fast it spoils.
Write the opening date
The simplest and most effective habit: write the date you opened it on the label.
The expiry date printed on the bottle counts from the date of manufacture, unopened. Once it is opened, the clock runs at a different pace, and only you know when that pace began.