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Perfume does have a shelf life, but not a fixed date. Oxygen, light and heat oxidise limonene and other light molecules, so the citrus top changes first; woody and resinous notes last far longer. A sealed bottle kept dark and cool holds its scent much longer than one left on a windowsill.

A bottle on a windowsill

Two bottles of the same scent, bought on the same day. One lies in its box, in a drawer. The other stands on a windowsill, clear glass, sun every afternoon. Five years later, opened, the two no longer smell alike.

The question of whether perfume expires has no short answer: it does, but the limit depends more on how the bottle is kept than on any figure printed on the box. Perfume changes because aroma molecules react with oxygen, and the reaction speeds up under light and heat. The first part to change is the top, where citrus molecules such as limonene sit. Woody, resinous and musky notes change far more slowly.

So an old bottle does not spoil evenly from start to finish. Most often the opening is less fresh, a little harsh or flat, while the base after an hour is almost as it was. A well kept bottle can pass many years with small changes.

What follows draws on the storage page of the Lê Mai library, written for essential oils. Perfume is concentrate dissolved in alcohol, and the molecules inside undergo the same reactions. Alcohol dilutes them, but does not shield them from oxygen and light.

Oxygen and limonene

Limonene is a monoterpene that smells of fresh orange peel. In sweet orange oil it makes up around 84 to 96 percent; in bergamot, around 27 to 52 percent. Nearly every perfume with a citrus opening contains it.

The limonene molecule has double bonds between carbon atoms, and double bonds are where oxygen from the air attaches. Oxidation produces hydroperoxides, then products such as limonene 1,2-oxide, carvone and carveol. The process is irreversible: once oxidised, there is no way to return the molecule to its former state.

The first consequence is smell. The fresh orange note thins out, replaced by something harsh, like turpentine or old paper. The second lies in the safety data: according to studies summarised in the library, oxidised limonene causes markedly more allergic contact reactions than fresh limonene, and combined data suggest an increase of about tenfold.

Limonene is not the only one. Linalool, abundant in lavender and bergamot, also oxidises in air and forms hydroperoxides. Linalyl acetate, the main ester of bergamot, is hydrolysed by water into linalool and acetic acid. These molecules gather in the top and the heart, so the opening of a perfume is where time leaves its clearest mark.

Light and heat

Light, especially UV, generates free radicals and speeds oxidation. It also creates molecules absent from the fresh material. According to data recorded in the library, sweet orange oil exposed to UV for 50 minutes developed piperitone, photocitral A and at least a dozen other new constituents.

Clear glass lets UV through. Many perfume bottles are clear to show the liquid inside, and for that reason the paper box that comes with them has a real function. A bottle kept in its box, in a cupboard, avoids most of the light the glass does not block.

Heat speeds up most chemical reactions. The library cites a comparison on lemon oil: kept at 5 °C with the cap opened three minutes a month, its monoterpene content barely changed over 12 months; kept at 25 °C with the cap opened three minutes a day, that content fell from 97 percent to under 31 percent in the same period. The two conditions differ in both temperature and opening frequency, and both matter.

A hot, humid climate makes this plainer. A bottle left in a car parked outdoors, near a stove, or in a bathroom takes heat and steam every day. A drawer in an air conditioned room is far more stable.

The air inside the bottle

A full bottle holds very little air above the liquid. With each use the level drops and air takes its place. When a third of the bottle is left, the oxygen in contact with what remains is far greater than when it was first opened.

So the last part of a bottle often changes faster than the first, even though it is the same liquid. For essential oils, the library's storage page suggests that once the level falls below 30 percent, decanting into a smaller bottle is worth considering to reduce the headspace.

A spray bottle has one advantage: a sealed atomiser does not open like a dropper cap, so less fresh air enters with each use. A splash bottle, opened at every use, meets more air. Small samples decanted from a large bottle are the same: each decanting is another opening.

Air entering the bottle carries moisture. Water hydrolyses esters, such as linalyl acetate into linalool and acetic acid, clouding the solution and promoting oxidation. In a humid climate each opening lets in a little more water vapour, and the larger the air space above the liquid, the more moisture it holds.

Shelf life in practice

Cosmetics sold in Europe often carry a small symbol: an open jar with a number such as 24M or 36M. That is the period after opening. Under European rules, cosmetics with a shelf life of 30 months or more need not carry an expiry date, but instead state how long the product can be used after opening. Many perfume bottles carry this symbol rather than a specific date.

The number on the symbol is the maker's commitment, not the day the scent disappears. A well kept bottle may be nearly intact after that period; a bottle left in the sun may change clearly before it. Real shelf life is the sum of formula and storage.

Formula decides a great deal. The library's data on opened essential oils, kept in dark, sealed, cool bottles, show wide gaps between groups: citrus around 6 to 12 months; light florals 12 to 24 months; spices and herbs 24 to 36 months; resins and woods 48 to 96 months; patchouli and sandalwood 60 to 120 months, sometimes deepening with age. A light citrus perfume is far more sensitive than a heavy woody resinous one.

The signs that a bottle has changed usually come in order. Smell is the earliest: a flat, harsh opening that has lost its layers. A deeper colour comes next, although colour change alone does not always mean spoilage. A bottle whose smell and colour have both changed is no longer the scent the perfumer built, and the safety data on limonene give reason not to use it on skin.

Keeping an opened bottle

The three things that change perfume are oxygen, light and heat. Storage means reducing all three at once, without any need for perfection. A bottle that will be finished in six months needs less care than one kept for years.

Avoid light: keep the bottle in its box or in a closed cupboard, not on a windowsill, not near a burning candle. Avoid heat: a cool, stable place, away from cars, stoves and bathrooms. Avoid air: close the cap right after use, and with a splash bottle, open it as rarely as possible.

A refrigerator is a good place for essential oils where space allows, according to the library's storage page. For perfume, a stable temperature matters more than a low one; a bottle moved between cold and warm many times may cloud as waxes and poorly soluble matter precipitate. A dark drawer in a cool room is usually enough.

The piece A batch of oil just arrived records how a lot of essential oil is checked and stored on arrival. Signs beyond the scent discusses what a bottle says beyond its smell.

A bottle of perfume does not stand still in time.

The top leaves first, the woods last.

Darkness and cool keep what remains.