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I know that deodorants and perfumes have alcohol as their main ingredient so I was thinking what happens when their expiry date passes?

I tried to search on google but got mixed results. Some say that they do not ever expire but some say that an expired deodorant can burn the skin.

What i think is that as they contain mainly alcohol so maybe a major part of the alcohol gets oxidised to ketones or acids which may in turn harm the skin. Does anyone of you know what happens when deodorants expire?

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  • $\begingroup$ I don't think ethanol gets oxidized in time, it won't react with air. Microbes can't use it either on higher concentrations (12%>) they will die. You won't do keton from it either, just aldehyde. The decay of the other components can cause the problems or maybe microbes if there is not enough ethanol in it. $\endgroup$ – inf3rno Jan 13 '16 at 12:16
  • $\begingroup$ why won't it get oxidized? I mean in India the expiry date is 2 years from the day we unpack the bottle(I don't know if its same in other countries too) so if ethanol gets oxidized to aldehyde which are highly prone to oxidation, it will easily get oxidized to ethanoic acid. And one of the perfumes(not body deodorant) that i have has 80% alcohol as per the label on it. $\endgroup$ – Pranav Rastogi Jan 13 '16 at 12:26
  • $\begingroup$ I have a different experience. I drank many years old ethanol several times... Only microbes can ferment it to aldehyde or lactic acid with enzymes by low concentrations. By standard conditions without these enzymes the transformation is very-very slow. You won't recognize it after decades or hundreds of years. $\endgroup$ – inf3rno Jan 14 '16 at 6:32
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The meaning of expiry dates

The date of expiry is widely interpreted by consumers as the date after which a product is known to detiorate. However, the meaning of expiry dates varies widely from product to product, and from country to country. Often times, the date of expiry is not a "first known bad" date, but a "last known good" date. In that latter case, the dates indicates that manufacturers know their product will last up until the expiration date, but after that, they don't know what happen. Their product could last 1000s of years, completely stable and undeteriorated. Or it could degrade instantaneously the day after expiry.

I don't know for sure what perfume manufacturers mean by their expiry dates. You could try asking the manufacturer for clarification. Ask for their data from their product stability studies.

Likely routes of perfume degradation

All of the above notwithstanding, let's suppose that perfumes do degrade over a time span of several years. Subject to that assumption, what could be happening happening? First, I agree with the commenters that ethanol oxidation to acetaldehyde or acetic acid is unlikely to be a major contributor to perfume degradation.

However, the same is not true of other perfume components.

Oxidation

  • Aldehydes such as octanal, benzaldehyde, citral, are an important ingredients in many perfurmes. Citral degradation has been studied and is accelerated by acid and oxygen, with the main ultimate product being p-cymene. Benzaldehyde is oxidized in air to benzoic acid, and octanal is oxidized to octanoic acid.

  • Terpenes are also important pefume components. Limonene is a component of many perfume blends and is a representative terpene. It is also oxidatively unstable, with products such as limonene oxide, carveols, and carvones being formed. Some of these oxidation products can trigger allergic reactions, which while not precisely "burning the skin", could lead to such a perception.

Other possible degradations

Perfume, especially if opened frequently, could also change by absorbing water from the atmosphere, leading to dilution and an alteration of its olfactory properties. Frequent opening or exposure to the atmosphere could also result in differential evaporation of the more volatile components, which would also change its olfactory properties.

The bottom line

Expired perfume or deodorant might be just as good as regular perfume. Or it might be degraded. The degradation products might possibly include compounds that have been reported to form rashes. It is unlikely, however, that the chemical mechanism for this is the formation of large amounts of acid.

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