Expiry dates and what they are intended to mean...

October 3, 2007 8:08 PM

Are "expiration" and "sell by" dates intended to communicate how long the product will last - even once opened? Or are they only meaningful in terms of shelf life before opened? A jar of peanut butter might have an expiration date 2 years off. But that's only if it remains unopened, right? It's not going to last 2 years once exposed to oxygen, etc. Right? I read these two pages and am aware that these dates are guidance only, not law, and that they are imprecise, and sometimes refer only to flavor / quality not safety. I'm just trying to understand the opened/unopened thing.
posted by scarabic to health & fitness (12 comments total)

Sorry - obviously the "sell by" date only refers to unopened products. I'm lumping it and "expiration" dates together.
posted by scarabic at 8:19 PM on October 3, 2007

I would assume that any expiry date is only valid until the packaging has been breached. Once you have contaminated the contents, all bets are off. For many things, they are not likely to damage you even if well past the use-by date, but they may not look/taste right. Some things (fish, shellfish, meat etc), I would be more concerned about.

But still, eat the sandwich anyway. What's the worst that could happen?
posted by dg at 8:20 PM on October 3, 2007

Unopened. Think of a can of tuna fish.
posted by smackfu at 8:34 PM on October 3, 2007 [1 favorite]

It depends on the product and the kind of packaging. Carton milk will go bad whether you open the package or not. So will bread in a bag.
posted by Steven C. Den Beste at 8:38 PM on October 3, 2007

Once opened, use within X days.
posted by Partial Law at 8:40 PM on October 3, 2007

I used to work for a major food company with "Best When Used By..." dates. Ours referred to unopened products. None of the products would make anyone sick if they bought them after the date, but the taste and consistency of the product would probably change (for example, mayonnaise might separate, etc.). Once you open it, oxygen is introduced into the product, and it will deteriate more rapidly.
posted by la petite marie at 8:46 PM on October 3, 2007

That's "deteriorate"
posted by la petite marie at 8:47 PM on October 3, 2007

an expiry date has no absolute meaning, only a meaning relative to the same product with a different date, fresher/staler.
posted by bruce at 9:12 PM on October 3, 2007

Opening it may introduce introduce microrganisms, fungi, slime molds, etc. along with oxygen.
posted by sebastienbailard at 10:32 PM on October 3, 2007

I once talked to a small brewer about the expiration on his beer, and he said his beer would stay good "just about forever" but he had to put an expiration date on his bottles "for insurance reasons"....
posted by DreamerFi at 3:34 AM on October 4, 2007

i tend to ignore the dates and just go by smell/appearance/texture...they are normally very good indicators...if in doubt don't eat it!
posted by koahiatamadl at 2:35 PM on October 4, 2007

Yeah that doesn't help for things like pills, though. I bought these giant Cosco bottles of ibuprofen a while back and my gf thinks we should throw them out because they're expired. I'm a little more tolerant of such things but eventually I noticed that they lost most of their efficacy.

The item in question that sparked this post is a kind of petromalt for our cat. It's a mixture of fish oil and mineral oil that comes in a tube. You put it in their paws, they lick it off, and it lubricates their innards so they can cough up their hairballs more easily.

It doesn't say refrigerate, so we don't. It's been open a while. 6 months or so? I'm a little concerned about feeding it to the cat. But the gf says the expiry date hasn't passed yet so we're good. I think she relies on them too literally.

Thanks for all the great answers!
posted by scarabic at 9:21 AM on October 5, 2007