How does pesto go bad? Will it silently kill me?

February 14, 2008 2:13 PM

How does pesto go bad? Will it silently kill me?

The pesto can says it stays good in the fridge for 3-4 days after opening. The can was opened well over a week ago, and I just ate some. Didn't notice any serious weirdness in taste or smell, so I suppose "no longer being good" means something else. Will I just get some gas? Hallucinations? Or should I put on my coat and start walking to the mortuary while I can still get there on my own?

posted by Anything to food & drink (27 comments total)

It's probably fine after a week. Garlic, basil, olive oil ... these things have the potential to go rancid, but since it was refrigerated and it's *just* a week, I think the worst you risk is a little gas.

P.S. I'm not a doctor so please don't sue me if I'm wrong.
posted by tastybrains at 2:21 PM on February 14 [1 favorite]

Pesto is a really good anaerobic breeding ground for botulism, because it's oily and has veggies. I never worry about that myself, and don't have any problem eating it for a week or so, but I know that I'm taking a risk. You might want to look at those frozen pesto cubes if you like pesto but don't want to keep throwing away most of a can or tub.
posted by ten pounds of inedita at 2:21 PM on February 14

Agreed that a week is not an issue. I take the leftover stuff, plop it in portions on a sheet of waxed paper, freeze that, then wrap the blobs up and keep in the freezer for later use. Should be good for many months that way.
posted by beagle at 2:25 PM on February 14

It usually takes me three weeks or so to finish a jar of pesto. No ill effects yet.
posted by frobozz at 2:26 PM on February 14

I've had pesto literally a few weeks after opening, but 1) they've been in jars not cans, not sure if the metal means anything, 2) and I think the way pesto would go bad would be by going rancid as oil ( and perhaps the oil in the nuts) is usually the only thing that would go "off" anytime soon, and even that should last a good amount of time.
posted by edgeways at 2:27 PM on February 14

(there is only about 26 cases of food born botulism per year in the US)
posted by edgeways at 2:29 PM on February 14

Botulism is the most likely problem from contaminated pesto, but it's unlikely to have cropped up at this point.

And, ditto the "Freeze it" chorus. I actually have quite a bit frozen right now - Mrs Deadmessenger grew an absolute pantload of basil in our garden last summer, and I made a good 3-4 liters of pesto from it, which I froze in an ice cube tray. Once frozen, take the pesto-cubes out, dust them with cornstarch to keep them from sticking together, and put them in a Ziploc bag for use later on. I'll pop a couple out every once in a while to use for, well, whatever.
posted by deadmessenger at 2:31 PM on February 14

Count me in with those who don't think there is much to fear from eating pesto a week after opening it. I've gone as late as a month after opening on jarred pesto and never once had any negative consequences come of it.
posted by mjgrady at 2:34 PM on February 14

I wouldn't keep a jar in the fridge more than a week or so after opening, but besides that it's perfectly edible. We're talking about a jarred, industrial (or semi-industrial) product anyway, so there really is no problem, as long as it's covered in oil (you can top up the jar with some) so that it does not oxidize. Fresh pesto, on the other hand, is really better if consumed in a few (a couple) days after preparing.
posted by _dario at 2:35 PM on February 14

I've had the same jar of pesto in my fridge for the better part of a year, no problems
posted by Oktober at 2:56 PM on February 14

(there is only about 26 cases of food born botulism per year in the US)

And most of those are in Alaska.
posted by clh at 2:57 PM on February 14 [1 favorite]

I've definitely gone 2-3 months without ill-effects. I've even skimmed off some fluffy mould and gone for the good stuff underneath.... I was hungry.
posted by tiny crocodile at 3:04 PM on February 14

I have an Italian friend who tops off the jar with an inch or so of olive oil. She claims that since it essentially keeps air from the pesto, it prevents bacteria.

I don't know if I buy it, but she's never gotten food poisoning from her pesto, that I know of. (And neither have I.)
posted by mudpuppie at 3:10 PM on February 14

Yeah, I keep my pesto for ages... months and months if I make a lot. I have to admit I never thought about botulism, cos my mum has always kept hers for ages and she seems to still be alive and kicking. Maybe I will freeze it in future though!

But to answer your question: commercially prepared pesto is quite unlikely to kill you. It usually has been heat treated to kill germs and it has a lot of preservatives in it.
posted by indienial at 3:15 PM on February 14

Fresh pesto can easily go bad because the water that's in the bits of basil leaves tends to stay there. That creates thousands of tiny oases for bacteria to grow in. Additionally, those fresh basil leaves and garlic are full of millions of bacteria that get treated to a feast when you blend/grind it into paste.

In jarred pesto, the manufacturer will have cooked it at a very high temperature (let's say 250F) to kill the bacteria and make it shelf stable. This will drive the water out of the basil and sort of equalize it throughout the mixture. After the water is equalized, there isn't enough in any one place to support much bacterial growth. With the 3-4 days they're probably talking about flavor. The oils that give basil its flavor are fragile and volatile, what little is left after the cooking will quickly evaporate away. Also, all that oil tends to pick up off flavors from the fridge.

Finally, bacteria that make you sick (pathogenic) have to come from somewhere. Usually, they come from soil, human, or animal waste that contaminates the food at the farm. The manufacturer has already killed them during the cooking process (including any botulism spores) so any spoilage bacteria will come from the air or anything you stick into the pesto. Pathogenic bacteria are really rare in those environments, so unless you're dipping with something that's been contaminated with, say, raw chicken juice, any bacteria that get in there will probably cause off flavors but be harmless to you.

Note: If you bought the pesto from the refrigerator section of your grocery, consider it to be the same as fresh. It's probably flash pasteurized which kills most bacteria, but leaves botulism spores intact. And you REALLY don't want to get botulism.
posted by TungstenChef at 3:18 PM on February 14 [1 favorite]

You're not going to die, but on the other hand YOU HAVE NOT LIVED unless you eat FRESH pesto.

Fresh pesto is defined as leaves that were crushed from a living basil plant, pine nuts that were ground, garlic that was pressed and parmesan that was grated only a couple hours before they all went into your dish. All these ingredients lose their flavours almost immediately due to oxidation.

Pesto is a shadow of its former-self if it wasn't prepared on the day, and the canned or bottled stuff bears no resemblance whatsoever to the real deal. If you buy it, buy it from an authentic italian deli. Don't even bother with the supermarket shit.
posted by randomstriker at 3:26 PM on February 14

If botulism was actually an issue, the pesto would have gone bad in the can before you even opened it. Botulinus is an anaerobe; oxygen kills it.
posted by flabdablet at 3:30 PM on February 14

OP here. It indeed is a jar, not a can. Excuse my non-native English, which occasionally fails me.

I won't mark a best response, but thanks to all who've answered!
posted by Anything at 3:31 PM on February 14

Oh and any pesto that has been pasteurized is going to be a far cry from the real thing. No heat whatsoever except that from the cooked pasta -- the crushed leaves must still be raw and the parmesan must never have been melted before!!!
posted by randomstriker at 3:34 PM on February 14

Yeah, I know it ain't the real thing. Then again, I'm Finnish, and we tend to settle for pretty much any gunk we can splash on our carbs. Supermarket pesto (or "pesto") fills that role nicely. I might try and make some of my own though, come summer.
posted by Anything at 3:49 PM on February 14

I've eaten pesto that was about a year and a half old.

That stuff keeps forever in the fridge, as long as you 'seal' it with a layer of olive oil on top.

For what it's worth, it was homemade pesto, and I was following the practice of the Italian family who made it. It's basically a way of preserving your forest of basil plants before they die off in the winter, and is a great source of no-effort pasta dinners.
posted by UbuRoivas at 4:10 PM on February 14

(disclaimer: those particular italians are a bit crazy)
posted by UbuRoivas at 4:12 PM on February 14

I've eaten pesto that was about a year and a half old.

That stuff keeps forever in the fridge, as long as you 'seal' it with a layer of olive oil on top.

From a food safety standpoint, this is risky. Garlic is a known carrier of botulism spores, and basil (along with everything else that comes from the soil) could carry it too. Botulism will grow at refrigerator temperatures, albeit slowly. A year and a half is definitely enough time.

I'd suggest dividing it up in an ice cube tray and freezing it. I haven't done this with pesto, but I have with Thai green curry which is similar in composition (delicate herbs mixed in oil). It keeps very well without losing flavor as long as you keep it in a good freezer bag.
posted by TungstenChef at 5:04 PM on February 14

well, it *did* give me the runs for a day.

at least, i assume the pesto was to blame.

i should point out that it's not exactly normal for them to store pesto in the fridge for that long - i was houseminding & found the jar right up the back of the fridge - but a month or two isn't abnormal.

having said that, freezing has to be a better way of doing things.
posted by UbuRoivas at 5:27 PM on February 14

The aforementioned topping-off with olive oil seems to keep it good for at least a week. I make my own and regularly do it this way.
posted by lackutrol at 5:53 PM on February 14

I have eaten pesto that had been in the fridge for a few weeks and I never got sick.
posted by HotPatatta at 5:43 PM on February 15

Yeah, storebought pesto is basically sterile and will last forever. They were storing it at room temperature in an anaerobic environment for months before you deigned to open it up and eat it. Thanks to a combination of parcooking, salting, and other preservatives, that stuff will go funky from fridge contaminants well before it goes bad. (This is also why it's not nearly as good, basil flavour molecules, like those of tarragon and cilantro, are very sensitive to temperature, and degrade terribly under significant heat)

Fresh pesto is a whole different ballpark, as covered above. Freeze or use quickly.
posted by mek at 9:32 AM on February 16