How can I tell if an egg has gone off? The 17 rules of food expiry dates

One news event you may have missed this month was that Tesco lost a judicial review over selling foods past their use-by date. The company had been taken to court by Birmingham city council, and was launching a challenge using the evidence of the food microbiologist Slim Dinsdale, a swashbuckler of the pathogenic world, who favours the "smell it and see" approach. All of this was over-ruled by judges, who decided that use-by dates were, overall, still important enough for public safety to be observed.

This was, of course, all overshadowed, by the coronavirus outbreak; but the use-by date controversy has never been more up to date. The cook, author and campaigner Jack Monroe runs ad hoc advice sessions called Lockdown Larder on Twitter, and says: "Since I've been doing it, by a country mile the most-asked question I've had is: 'I've got this yeast, it went out of date two months ago, can I use it? I've got these sardines, they went out of date a year ago, can I use them?' People want someone they perceive to be an expert to reassure them. As a species, we seem to have lost the ability to trust our own senses."

This gets to the core of the matter. If you want a blanket rule for everyone, the use-by date is that rule. I can only tell you what I would eat. If it happens to be the same as Theresa May (who famously admitted to scraping the mould off jam), that's just coincidence.

Erik Millstone, emeritus professor of science policy at the University of Sussex, lays out the ground rules: ignore sell-by dates. "Those are a guidance by the retailers for the retailers, so they can manage their stock. The indications concerning food safety, particularly microbiological food safety, that's use-by dates."

But are all use-by dates created equal?

Eggs

Eggs, strictly speaking, don’t have a use-by date but a best-before-end (BBE), which is mostly to do with quality and taste rather than safety. Either way, this is a hill I’m prepared to die on: dates are irrelevant to eggs, because of the float test. Drop the egg in a glass of water. If it floats to the surface, it’s definitely off. If it sits flat on its side, it’s definitely fresh. If it sits vertically, it’s somewhere between fresh and off (I would still eat it). A fully off egg will also smell disgusting, but you can save yourself that trauma by conducting this test.

The sulphurous smell of an off egg triggers a useful disgust mechanism, because the bacterium causing it – Pseudomonas – can be injurious. Salmonella is far more dangerous, being undetectable, but British hens are inoculated.

Bread

If a loaf of bread past its BBE date is mouldy, or smells of mould, don’t eat it. Don’t just chop off the mouldy bit, because there is a twilight when mould spores are present but not yet visible. Mould on bread isn’t the most dangerous of the fungi, but it’s still revolting.

Spices

I remember being astonished to read (in Delia) that you should do a complete spice overhaul every year, because they won’t taste as good. As I have spices that are older than my children; this seemed extremely wasteful. There are spices that most cooks won’t get through in a year (za’atar) unless you live with Yotam Ottolenghi. But Delia’s right, if you want them to last longer, buy them whole and grind yourself to order, or go back in time and store them in brown rather than clear glass.

Nuts

Ah, this is where things get interesting. Millstone says: “Some of the compounds formed from mould can be seriously toxic, especially aflatoxins, which can be carcinogenic.” These are found on mouldy grain and nuts.

You can’t buy dangerous grains and nuts (because of that blasted EU), but you should take very seriously the use-by date on what you have bought. There is no meaningful trade-off between risk now and cancer later. It’s not like pharmaceuticals; it’s just a bar snack.

Harissa, wasabi paste and other things that discolour

Some time before anything goes mouldy, it may go a bit grey (if it started out green) or just a murkier version of itself (if it started out red). This is plain pigment oxidisation and is not dangerous, though it is a bit unappetising – if the fat has oxidised, you will be able to smell it.

Yeast, baking powder and other things that react

Monroe raises the stakes here: “I’ve personally used yeast that is 10 years out of date. But it’s not been opened, and was stored away from direct light in cool conditions. It didn’t rise as well as normal yeast would. If you’re using old yeast, I would always recommend using 1.5 times the amount the recipe states, leave it for an extra hour, and if you’re really running low on flour, make a quarter batch, then if it doesn’t rise, or doesn’t go so well, you can work it into a soda bread and you’re not going to waste half a bag of what is currently quite a precious resource.”

Baking powder and bicarbonate of soda typically have shelf lives of one to two years, but storage is more important than numbers, and there is a school of thought that says bicarb lasts for ever. Yup, for ever.

Fresh meat and fish

These are “particularly vulnerable to bacterial spoilage,” Millstone says, “and if it smells off, it’s definitely off.” However, some forms of spoilage are smellier than others, so just because it doesn’t smell doesn’t mean it’s safe. “If I had a piece of chicken one day past its use-by date,” Millstone continues, “I wouldn’t simply barbecue it and eat it, but I might put it in a casserole and cook it at a high temperature for quite a long time.”

Vulnerability to rottenness is partly about surface area – how much the meat has come into contact with the outside world, and how much of it is subject to direct heat while you’re cooking. This is why mince is such a minefield, because it has a huge surface area, and if you’re cooking a burger medium rare, the middle won’t have been thoroughly cooked. A whole bird has a much smaller surface area. Game birds are a subtle enterprise, the window between how it’s supposed to smell and “off” being so narrow. It’s eater-beware in this case.

Cured meat and fish

Salt, naturally, has a preservative effect, and the sniff and overall appearance test should work with things such as salami. These things have a pretty good shelf life to begin with, and are very low effort (you don’t have to cook them, you just have to pop them in your mouth). So there is something to be said for using your memory rather than your judgment. Eat them before the use-by date, and you won’t have to play ham-roulette.

Pesto, jam and other jars full of mould

The food safety scene is united on this – nothing with mould should be consumed, except where it has been introduced deliberately (blue cheese). It does, however, seem a bit absurd, if you have a jam with a bit of mould on, stuffed with sugar so you know it’s fine underneath, to have to chuck the whole lot out. Millstone is slightly forgiving on this. “If there’s a bit of mould on the top surface of jam that can be safely removed, then the jam lower down might well be safe. But if the entire top surface is covered in mould, you should discard it.” Pesto will taste rank if it’s mouldy as pine nuts go rancid very fast.

Dried fruit

Again, frozen, tinned and dried food gets a BBE date rather than a use-by date. You can sail past the BBE on dried fruit by as much as a year, though the higher the water content, the less time you have – figs and dates won’t last as long as currants and raisins. Do yourself a favour, though, and taste these things if they are very old. If they no longer taste of themselves, that’s just a waste of calories.

Oils

Certain oils (walnut, sesame) seem to go off much faster than others (vegetable, olive oil) but this is usually because we don’t use them fast enough. The smell and taste of a rancid oil is subtle but very insistent, and you can easily ruin a whole dish with a very small amount of it. Don’t give it the benefit of the doubt; smelling it, tasting it, smelling it again, not wanting to chuck it out because it was expensive. It’s much more wasteful to ruin more food than to throw it away.

Dried pulses and beans

We think of these as forever foods, but by the end of the best-before date, they will be tough and won’t taste as good. Vexingly, they will have toughened at different rates within the packet, so it will be impossible to adjust your cooking times for toughness without some of them going mushy. Stick with the guidelines.

Flour

Your real enemy is the weevil. Some people say just sift them out, others (Millstone) remind us that you don’t know what else those bugs have been crawling over.

Cheese, milk and yoghurt

Hard cheeses are less vulnerable to mould and bacteria than soft ones, since their higher salt content has a protective effect. All bets are off for cheeses in which the mould is deliberate. Those are delicious. I would happily saw the mouldy end off a cheddar and eat the rest. Milk has the fabulous trait of smelling awful – and shortly after that, separating – to warn you that it’s off. Yoghurt is the great unknown (I have never met anybody who takes yoghurt use-by dates seriously). Monroe will eat a yoghurt a week after its expiration. I once found one at the bottom of my daughter’s school bag, decided it would probably be fine, and it exploded. So don’t listen to me (on this one occasion).

Oat and almond milks

These new fangled milks will keep in pantry conditions for several months, then for seven days in the fridge. There hasn’t been a huge amount of consumer experimentation; the best anecdotal evidence is that they lose their flavour, rather than becoming disgusting. Oat milk is lower in fat and higher in sugar, so will last longer. Almond milk won’t have a very strong smell, even if it’s off, so taste it tentatively.

Crisps

Best-before dates on crisps are so long, and crisps themselves so delicious, that it’s actually quite hard to find any that are out of date. Monroe got some two-year-out-of-date Doritos from a vending machine the other day (“Were they the best Doritos I’ve ever had? Probably not, but they were fine”); I had some Quavers this morning that should have been eaten by February. They are loaded with salt and preservatives. Probably the worst they can bring is disappointment, of not being crunchy enough.

Tins

The best-before dates on cans range between three and five years from purchase, but this is essentially a way of saying “for ever, all things being equal” – if you’re dealing in units of half a decade, you’re not really talking about the food perishing, but rather the tin. Acidic foods such as tinned tomatoes can attack the can from the inside. Otherwise, rust, dents, any attack on its structural integrity can interfere with its powers of preservation. If you have a tin with a dent in it, even if you can clearly remember the time you dropped it, it’s still better not to eat it. I once had some Swedish anchovies that swelled up like a bloater fish. I don’t know how that happened, but I didn’t try to eat them. – Guardian

Jack Monroe's Good Food for Bad Days is out at the end of May

How can I tell if an egg has gone off? The 17 rules of food expiry dates

Use-by dates: a guide to where you do – and definitely don’t – have some leeway