Would you eat a six-month-old yoghurt? This is a question you may have asked if you read the recent story about a US grocer and his year-long experiment eating expired food.

It started in October 2016, when Scott Nash, founder of the Mom’s Organic Market chain of grocery stores, wanted to make a smoothie. He likes his with yoghurt. As he was at his holiday cabin in Virginia, though, the only pot he had to hand was one he had inadvertently left behind on his last trip there, six months earlier. He opened it. No mould, no smell. He decided to take the plunge and dumped the yoghurt in the blender. “I drank and waited,” he wrote on his blog. And nothing happened.

Nash had always been averse to wasting food, but now he started documenting his experiences. He whipped up cream to use, uncooked, almost four months past the date on the carton, and stirred artichoke lemon pesto through pasta seven and a half months in. There was also minced beef (15 days old), smoked trout (24 days past sell-by), smoked turkey (six weeks past use-by), chicken broth (more than three months past best-before), roasted tomatoes (seven months past sell-by) and tortillas (practically a year old). Still nothing happened.

It raises the question: were the dates just wrong? Have more compliant people the world over been binning perfectly good food this whole time? Should we be eating expired goods?

The first thing to point out is that Nash is based in the US, where regulations on food dating differ significantly from those in the UK. While British foods carry just one date – either “use by” or “best before” – Nash was confronted by “expiration, use by, best by, sell by, best if used by …” He sells food for a living, and even he doesn’t understand the system. And while fresh chicken and fish went bad exactly when the dates suggested they would, dates on everything else seemed arbitrary. “I don’t think any of them are rooted in reality,” he says.

Nash points out that even things that aren’t food – baby wipes, toothpaste, soap, lotion – are dated, as are jarred and canned goods. A specialist from the US Department of Agriculture’s food safety and inspection service (FSIS) told the food website the Takeout in February that as long as a can is kept in good condition (ie, it is not swollen, rusting, leaking or heavily dented), its contents are safe to eat, for ever. “They will never make you sick,” she said. The FSIS’s own website, however, appears to contradict that advice, stating in its shelf-stable food-safety guidance that there are limits to how long canning will preserve food.

Clearly, this lack of clarity has implications for both the health of the environment and the health of the nation. What you don’t eat, you’ll end up binning, even if you could have safely eaten it; and what you don’t know not to eat could make you sick. A joint report from the Natural Resources Defense Council and Harvard Law School in 2013 said that 40% of American food goes uneaten each year, and the disorienting effect of the US date labelling system is in large part to blame. At the same time, said the report, that system fails to convey important food safety information, “despite the appearance of doing so”.

Facebook Twitter Pinterest Scott Nash ... not afraid of old yoghurt. Photograph: Tamzin B Smith

British rules are clearer. The use-by date concerns safety (ignore it and you could get food poisoning), while the best-before date is about quality (you’re probably fine to eat it afterwards; it may just no longer taste or look as good). Of course, “afterwards” here doesn’t necessarily mean indefinitely: best-before dates are applied to both long-life products (biscuits, say, or Marmite) and very fresh ones, such as bread and eggs, which can go off. Only, it’s really obvious when they do (they dry out, they smell bad, they go green), so it’s easy for you to avoid eating something that might make you ill. (For uncracked eggs, use the bowl of water test: if it sinks, it is good; if it floats, it is bad.) In other words, a best-before date means that you, like Nash, have what it takes (your senses, and common sense) to make the call. The main caveat is that the accuracy of that best-before date depends on your abiding by any storage and “once opened” guidance on the packaging.

Yet date labelling has been accused of generating both confusion and food waste in the UK, too, or of simply being ignored. As recent research by the makers of the food-waste app Too Good To Go shows, British home cooks threw away a whopping 720m eggs in 2018, with one in three saying they will bin any carton that is out of date. Yet eggs in the UK carry a best-before date, not a use-by.

All those discarded eggs show that most people still don’t understand the difference. If 74% of respondents to a 2016 Women’s Institute (WI) survey knew that “use by” was about safety, only 45% knew that “best before” wasn’t. The waste reduction charity Wrap has found that as much as 30% of the food binned for being “past date” had a best-before; ie, it probably didn’t need to be binned. And we throw away an awful lot of food in the UK: upwards of 7m tonnes a year. Clearly, understanding dates is crucial.

Andrew Parry of Wrap says that a lot of thought goes into how a business decides on a date: what something is made of; where and how it is made; how hygienic the space in which it is made is; how consumers will treat it; how cold (or not) their fridges will be. Wrap’s research has found that only one in three of our fridges is cold enough (at 5C or lower); a degree can shave a day off the life of something. And then there is the question of liability. The microbiological risk assessment that products have to go through is hefty; businesses have to provide “robust evidence”, says the Food Standards Agency (FSA). So, sometimes a very conservative use-by date is, as Parry puts it, just a business being overly cautious. Nash thinks businesses might be being more than cautious: “At best [those dates] are a neurotic, cover-your-ass thing; at worst, it could be planned obsolescence.” The food industry is in the business of selling you food, after all: the more you throw away, the more you’ll need to buy. Parry agrees that manufacturers’ main job is to sell food, but says that for the most part they actually want the longest shelf-life possible.

He, along with WI vice-chair Ann Jones, the British Nutrition Foundation and the FSA are categorical: as a consumer, you don’t ignore a use-by date. The pathogens that cause food poisoning, from listeria (which the NHS states is found most commonly in things such as butter, cooked meats, smoked salmon and certain soft cheeses) and salmonella (meat and poultry, eggs) to campylobacter (raw milk, raw chicken) and E coli (meat, raw dairy, raw leafy vegetables) are undetectable without a microscope. Even when these bacteria have grown to dangerous levels, food could still look and smell just fine.

Wrap surveys businesses to check whether they’re “absolutely sure” (as Parry puts it) that their products need to carry a use-by date. It has had notable success with hard cheeses and fruit juices – more than 95% of each now have best-before dates after the tech guys in each sector did new tests and realised they didn’t need use-by dates. Which means, as a harried home cook, you are no longer on the clock to use them up quickly or face sending them to landfill. You can just use your nose.