Under the right conditions, those probiotics break down the lactose in milk and churn out lactic acid, which lowers the pH (giving yoghurt its tartness) and unfurls and knits the milk’s proteins into a stable gel network (giving it its thick, lush texture).

Both of these qualities – the semi-solidity and the tang – make yoghurt unwelcoming to bacteria and other microbial growth aside from the probiotics themselves. Although there are limits, “the act of producing yoghurt keeps milk fresh longer than it would ever otherwise last without guided intervention,” said David Zilber, chef, food scientist and co-author of The Noma Guide to Fermentation. But how long, exactly?

How long can yoghurt safely sit out?

Although some government agencies caution against eating yoghurt that has been out of the fridge (that is, held at 4C or above) for more than two hours, experts agree that these guidelines are written very conservatively, keeping in mind the possibility of imperfect storage conditions and populations who are more vulnerable to foodborne illness (children younger than 5, adults 65 and older, pregnant and other immunocompromised people).

“Because not all yoghurt is equal or consistent, it is likely easier for the agency to take the worst-case scenario,” said Ben Chapman, the department head of Agricultural and Human Sciences at North Carolina State University and a host of the podcast Food Safety Talk. “I wouldn’t even start to toss them out until four hours.” He added that including a fruity yoghurt in his child’s lunch wouldn’t be a concern for him – as long as it’s being eaten that day.

For comparison (and comfort), it’s only after four hours without refrigeration that the Food and Drug Administration requires restaurants to throw away foods notoriously susceptible to food-poisoning outbreaks, such as raw sprouts or cut rockmelon. “Four hours for me is really the number,” Chapman said.

Which types of yoghurt are the most (and least) perishable?

Non-fat or whole, plain Greek or flavoured, all dairy-based yoghurt sold in the United States has two safety features built into the process.

First, the milk is heated to 185 degrees Fahrenheit (85C) for 30 minutes to unravel the proteins to help them mingle and form a thick, creamy texture (pasteurising fresh milk, in comparison, is most often done at a lower temperature for only 15 seconds), according to Nicole Martin, assistant research professor in dairy foods microbiology at Cornell University. “This heat treatment is the pasteurisation,” Martin said.

In addition, the FDA requires the final product to have a pH of 4.6 or lower. At this acidity, conveniently, milk proteins begin to clump (and yoghurt to thicken), and most foodborne pathogens can’t grow.

That said, some yoghurts will start to go off more quickly than others. “Fruit (and even just sugar added to yoghurt) really offers spoilage bacteria and fungi a truly easy-to-consume food source,” Zilber said. Making the fruit less accessible may help; fruit-on-the-bottom style is more than a clever marketing tactic.

Stirring in your own fresh ingredients can especially throw off the protective low pH, so those batches are even more important to return to the refrigerator promptly. To maximise the safety and longevity of his yoghurt, Sandor Ellix Katz, fermentation revivalist and author of The Art of Fermentation, adds fruit or herbs just before serving.

And any yoghurt product labelled in the United States that “does not contain live and active cultures” has been pasteurised after fermentation, so there will be no probiotic activity to outcompete pathogens and spoilers. That said, the pH is still required to be at or below 4.6, which makes even inactive yoghurt acidic enough to be unwelcoming to most pathogens.

Is it safe to eat yoghurt past its best-by or sell-by date?

“Let’s just say I certainly have, but don’t go crazy,” said Cheryl Sternman Rule, author of the cookbook Yogurt Culture.

All food expiry dates in the United States, other than those on infant formulas, are unregulated and an estimate from the manufacturer of quality, not safety. (The official storage recommendation from the Department of Health and Human Services of one to two weeks in the fridge, regardless of expiry date, is quality-driven, too.)

So when you’re pondering which dregs in your fridge might go bad first, remember the types of yoghurts that are more vulnerable to spoilage – the sweetened, the fruit and granola-speckled, the tube your child sipped and then abandoned two weeks ago.

With hardier plain yoghurt, you have more time: “It can slowly get more sour, which for some people could render the flavour too strong, but the additional acidity will further protect it from pathogens,” Katz said.

Food safety experts also agree that although you can’t see the salmonella in an egg or E. coli on a lettuce leaf, with yoghurt, you can believe your eyes (and nose).

How can you tell if yoghurt has gone bad?

“I always recommend that folks use their senses first when consuming yoghurt (or other dairy products) after the labelled date,” Martin said. “Does it look and smell good? Then go ahead and eat it!”

Although there are dangerous pathogens that you can’t see, smell or taste in addition to the spoilage microbes that make foods unappetising (fuzzy moulds, hair-raising stink), there are only two harmful bacteria that could possibly survive in yoghurt: certain acid-resistant strains of listeria and E. coli. Martin and Chapman aren’t concerned about either in yoghurts made in commercial settings – where milk is pasteurised and fruit inclusions are monitored for listeria. “No food is zero risk,” Chapman said, but “we have thousands of years of fermentation history of yoghurt making a safe product and very, very, very few illnesses”.

And even though moulds can produce toxins that can make you sick, “You have to eat a lot of them, often, to get symptoms from them,” Zilber said.

How long can plant-based yoghurts safely sit out of the fridge?

According to Anita Shepherd, founder of Anita’s Coconut Milk Yogurt, which closed in 2022, the biggest problems facing plant-based yoghurts are spoilage-causing yeasts and moulds. But if a non-dairy yoghurt is packed on the same equipment as dairy yoghurt, and “there’s one gasket with a crack in it that doesn’t get fully cleaned in between,” there could be a cross-contamination risk.

Chapman recommends sticking to the same four-hour limit that the FDA Food Code advises for other perishable foods sitting out of the fridge in restaurants. Since the plant-based yoghurt industry is relatively new, there isn’t a regulated definition, “so the pH could be kind of all over the place,” he said.

Can you freeze yoghurt?

You can, but the texture will suffer. “You thaw it out and eat it, you’ll be fine, but I’m a purist on texture,” said Dashtaki. “I would personally only freeze it if you’re making frozen yoghurt and you churn it.” Otherwise, it will develop long ice shards, and the creamy texture will be lost, becoming weepy and flat as it thaws. “Freezing yoghurt would diminish the viability of its probiotic bacteria,” notes Katz.

If needed (or if you find yourself with an accidentally frozen batch), Dashtaki recommends blending thawed yoghurt into a smoothie or cold green soup – for example with spinach, cucumber, cumin and dill – to mask the texture. For those who make and strain their own yoghurt, she points out that the whey freezes beautifully in ice pops.

What are some good ways to use up yoghurt before it goes bad?

In Dashtaki’s home, yoghurt isn’t just a breakfast food, but a significant portion of savoury meals. It’s “like a third of the plate,” either mixed with finely diced cucumber, dried mint and salt, or as mast-o moosir, with the earthy, garlicky Iranian dried moosir (the bulb of the Leopoldia flower, available at Iranian markets or online) rehydrated and stirred in. “You douse anything that’s uninspiring with mast-o moosir, crinkle potato chips on top of that, and you’ve just saved your totally depressing leftovers, or spruced up any sort of sandwich.”

When Zilber wants to use up a lot of yoghurt, quickly, he makes Indian kadhi, “a straight-up yoghurt curry stew that simmers it down with cumin, turmeric, asafetida, mustard and fenugreek” (or raita or mango lassis).

Sternman Rule says yoghurt makes the best pancakes and excellent marinades for proteins. “And if you’ve never made yoghurt whipped cream, you should do so right now,” she said. “It has more structure than standard whipped cream and won’t collapse into a sad puddle (at least for a few days).”

This article originally appeared in The New York Times.

Written by: Kristen Miglore

Photographs by: Christopher Testani, Andrew Scrivani

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