How Long Can Yogurt Sit Out? And More Food Safety Questions, Answered

Yogurt was born as a means of preserving fresh milk, thousands of years before refrigerators were invented. But forget stashing a tub in your backpack or on the counter. The United States Department of Health and Human Services suggests throwing it out after two hours, the same deadline it gives fresh milk and other perishables. Why? And what are the true risks?

Not all yogurt is created traditionally or equally. Read on for what to keep in mind when determining the likely resilience of your own supply, and how to know when you’ll want to move it along to the compost.

Understanding yogurt

Yogurt’s semi-solidity and tang make it unwelcoming to bacteria.Andrew Scrivani for The New York Times

“Making yogurt is a magical, wonderful, historical, nutritious process,” said Homa Dashtaki, the founder of the White Moustache yogurt brand and author of the cookbook “Yogurt & Whey.” “You add a bunch of probiotics — bacteria — to a bowl of milk, and then you turn it into something so nutritious for your gut.”

Under the right conditions, those probiotics break down the lactose in milk and churn out lactic acid, which lowers the pH (giving yogurt 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 yogurt unwelcoming to bacteria and other microbial growth aside from the probiotics themselves. Although there are limits, “the act of producing yogurt keeps milk fresh longer than it would ever otherwise last without guided intervention,” said David Zilber, chef, food scientist and a co-author of “The Noma Guide to Fermentation.” But how long, exactly?

How long can yogurt safely sit out?

Food experts suggest that yogurt can sit out at room temperature for four hours.Christopher Testani for The New York Times. Food Stylist: Simon Andrews.

While some government agencies caution against eating yogurt that’s been out of the fridge (that is, held at 40 degrees Fahrenheit 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 food-borne illness (children younger than 5, adults 65 and older, pregnant and other immunocompromised people).

“Because not all yogurt 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 yogurt 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, like raw sprouts or cut cantaloupe. “Four hours for me is really the number,” Dr. Chapman said.

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

Add-ins can increase the chances of spoilage for yogurt.Karsten Moran for The New York Times

Nonfat or whole, plain Greek or cotton candy-flavored, all dairy-based yogurt sold in the United States has two safety features built into the process.

First, the milk is heated to 185 degrees Fahrenheit for 30 minutes to unravel the proteins to help them mingle and form a thick, creamy texture (pasteurizing 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 pasteurization,” Dr. Martin said.

In addition, the F.D.A. requires the final product to have a pH of 4.6 or lower. At this acidity, conveniently, milk proteins begin to clump (and yogurt thicken), and most food-borne pathogens can’t grow.

That said, some yogurts will start to go off more quickly than others. “Fruit (and even just sugar added to yogurt) really offers spoilage bacteria and fungi a truly easy-to-consume food source,” Mr. 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 maximize the safety and longevity of his yogurt, Sandor Ellix Katz, fermentation revivalist and the author of “The Art of Fermentation,” adds fruit or herbs just before serving.

And any yogurt product labeled in the United States that “does not contain live and active cultures” has been pasteurized 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 yogurt acidic enough to be unwelcoming to most pathogens.

Is it safe to eat yogurt 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 expiration 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 Health and Human Services department of one to two weeks in the fridge, regardless of expiration date, is quality driven, too.)

So when you’re pondering which dregs in your fridge might go bad first, remember the types of yogurts 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 yogurt, you have more time: “It can slowly get more sour, which for some people could render the flavor too strong, but the additional acidity will further protect it from pathogens,” Mr. Katz said.

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

How can you tell if yogurt has gone bad?

Only two harmful bacteria can survive in yogurt.Andrew Scrivani for The New York Times

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

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

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

Is yogurt safe from bird flu?

“There is now a plethora of evidence that the bird flu virus currently circulating in dairy cows in the U.S. is not heat-resistant, which is great news,” Dr. Martin said. “That means that heat-treated dairy products are safe to consume,” including yogurt, which is subjected to higher temperatures, for longer periods of time than many other dairy products.

Trying to ferment your own raw milk yogurt at home, on the other hand, would be inherently risky, Dr. Chapman said, not only for bird flu, but “because of the acid-tolerant E. coli potential.”

“It doesn’t matter how well you’ve fermented,” Dr. Chapman said, “because the pathogen can tolerate the acid, and it’ll survive.” (It doesn’t tolerate pasteurization.)

How is the production of dairy products (and other foods) monitored to keep consumers safe?

“The food industry doesn’t rely on the government to keep food safe,” Dr. Chapman said. “They’re focused on keeping pathogens out of food because it’s really bad business.” He also noted that, even if there were to be disruptions at the federal level, state and local governments are still inspecting food plants and restaurants.

In the example of yogurt, an F.D.A.-regulated food, an inspector typically visits factories no more than once a year. “So there are hundreds of days of operation when there’s no inspector,” when companies have been largely responsible for monitoring their own food safety, Dr. Chapman said.

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

According to Anita Shepherd, the founder of Anita’s Coconut Milk Yogurt, which closed in 2022, the biggest problems facing plant-based yogurts are spoilage-causing yeasts and molds. But if a nondairy yogurt is packed on the same equipment as dairy yogurt, 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.

Dr. Chapman recommends sticking to the same four-hour limit that the F.D.A. Food Code advises for other perishable foods sitting out of the fridge in restaurants. Since the plant-based yogurt 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 yogurt?

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 Ms. Dashtaki, of the White Moustache yogurt brand. “I would personally only freeze it if you’re making frozen yogurt 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 yogurt would diminish the viability of its probiotic bacteria,” notes Mr. Katz, the author of “The Art of Fermentation.”

If needed (or if you find yourself with an accidentally frozen batch), Ms. Dashtaki recommends blending thawed yogurt 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 yogurt, she points out that the whey freezes beautifully in ice pops.

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

Mango lassis are a great way to use up a lot of yogurt.Christopher Testani for The New York Times. Food Stylist: Simon Andrews.

In Ms. Dashtaki’s home, yogurt isn’t just a breakfast food, but a significant portion of savory 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 Mr. Zilber of “The Noma Guide to Fermentation” wants to use up a lot of yogurt, quickly, he makes Indian kadhi, “a straight-up yogurt curry stew that simmers it down with cumin, turmeric, asafetida, mustard and fenugreek” (or raita or mango lassis).

Ms. Sternman Rule of “Yogurt Culture” says yogurt makes the best pancakes and excellent marinades for proteins. “And if you’ve never made yogurt 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).”