How long can yogurt sit out?

NEW YORK – Yogurt was born as a means of preserving fresh milk, thousands of years before refrigerators were invented.

But forget putting 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.

Here is what to keep in mind when determining the likely resilience of your own supply and how to know when you will want to discard or compost it.

Nutritious for the gut

“Making yogurt is a magical, wonderful, historical, nutritious process,” said Homa Dashtaki, 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. This lowers the pH – giving yogurt its tartness – and unfurls and knits the milk’s proteins into a stable gel network, giving it a thick, lush texture.

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 co-author of The Noma Guide To Fermentation. But for how long, exactly?

How long yogurt can safely sit out

Although some government agencies caution against eating yogurt that has been out of the fridge – that is, held at 4.4 deg C or above – for more than two hours, experts agree 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.

These would include children younger than five, 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 Dr 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 after four hours.”

He added that including a fruity yogurt in his child’s lunch would not be a concern for him, as long as it was being eaten that day.

For comparison, it is only after four hours without refrigeration that the Food and Drug Administration (FDA) requires restaurants to throw away foods notoriously susceptible to food-poisoning outbreaks, such as raw sprouts or cut cantaloupe.

“Four hours for me is really the number,” Dr Chapman said.

Types of yogurt that are most and least perishable

Non-fat or whole, plain Greek or cotton candy-flavoured, all dairy-based yogurt sold in the US has two safety features built into the process.

First, the milk is heated to 85 deg C for 30 minutes to unravel the proteins to help them mingle and form a thick, creamy texture, said Dr Nicole Martin, assistant research professor in dairy foods microbiology at Cornell University.

“This heat treatment is the pasteurisation,” she said.

The FDA also requires the final product to have a pH of 4.6 or lower. At this acidity, milk proteins begin to clump and most food-borne pathogens cannot 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,” chef 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 yogurt, fermentation revivalist Sandor Ellix Katz, author of The Art Of Fermentation, adds fruit or herbs just before serving.

And any yogurt product labelled in the US 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 yogurt acidic enough to be unwelcoming to most pathogens.

Check best-by or sell-by dates

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 US, other than those on infant formulas, are unregulated and a manufacturer’s estimate 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 expiration date, is quality-driven too.

So, when you are 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, and the tube your child sipped and abandoned two weeks ago.

How to tell if yogurt has gone bad

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

Although there are dangerous pathogens that you cannot see, smell or taste besides the spoilage microbes that make foods unappetising – such as fuzzy moulds and 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 are not concerned about either in yogurts made in commercial settings, where milk is pasteurised 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 moulds can produce toxins that can make you sick, “you have to eat a lot of them, often, to get symptoms”, chef Zilber said.

Monitoring the production of dairy products

“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 FDA-regulated food, an inspector typically visits factories no more than once a year. “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. NYTIMES