How long can yogurt be left out?

US food authorities suggest discarding it after two hours, but some yogurt spoils more quickly

- Kristen Miglore

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.