Heat and acid help keep yogurt stay safe during its formation; incubators maintain heat, typically around 110 degrees Fahrenheit, and beneficial bacteria convert the sugars in milk into lactic acid.

But although natural-foods enthusiasts suggest that yogurt is safe at room temperature for extended periods, there are plenty of reasons for you to err on the side of caution and treat yogurt as a potentially hazardous food.

Keep yogurt refrigerated after it has coagulated or when you bring it home from the store, and never leave it at room temperature for longer than 2 hours.

  • The Foodsafety.gov website reports that 3,000 Americans die each year from food-borne illness; that 100,000 Americans go to the hospital each year with food poisoning; and that harmful bacteria in foods multiply rapidly at room temperature, doubling every 20 minutes.
  • Governmental, nonprofit websites and food websites all recommend limiting yogurt to 2 hours without refrigeration once it has thickened. These include the National Center for Home Food Preservation, the U.S. Department of Health and Human Services at Foodsafety.gov. and epicurious.com.
  • Some harmful bacteria, such as E. coli, and varieties of salmonella and listeria, are acid-tolerant and can grow in acidic environments like yogurt. Unlike molds that you can see, these bacteria are undetectable.
  • The addition of sugar and fruit to some yogurts gives bacteria a favorable environment in which to grow.
  • Refrigerate yogurt after 2 hours without refrigeration or after 1 hour if the temperature is above 90 F.
  • Refrigerate homemade yogurt within 2 hours of its setting and becoming firm.
  • Discard homemade yogurt that fails to set. This yogurt has never produced beneficial lactic acid and has allowed harmful bacteria an opportunity to grow.

Tips

When stored in the refrigerator, yogurt stays safe for between 10 and 21 days, according to the National Center for Home Food Preservation. If your yogurt becomes extremely runny, or if it has a sour smell or bits of black or green mold, throw the entire carton away.