Plain yogurt naturally has bacteria, albeit specially formulated bacterial cultures that convert the sugars in milk to lactic acid. Safe handling of yogurt can prevent the growth of harmful bacteria and other microorganisms that can cause food-borne illness. It also ensures that your plain yogurt retains its creamy texture and slightly tart flavor.
Reading the Label
Most yogurt manufacturers stamp a date on containers of plain yogurt, although this date stamp is not mandated by federal law. If there is a date on the package, however, the USDA Food Safety and Inspection Service requires there to be an explanation of the date. Right next to the date on your package, you should see either a "sell by" or "use before" indication.
"Sell-By" vs. "Use-Before" Dates
If your yogurt has a "use-before" or "use-by" date, the date stamped on your yogurt is the last date that your yogurt will be at its peak, in terms of flavor and texture. A "sell-by" date, on the other hand, indicates to a store the last day that the product should available for sale. Both of these dates refer to the quality of the yogurt, not its safety.
The length of time you can safely store plain yogurt at home depends on your following safe-handling procedures. While you're doing your shopping, pick up the yogurt -- along with other refrigerated foods -- just before heading to the checkout. Place the yogurt in the fridge as soon as you get home. When you serve yogurt, always use a clean utensil and recover the container before placing it back in the refrigerator. At no time should you leave the yogurt out at room temperature for over two hours. The microorganisms that cause food-borne illness thrive at room temperature.
If you store and handle your plain yogurt properly, you can keep it for up to 10 days past the sell-by date. For use-by dates, you can keep the yogurt up to that date. Don't freeze yogurt to extend its life. Freezing and thawing the yogurt will change its texture.
When to Toss
Yogurt that smells or tastes funny or that has visible mold on the surface should be tossed, even if you haven't passed the sell-by or use-by date. A change in texture, however, doesn't necessarily mean that your yogurt has gone bad. If your plain yogurt separates -- leaving a pool of liquid on the surface of the yogurt -- it's still safe. Just give it a good stir before digging in.