As a sour and fermented dairy product, it can be difficult to tell when yogurt has curdled. Checking the sell-by date on the yogurt container offers a clue as to your yogurt's age. Most yogurt manufactures guarantee their products quality seven days past this date, but its freshness depends on how the yogurt was stored. Yogurt can keep in the refrigerator for one to two weeks and in the freezer for one to two months. Once opened, sealing the yogurt in an airtight container keeps out the moisture and air-borne contaminants that hasten curdling.
Dip a spoon into your yogurt container and gently stir the yogurt. Look for small, balls that look like cottage cheese, which indicate curdling. This usually happens first around the edges and along the bottom of the yogurt container.
Feel for a rubbery resistance when you stir or press a spoon against the yogurt. Even if the yogurt doesn't look curdled, a change in the yogurt’s texture is a sign of spoilage. Good yogurt is soft, creamy and smooth.
Scoop up some yogurt on your spoon, then tip the spoon, and watch how the yogurt drops back into the container. If the yogurt breaks apart as if falls from the spoon, and seems watery and runny, this is a sign of curdling. Fresh yogurt will slide off the spoon and keep its shape. The small amounts of liquid pooled on the surface of the yogurt is a beneficial byproduct called whey, and should be stirred back in before serving.
Smell the yogurt to detect an acrid odor instead of a pleasantly sour one. If the yogurt is near the bottom of the container, it might be the container itself that smells bad. Transfer the yogurt to a smaller container and smell again. If you suspect your yogurt has curdled but aren’t certain, do not taste it. Instead, throw it out.