Yogurt is full of life, and that’s good as long as it’s under control. The milk in yogurt is fertile material for growing microbes like bacteria and yeast and mold, so yogurt producers take great care in processing and packaging so that only the desired microbes are in the yogurt. Some of the information here can also apply to other dairy products like sour cream.
These tips cover commercially-produced yogurt which must follow certain rules covering production and testing. Home-made yogurt can be more unpredictable, for example not having an expiration date. The same signs of spoilage still apply, though.
What could be growing in that yogurt?
Yogurt can spoil in more than one way, depending on what might contaminate it. Yogurt production combines sterilization and deliberate inoculation with desired bacteria. The process faces risks of contamination. The Neogen Blog, a food-safety publication, explains that many wild species of bacteria, mold and yeast exist in ambient air. We go around inhaling them all the time. This accounts for the majority of contamination cases in yogurt processing.
Several species of bacteria can culture yogurt, usually strains of Streptococcus thermophilus and Lactobacillus acidophilus That name is descriptive; it’s a bacillus (rod-shaped bacteria) that lives on lactose and likes an acidic environment. When yogurt is cultured with the right bacteria, they raise the level of lactic acid which makes the yogurt less hospitable to other bacteria. But mold and yeast are fungi, which are completely different kinds of organisms.
Mold grows on the surface of the yogurt and spreads by threads. It may appear as green or black or orange spots, or as a blue fuzz. Mold can produce toxins, so it’s a sign that the yogurt is not safe. But is that absolute? Since the mold stays on the surface, if there’s just a little dot of it on the surface of a big tub of yogurt, you could spoon it out (and wash the lid to remove any spores). In that case, use it up before more mold grows.
Yeast is less visible, partly because it shows as white spots, which Neogen says are difficult to identify because they blend with the appearance of a dairy product. Yeasts can grow down into the product, below the surface. They can produce gases, which can make a product fizzy or bubbly.
Shelf-life expectancy of various yogurt types
The Colorado Integrated Food Safety Center of Excellence  lists these shelf lives for yogurt stored at 40 ° F (4.4 °C):
“Swiss” custard-style yogurt 10 – 21 days
Drinkable yogurt 4 – 10 days
Yogurt cheese 7 – 14 days
Other products like frozen yogurt or yogurt snack bars have life expectancies longer than the fresh yogurt we’re considering here. Some of the following tips may apply to them, too.
Check the expiration date
The most obvious thing to check is the expiration date. If the container is intact and the date has not passed, the yogurt is probably still good. Yogurt may still be good after this date, as long as it has been stored properly at a cool temperature.
The United States Department of Agriculture (USDA) food product dating fact sheet  says the selected expiration dates relate to food quality and not to food safety. The best-quality date is set by the manufacturers and retailers according to conditions like packaging, handling and storage.
Yogurt may spoil before the date if not stored properly; the stores keep it cold like any other dairy item so if the yogurt has been sitting at room temperature, it may spoil like other dairy items.
Check the container
The care taken in yogurt production can’t protect the product once it’s out in the world. When you inspect a container, look for any signs that its original seal has been broken in any way – and that includes a container that has been sitting around after being opened. If the seal is compromised, look for more warning signs.
Also, check the container for cleanliness. Sometimes an intact container can be spattered with product from some other container that failed. This can also be true for other foods like plastic-wrapped meats.
Check the product
Look for any signs of mold spots or scum on the surface. Don’t forget to check the underside of the lid. If you see signs of mold, maybe you could skim it off, but it’s better to not take chances, and just throw it away.
Yeast is not as easily visible as mold but it produces gas, so intact seals on infected containers will show pressure by puffing out. (Pressure in sealed food containers is often a warning sign anyway.) If the container seal has been compromised, for whatever reason, another way to detect yeast is to smell the yogurt. The aroma should be slightly acidic, but not tangy like the aroma of rising bread dough.
Examine the texture. This brings up a distinction between yogurt products with or without active cultures. Pasteurized yogurt is like “processed” cheese which has no active culture so it’s stable. Natural yogurt, like “natural” cheese, has live cultures which continue to grow, just as natural cheese continues to ripen.
The US Food and Drug administration requires natural yogurt to list what cultures are present and their concentration in “colony-forming units.” The longer the cultures grow, the more the yogurt will thicken and taste acidic. It also develops some separation between the milk solids and the whey. You’ve probably seen some loose liquid on the top of older yogurt, and that’s not necessarily a sign of spoilage. You can simply pour it off.
You be the judge
These basic tips should help you make a better-informed decision whether some yogurt is “bad” or still useable. Even if the yogurt isn’t in “best” condition, it might still be acceptable. As USDA says, “Consumers must evaluate the quality of the product prior to its consumption to determine if the product shows signs of spoilage.