How long does an opened and unrefrigerated tub of yogurt last?
When you open a food product, you are exposing it to microbes in the air. The food is sealed in a sanitary environment and bacteria is kept out. Once you open the container of yogurt, those microbes begin working on the yogurt. Further, when you leave the yogurt at room temperature the bacteria can rapidly multiply as the yogurt is in the danger zone between 40 F and 140 F. At temperatures in this range, any yogurt exposed to bacteria in the air will become rancid rather quickly.