What causes spoilage?

Yeast is the most common spoilage organism in yogurt. The ironinic part (and this is common in microbiology) is that yogurt seems to thrive on a small amount of yeast, but is easily transformed into "spoiled" when yeast becomes too abundant. Yeast spoilage can be considered a pre and post production spoilage organism, meaning it is there during yogurt fermentation and can grow after. Some post production spoilage organisms, which means they occur after yogurt is made, can be psychotrophs and coliforms. These are both umbrella terms for a type of organism. Both psychotrophs and coliforms only occur due to package damage, or direct microbial contact in processing plant from worker etc. In other Words, these two are not common, but occur due to error. Yeast, however is the most common spoilage organism.