What kind of bacteria is in spoiled milk?

Paenibacillus is the spore-forming bacterium that is found in spoiled milk. It is responsible for spoiling milk and causes curdling as well. Paenibacillus also contributes to off flavors in a variety of other foods.

Paenibacillus can exist as a spore in certain stages of its life cycle, such as when it must become dormant for survival. Paenibacillus may withstand the extreme conditions of pasteurization and subsequent cooling in its spore state, allowing it to survive in milk and other foods. The high temperatures of pasteurization may actually encourage the bacteria to germinate, as Paenibacillus-related spoilage are more pronounced in experiments with higher temperatures.