Why foul smell from rotten food?

The foul smell from rotten food is primarily due to the breakdown of proteins and other organic materials by bacteria and fungi. As these microorganisms decompose food, they produce volatile compounds, such as ammonia, hydrogen sulfide, and various fatty acids, which emit unpleasant odors. Additionally, the fermentation process can generate gases that contribute to the offensive smell, signaling the food's spoilage and potential danger to health.