Final answer:
The stomach is the part of the digestive system essential for churning food, with peristalsis aiding in mechanical digestion and gastric acid facilitating chemical digestion.
Step-by-step explanation:
The part of the digestive system that is essential for churning food is the stomach. The stomach is a J-shaped organ that mixes and churns the food with muscular contractions, a process known as peristalsis. This mechanical digestion breaks the food into smaller pieces and combines it with digestive juices to create a mixture called chyme. The gastric acid, mainly hydrochloric acid, in the stomach creates a highly acidic environment which is necessary for the digestive enzyme pepsin to commence the digestion of proteins.