By which force, the food passes through the oesophagus?

After swallowing a bolus of food, it travels down a muscular tube in the chest called the oesophagus, where the process of peristalsis begins.

Peristalsis is a series of wave-like muscle contractions that cause a wave-like motion (force) to move food to different processing locations in the digestive tract. 

This movement helps food to travel through the stomach, where it is churned into a liquid mixture called chyme.

The word Parastalsis comes from ‘New Latin’ and is derived from the Greek “peristellein”, meaning "to wrap around".