What is the Ring of muscle that controls movement of food from the stomach to the small intestine?

Its the pyloric sphincter.

At the bottom of the esophagus, just before the opening to the stomach is a ring-shaped muscle known as the lower esophageal sphincter (LES). This muscle relaxes (opens) to let food into the stomach...

The food in the stomach are known as chyme at this stage. The pyloric sphincter (a muscle) ensures that chyme is released bit by bit into the small intestine.

The pyloric sphincter

The valve is called the sphincter or better known as the pyloric sphincter