What part of the intestinal tract prevents backflow of food from the stomach into the esophagus?

The lower esophageal sphincter, or

cardioesophageal sphincter, or

gastroesophageal sphincter

Explanation:

As the name suggests, the lower esophageal sphincter or the gastroesophageal sphincter, is located between the esophagus (esophageal) and the stomach (gastro-). It is a group of smooth muscles (ring-like in structure) that prevents the backflow of food and gastric juices from going back up the esophagus. If this is impaired or weakened, then you may develop GERD or gastroesophageal reflux disorder, or heartburn.  

organ system-an organ system is a group of organs that work together to perform one or more functions. each does a particular job in the body, and is made up of certain tissues.