Achalasia is a rare disorder that makes it difficult for food and liquid to pass from the swallowing tube connecting your mouth and stomach (esophagus) into your stomach. Achalasia occurs when nerves in the esophagus become damaged. As a result, the esophagus becomes paralyzed and dilated over time and eventually loses the ability to squeeze food down into the stomach. Food then collects in the esophagus, sometimes fermenting and washing back up into the mouth, which can taste bitter. Some people mistake this for gastroesophageal reflux disease (GERD). However, in achalasia the food is coming from the esophagus, whereas in GERD the material comes from the stomach.