It is widely assumed that acid provokes heartburn. Infusion of dilute hydrochloric acid through a tube into the acid-sensitive esophagus reproduces heartburn. However as in all things biological, it is not that simple. Even when the pH of tomato juice is adjusted to neutrality, it evokes heartburn. Coffee, with its pH adjusted to 7, still provokes heartburn in people with a sensitive esophagus. One study determined that fluids caused heartburn in proportion to their osmolality.
Coffee's effect is not due to caffeine or acidity, yet it reliably causes heartburn in susceptible people. Except for buttermilk, milk is neutral. Its tendency to induce heartburn is proportional to its fat content. Alcohol stimulates acid and loosens the LES. In these cases, it is impossible to be certain that they irritate as well as promote reflux. Accidental or suicidal ingestion of lye or strong acid devastates the esophagus. If not immediately fatal, there are inevitably severe complications. Occasionally certain pills lodge in the esophagus and cause a painful pill esophagus.