Apparently, coffee contains compounds other than caffiene, that cause gastric acid secretion: tryptamides and catechols.
“Espresso and French press both tend to extract higher concentrations of tryptamides,” Sebastian says. “Meanwhile, tryptamide concentrations in filter coffee are usually quite low because they are absorbed by the paper filter.
“However, tryptamides are only some of the compounds which contribute to the increased secretion of gastric acid,” he adds. “In our research, we are also analysing the effect of chlorogenic acids [on the stomach], but more evidence needs to be gathered.”