Japanese scientist Kikunae Ikeda first proposed umami as a basic taste — in addition to sweet, sour, salty and bitter — in the early 1900s. About eight decades later, the scientific community officially agreed with him.

Now, scientists have evidence of a sixth basic taste. In a study published recently (Nature Communications), researchers have found that the tongue responds to ammonium chloride through the same protein receptor that signals sour taste. Scientists have for decades recognized that the tongue responds strongly to ammonium chloride. However, despite extensive research, the specific tongue receptors that react to it remained elusive. But in recent years, they uncovered the protein responsible for detecting sour taste. That protein, called OTOP1, sits within cell membranes and forms a channel for hydrogen ions moving into the cell. To confirm that their result was more than a laboratory artifact, they turned to a technique that measures electrical conductivity, simulating how nerves conduct a signal. Using taste bud cells from normal mice and from mice the lab previously genetically engineered to not produce OTOP1, they measured how well the taste cells generated electrical responses called action potentials when ammonium chloride is introduced.

Taste bud cells from wildtype mice showed a sharp increase in action potentials after ammonium chloride was added while taste bud cells from the mice lacking OTOP1 failed to respond to the salt. This confirmed their hypothesis that OTOP1 responds to the salt, generating an electrical signal in taste bud cells.