For decades scientists have recognized five basic tastes for humans: sweet, sour, salty, bitter, and umami (or savory). According to the latest research, we may need to add another basic taste to that list.

The as-yet-unnamed new taste is a response to ammonium chloride, and the particular way it activates receptors in the tongue has been revealed by a team from the University of Southern California (USC) and the University of Colorado.

When it hits your tongue, ammonium chloride produces a taste sensation described as bitter, salty, and a little sour. While animal reactions to it have long been studied, the precise way human taste buds react to it hasn't been fully analyzed before now.

As well as being used in medicines and fertilizers, ammonium chloride is also used to add crispiness to snacks in some Asian countries, and as salmiak salt added to flavor dark sweets called salty liquorice in parts of Europe.

"If you live in a Scandinavian country, you will be familiar with and may like this taste," says neuroscientist Emily Liman, from USC Dornsife.

The research builds on a previous study of the protein otopetrin1 (OTOP1), which confirmed how it was used to detect sour tastes and how it affected acid levels in cells. Now, they've shown its response to ammonium chloride too.

Exposing cultured human cells to the sour taste of an acid and ammonium chloride separately showed the OTOP1 receptor responding in the same way to both stimulants. What's more, in animal tests, mice without the OTOP1 gene didn't mind tasting foods that had ammonium chloride in them, while those that did have the gene avoided these substances.

This previously unknown activation pathway is a sign that we may have to add a sixth taste to the list – though more research is going to be needed to know for sure. Potential new tastes have previously been identified without being made official.

In the future, the researchers want to look in more detail at how the OTOP1 receptor responds to ammonium chloride, and how this response might vary among species – the team notes that different animals react differently to ammonium chloride.

Considering creatures as simple as the Caenorhabditis elegans worm are able to recognize this taste and avoid certain types of potential food because of it, it's possible that natural evolution has trained us to steer clear of the chemicals that stimulate it because of their potential toxicity, even if humans have now adapted to eating it in small amounts.

"Ammonium is somewhat toxic, so it makes sense we evolved taste mechanisms to detect it," says Liman.

The research has been published in Nature Communications.