If you’re celebrating in a Scandinavian country, anyway.

Let’s backtrack. First off: What is ammonium chloride, and what does it taste like? Also known as “salmiak salt,” it’s formed by ammonia and hydrogen chloride and serves as a key ingredient in salt licorice—a treat popular in countries like Sweden, Finland, Greenland, Norway, Iceland, and Denmark since the early 1900s. 

Its taste, though difficult to pinpoint, can be described as “a combination of bitter, salty, and a little sour,” says University of Southern California neuroscientist Emily Liman, whose team proposed the potential sixth taste in a recently published article in Nature Communications. Some have suggested that it “tastes like window cleaner.”

When a taste means safety

Scientists have known for decades that the tongue responds to ammonium chloride. But just how and why it does remained elusive, until Liman and her team in 2018 discovered a protein channel responsible for detecting sour taste: OTOP1.

Sitting in the outer membrane of taste-receptor cells, OTOP1 forms a channel for hydrogen ions—responsible for the sour taste of acids—to move inward. Hydrogen atoms from sour foods like lemonade (rich in citric and ascorbic acids) and vinegar (acetic acid) enter taste-receptor cells through a channel created by the newly discovered protein.

But it’s not just hydrogen that activates OTOP1, Liman’s team found. So does ammonium chloride—and it does so just as well as hydrogen, if not more so. What’s more, they also found that ammonium chloride gives off a small amount of ammonia, which contributes to its cleaner-like taste.

Just why did humans develop the ability to taste ammonium chloride? It’s a “somewhat toxic” substance found in waste products like fertilizer, Liman points out. So it makes sense that humans would benefit from the ability to detect and avoid it—at least in larger-than-licorice quantities.

Along those lines, chickens are much more sensitive to ammonium chloride than zebrafish, Liman adds. That may be because chickens encounter a good deal of ammonium chloride in chicken coops—which are filled with feces and, thus, ammonium chloride. But zebrafish aren’t likely to encounter it much, making their ability to detect it less crucial.

Japanese scientist Kikunae Ikeda proposed umami as a new taste in the early 1900s. It took about 80 years before the scientific community agreed with him.

Perhaps in time, the scientific community will also agree with Linman and her colleagues, dubbing ammonium chloride the official sixth taste.