Sour, sweet, bitter, salty, and fresh. Is there an alkaline taste?

In ancient times, the Chinese summarized "sour, sweet, bitter, spicy, and salty" as the "five flavors." Of course, we now know that this statement has some flaws: spicy is not a taste but a pain sensation, because there are no "spicy taste receptors" on our taste buds, and what feels spicy is actually the pain receptors. The real five tastes are sour, sweet, bitter, salty, and fresh - our taste buds have their own specific taste receptors to perceive these five flavors and generate taste signals.

Now, this list of tastes may have a new member. A study published today in Nature Metabolism found a new taste receptor that can sense alkaline foods in fruit flies. The authors said that more future research will tell us whether this "alkaline taste receptor" also exists on the tongues of other animals, especially mammals.

I wonder if you have ever thought about this question. We are all familiar with the taste of sourness. Acidity is essentially a high concentration of hydrogen ions, or a pH value less than 7. The receptors that sense sourness are also proton channels that can sense pH. Then, on the other side of the acid-base balance,

Do we also have corresponding alkaline taste receptors on our tongue that allow us to taste foods with a pH greater than 7?

As early as the 1940s, experiments found that the tip of the tongue seemed to be more sensitive to alkaline sodium hydroxide than other parts of the tongue - this became early evidence that alkaline taste receptors might exist. However, if such receptors really exist, what are their molecular and cellular basis? The scientific community knows very little about this.

In the latest study, Dr. Yali Zhang's team at the Monell Chemical Senses Center in the United States first provided normal fruit flies with neutral and alkaline (pH=12) food respectively. As a result, the fruit flies all chose neutral food, while few fruit flies were interested in alkaline food.

This result is not surprising. Animals' ability to sense alkalinity is important because they can use it to judge the nutritional properties of food: just as sour taste often corresponds to dangers such as food spoilage,

Alkalinity is also a harmful signal that is unfavorable to health and may correspond to pathogens in toxic and basophilic environments.

After eating strongly alkaline foods, humans may experience symptoms such as muscle cramps, nausea and vomiting; fruit flies will even suffer a shortened lifespan as a result.

However, a small number of fruit flies did not actively avoid alkaline foods, so they were likely carrying a genetic mutation that caused an abnormal ability to sense alkalinity. Sure enough, the research team isolated a gene from these fruit flies that

The alkaliphile (alka for short, meaning basophil) gene encodes a chloride ion channel protein called Alka.

The study found that Alka is expressed in the taste receptor neurons of the fruit fly labellum (equivalent to the taste receptors of mammals) , which is a necessary and sufficient condition for fruit flies to avoid alkaline foods: fruit flies cannot do without Alka to detect alkaline foods, and the presence of Alka is sufficient.

This study identified Alka as a new taste receptor that helps fruit flies detect the pH value of food. " Our work resolves the debate about whether alkaline taste exists. " said Dr. Yali Zhang. This progress lays the foundation for more research on the mechanism of how animals perceive alkaline foods. Do we also have such alkaline taste receptors on our tongue? I hope that research in the near future can give the answer.

References: [1] Tingwei Mi et al. Alkaline taste sensation through the alkaliphile chloride channel in Drosophila. Nature Metabolism (2023). https://doi.org/10.1038/s42255-023-00765-3[2] Monell Center team discovers molecular basis for alkaline taste. Retrieved March 20th, 2023 from https://www.eurekalert.org/news-releases/983210