How do we taste, and how many tastes are there?

(Click on figures to enlarge.)

How many tastes can the human tongue distinguish? Some people will say four: salt, sweet, sour, bitter. Some will claim five with a new-ish one in recent years hitting the limelight: umami (Japanese word for savory, and some would even say meaty is a part of it). But there may likely be six or seven, or even more. How does that figure? What do we know about these and about taste in general?

From Getty Images

Taste is one of the five traditional senses (followed by touch, hearing, sight, and smell). But humans have more than five senses, and no, the sixth sense is not an ESP perception. We have a sense of balance, a sense of movement and position of our limbs and muscles (proprioception), a sense of internal body needs (like hunger, thirst, or going to the toilet), etc. But let's stick with taste. 

Aristotle designated sweet, sour, bitter, and salty in the 4th century B.C. but also considered three others: astringent (slightly acidic or bitter), pungent, and harsh. Many other people created their own lists, as you can see below.

Table from Handbook of Perception, volume VIA, Tasting and Smelling, 1978)

In more recent decades, we were taught incorrectly something about our ability to detect a handful of tastes. Yes, incorrectly. The following diagram you might be familiar with is wrong, yet it has been provided through the years to schoolchildren to show not only what we can taste, but where on the tongue the sensors are located. I repeat, this is incorrect information.

Widely used but inaccurate model of taste locations on the tongue

Many scientists in the 1700s ad 1800s were investigating the types of tastes we can detect and their locations on the tongue. In 1891, L.E. Shore published a mapping study he did ("A contribution to our knowledge of taste sensations") using the four accepted tastes with the following materials:

• sweet (glycerine)
• sour (sulfuric acid)
• bitter (quinine)
• salty (sodium chloride, table salt)

He determined the minimum amount of each one that could be detected and where.

Following that, in 1901, German scientist David P. Hänig published a paper, "On the Psychophysics of the Sense of Taste", in which he described similar locations on the human tongue and showed the data in graphical form. 

Results of Hänig's 1901 paper

 The differences in sensitivity seem really small for salt and sour, but he drew a map of where all four tastes were detected:

From Hänig, 1901

If you look at the drawing, you might be able to see the minute differences around the tongue's surface, where the dots representing tasting points are grouped closer together and therefore agree with the graph. In 1916, another German researcher Henning sketched a "taste tetrahedron" to show yet another view of the four recognized tastes (saline = salt). His picture meant to symbolize not just the four, but that along each edge and face of the shape, gradations of each taste could be found. Some of those gradations were listed as salt-bitter, salt-sour, salt-sweet, sour-sweet, sour-bitter, and sweet-bitter.

Almost three decades later, Harvard psychologist Edwin Boring compiled massive amounts of data and investigated on his own, ending up with a 644-page book in 1942, Sensation and Perception in the History of Experimental Psychology. It is stated by many people that Boring was the first to actually draw the incorrect taste map, probably based only on where the major locations were, but I cannot find any source that mentions the first tongue taste map.

If you look a your tongue, you can distinguish many bumps on its surface. These are called papillae (singular: papilla). There are 4 types, but only 3 contain groups of cells called taste buds. The buds consist of hair-like projections into the mouth, pores, and various types of gustatory (taste sensing) cells. These are the actual taste receptors that identify sweet, sour, salt, and bitter at different levels and send signals to the brain. 

Papilla, taste buds (cross section), and taste receptor cells (Chandrashekar et al., 2006, Nature)

To see the taste buds in better detail, the same authors as above provided the following colored version with various taste receptor cells. It's still not clear which of the three proposed models is accurate.

Model A: each taste bud receptor cell detects a separate taste and has its own nerve

Model B: each cell is "tuned" for different tastes but has its own nerve

Model C: each cell detects one taste but has nerve connections to the other cells for mixed taste potential

So, after decades of research, things were pinned down to four basic tastes, right? No. The above diagrams show five. Even though Boring seemed to indicate sweet, salt, bitter, and sour were definite distinct tastes, he wrote in 1942 that those may not be the only ones. Well, he was correct. Japanese researcher Kikunae Ikeda around 1908 had discovered a fifth taste from their diet: umami. He was searching for something to "make healthy, bland food more palatable" and essentially discovered MSG (monosodium glutamate) in sea kelp (konbu), which is used widely in broths in Japan. Other Japanese scientists found it dried bonito fish (1913) and in shiitake mushrooms (1957). It has since been associated with many savory foods like meats, cheeses, soy sauce, fish, yeast extract, mushrooms, tomatoes, and more. Umami comes from the Japanese casual word umai for delicious plus mi for taste.

Kikunae Ikeda (ajinomoto.com)

Ikeda got a patent for making umami (MSG) and shared it with Saburosuke Suzuki, who started the Suzuki Chemical Company and began manufacturing it in 1908 as "Aji-no-moto" (meaning "essence of taste"). But, popular as it was as a food ingredient in Asia, it wasn't until 1990 that umami was finally recognized as a separate taste at the International Symposium on Glutamate, which means it does not fit the "taste tetrahedron" from Henning. And the taste bud receptor for glutamate was located in only as early as 2006.

So, is that it, then? Five tastes plus whatever mixtures we can sense? No, again. In 2005, French researchers found that mice have a sense of tasting fatty foods. As for humans, we don't know. However, enzymes that break down fatty acids are found on the tip of the tongue, so Danish researcher Camilla Andersen tested types of milk on volunteers who were also hooked up to a monitor that measured brain activity. She determined that we can discriminate two milk fat extremes (0.1% and 38% fat).This sense of taste has not yet been fully confirmed.

Testing fatty acid taste from milk (Andersen et al., 2020)

The sense for umami on the tongue was first thought to be in the big blank area in the center where none of the other tastes had been detected. But like Shore's and Hänig's data from 1892 and 1901, it is widely accepted now that taste buds all over the tongue can detect all of the flavors we can describe; the sense is just stronger in certain parts of the tongue.

In addition to these tastes, scientists have also begun to feel we may have two senses for salt: high-salt and low-salt tastes. Salt itself helps regulate blood pressure and controls nerve impulse transmission. It may be that we have taste detectors which tell us that the amount in a food is acceptable ("just right") or unacceptable ("too salty"). The sodium in table salt seems to regulate our receptors' feeling for low salt acceptance, but recent research is emerging that suggests the chloride part of salt might be responsible for us rejecting foods that are too salty. All the data is not in yet, but wouldn't it be interesting to have two salt detector cells!

So, what is the most up to date tongue map that we should use? Perhaps the 2006 paper in Nature shows it best. In other words, we taste everything everywhere on the tongue.

From Chandrashekar et al., 2006, Nature