For millennia it was thought that there were only four basic tastes perceptible to humans: sweet, salty, sour and bitter. Plato and Aristotle were on board with this idea, as was their predecessor Democritus, who contributed bitter to the short list of what are now called "taste qualities." It wasn't until about a hundred years ago that another taste, umami, was proposed for inclusion in 1909 by Kikunae Ikeda, a professor at Imperial University of Tokyo. Ikeda wrote, "I believe that there is at least one other additional taste which is quite distinct from the four tastes. It is the peculiar taste which we feel as 'UMAI [meaning brothy, meaty, or savory],' arising from fish, meat and so forth. The taste is most characteristic of broth prepared from dried bonito and seaweed ... I propose to call this taste 'UMAMI' for convenience."
The aforementioned broth is called dashi, and the ingredient bonito that he named, aka bonito flakes, are shavings of dried, fermented, smoked tuna. Although Japanese chefs may not have understood the hows and whys, they knew that preparing fish this way and adding it to kelp broth made for a very satisfying bowl of soup, as well as a base for many other dishes.
Ikeda's landmark paper, "New Seasonings," details the process by which he identified and extracted the essence of umami from the savory broth. He concludes, "This study has discovered two facts: one is that the broth of seaweed contains glutamate and the other that glutamate causes the taste sensation 'UMAMI.'"
The power of umami was already long understood by cooks the world over. But nonetheless, it wasn't taken seriously as an official fifth taste quality until glutamate receptors were discovered on the human tongue in 2000. This proved that humans are engineered to appreciate umami.
"The sequencing and functional expression of a human taste receptor for glutamate determined by these studies provides a first molecular basis for Ikeda's pioneering work," noted the Journal of Chemical Senses, in 2002.
Several more of Ikeda's observations on umami have withstood the tests of time as well. He noted, for example, that the taste of umami is enhanced with salt, but muted with sugar. John Prescott, in his book Taste Matters, wrote that this is precisely why tomatoes are rarely found in fruit salads and other sweet dishes. There is no tomato shortcake. Rather, the high levels of glutamate in tomatoes make them more suitable for savory applications.
Ikeda also noted the distinction between glutamate that is part of a protein molecule, aka bound glutamate, and glutamate that is floating around unattached, known as free glutamic acid. The bound form of glutamate, as is found in muscle protein, isn't available to the receptors. Thus, raw meat has very low levels of umami.
Some foods have naturally occurring high levels of free glutamic acid, and hence more umami taste. Parmesan cheese and anchovies have helped Italian food get its umami on. The French do it with veal stock, in which flesh and bones are simmered long enough to disassemble the tightest proteins, thereby freeing maximum glutamate. In Southeast Asia umami comes via fish sauce. In America look no further than a charbroiled bacon cheeseburger with ketchup.
In addition to being tasty, free glutamic acid is also a neurotransmitter. The cells of Huntington's Disease sufferers can become overstimulated by glutamic acid, making these people potentially sensitive to it. But in the general population, little scientific support has been found for the idea that MSG can cause headaches or other adverse reactions.
When glutamate receptors were found, it not only proved that umami is a basic taste, but was taken as evidence that a taste for glutamate offered some kind of evolutionary advantage—otherwise the receptor wouldn't be there. Subsequent studies found a similar glutamate receptor in mice. The mouse receptor was part of a clump of receptors that target a variety of amino acids. Because amino acids are the building blocks of proteins, having a taste pathway dedicated to their detection, the authors of one study note, "... probably had significant evolutionary implications."
The multi amino acid receptor in mice is thought to encourage them to consume a variety of amino acids whenever they're available. The glutamate receptor in humans, by contrast, doesn't bind to any other amino acids. This is curious, because glutamate is a non-essential amino acid that our bodies can manufacture from scratch. Why would humans have multiple receptors for it?
Many experts believe that, in humans, glutamate has become a signal for the general availability of amino acids. In Taste Matters, Prescott writes that because glutamic acid is an amino acid present in animal and vegetable protein, "... umami taste may act as a signal for the presence of protein, with its effects on palatability therefore promoting consumption."
But paradoxically, the umami switch is not triggered by the most protein-dense food of all: meat. As the glutamate is bound up in muscle protein, it isn't free to impart its umami deliciousness. Thus, a taste for umami would not have enticed our ancestors to gorge themselves on a fresh kill.
The most convenient and delicious way of releasing free glutamate from meat is to cook it. The processes of heating and browning meat make glutamate, and other amino acids, available to the body, including its glutamate receptors.
Cooking also makes the calories in food more accessible, which offered clear evolutionary advantages to our ancestors. Many scientists believe that cooking, and the extra calories it made available, is what led to a dramatic increase in the size and power of the human brain.
Perhaps a cultivated taste for glutamate helped seal the evolutionary deal between man and fire. From browned meat to umami-rich broth, fire has allowed the creation of some of the most savory, delicious foods we know. And we can thank our umami receptors for encouraging us to keep cooking.