Yummy: The Neuromechanics of Umami

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It's called the "fifth taste," and it's loved, feared, and innocently sprinkled on food the world over, even though many people believe it's a peculiarity of Asian food. I'm talking about umami, the savory essence of seaweed, dried fish, mushrooms, yeast, meat, cheese, tomatoes, and many other tastes.

And yet, ubiquitous as it is, it took until the early twentieth century for a Japanese chemist to isolate umami and recognize it as the fifth fundamental human taste—joining the select company of sweet, sour, salty, and bitter. In an endearing bit of science history, the discoverer, Kikunae Ikeda, named the substance umami—Japanese for "yummy."

You may know the flavor better as monosodium glutamate (MSG), the infamous synthetic form of glutamate, the chemical largely responsible for umami taste. Glutamate is an amino acid that occurs as a building block in many proteins (it's actually one of the most common neurotransmitters in the human body). But it only triggers the umami taste when it reaches the tongue in a free state, unbound to other molecules.

This week, scientists writing in the Proceedings of the National Academy of Sciences have puzzled apart the way glutamate activates nerves on the tongue. The findings help explain why umami taste can be accentuated by the addition of either of two other compounds: inosinate (found in meat) or guanylate (found in mushrooms).

Scientists call what happens during umami tasting a "Venus flytrap" mechanism: Glutamate lands on your tongue and nestles into a glutamate-shaped depression on an umami receptor. Upon contact, the receptor—an enormous, folded protein—changes shape and grasps the glutamate. That shape change also activates the neuron that tells your brain you are tasting umami.

The scientists also learned that inosinate and guanylate can bind to a separate part of the umami receptor. Once bound, they tighten the receptor's grip on glutamate, increasing its ability to "taste" glutamate by up to 15-fold before the receptor relaxes its grip. The finding explains, perhaps, why a good Japanese broth contains both glutamate-rich seaweed and inosinate-rich dried fish flakes.

MSG—and by extension, umami—has gotten a bad rap over reports of people getting headaches or tingling sensations in the head and neck after eating foods containing the additive. But the FDA has not been able to identify MSG as the cause of such symptoms (so-called "Chinese restaurant syndrome").

Even more reassuring than the FDA's pile of inconclusive medical studies are the legions of people who blithely eat glutamates every day, the world over, in the form of hydrolized soy protein and yeast extracts. As a properly raised half-English kid, I spread glutamates on my toast every time I enjoy some delicious Marmite. When I settle in to watch Doctor Who reruns, the savory-cheesy nutritional yeast I sprinkle on my popcorn is glutamate central.

And it's not just niche foods. Ever wonder what compels you to eat an entire bag of Doritos all by yourself? They may not contain MSG, but they're packed with five separate sources of glutamate.

Head over to Umami Mart for more examples of this great flavor. (Star UM-er Kayoko has been on an umami binge in Japan for several weeks now, and I'm getting to the point where I'm too envious to keep reading her posts.)

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