Sour? Sweet? When the primary tastes collide, lemonade is just delicious.

The Primary Tastes

Until recently, scientists have accepted four basic tastes. You know them well -- sweet, salty, sour and bitter. They are the building blocks of flavor and at the root of other tastes. Each primary taste triggers a particular gustatory receptor (although receptors can, and frequently do, respond to multiple tastes). The basic tastes went unchallenged for years, perhaps because of their familiarity -- name another taste that is as distinctive as one of the four.

In the early 1900s, however, a Japanese scientist sought to detect another taste -- that of the savory seaweed common in Japanese cooking. Kikunae Ikeda eventually isolated glutamic acid as a distinct fifth taste -- one with its very own gustatory receptor. Ikeda named this fifth taste umami, a Japanese word meaning delicious, savory taste. You can taste umami in meats and tomatoes.

Researchers continued to study umami throughout the 20th century. An important breakthrough came in 1985 when scientists trying to mimic the controversial, flavor-enhancing substance MSG failed to replicate the taste with any combination of the basic four.

But because Ikeda's study on taste was not translated into English until 2002 and because the taste of glutamic acid is subtle and less common in Western food, umami has only recently entered the taste canon. Now that the gate is open, however, it's unlikely that scientists will ever be so secure in the limits of primary taste. French researchers even identified a potential gustatory receptor for fat. Fat could actually be the sixth taste.

The primary tastes gave early humans clues about what food was good to eat and what was harmful. Sweet foods usually had calories. Salty foods had important vitamins and minerals. Sour foods could be healthy, like oranges, or spoiled, like rotten milk. Bitter tastes were often poisonous. The enhanced flavor of processed food could signify nutritional value that isn't actually there, but our preferences have remained. We still crave and respond to our ancestral favorites, even to our detriment.

So if there are at least five primary tastes, what's up with the tongue map? In the next section we'll learn about the biology-book mainstay and why it might be completely wrong.