The answer used to be four -- until a French chef named Auguste Escoffier came along during the late 19th century. Soon, researchers decided there was a taste that didn't fall into the previous four categories of sweet, salty, bitter and sour. According to NPR:
. . . (Escoffier) created meals that tasted like no combination of salty, sour, sweet and bitter; they tasted new. Escoffier invented veal stock.You can hear the full audio of the story that aired this morning on NPR.
. . . halfway across the world, a chemist named Kikunae Ikeda was at the very same time enjoying a bowl of dashi, a classic Japanese soup made from seaweed. He too sensed that he was tasting something beyond category. Dashi has been used by Japanese cooks much the way Escoffier used stock, as a base for all kinds of foods. And it was, thought Ikeda, simply delicious.
But what was it? Being a chemist, Ikeda could find out. He knew what he was tasting was, as he wrote, "common to asparagus, tomatoes, cheese and meat but … not one of the four well-known tastes."
Ikeda went into his lab and found the secret ingredient. He wrote in a journal for the Chemical Society of Tokyo that it was glutamic acid, but he decided to rename it. He called it "umami," which means "delicious" or "yummy" in Japanese.