When I was in cooking school 20 years ago, we were taught that the most delicious recipes carefully balanced the four flavors: salty, sweet, bitter and sour. The French chefs who taught us were classically trained and extremely chauvinistic about French cuisine, which may be why, even if they'd ever heard of the fifth flavor -umami- they didn't bother teaching us about it.

Umami, after all, had been discovered in 1908 by a Japanese scientist who was trying to discern what made a bowl of seaweed broth so delicious. Had I known about umami in my cooking school days, had I asked about it, I would have been met with that particularly irritating Gallic chortle the chefs used whenever they dismissed what wasn't in the Escoffier playbook.

Seaweed, ma petite? Iz zat not food for feesh? For plompe leetle whales? Are we not youman beanz? Oomoomoo? Do not mention zees word in zees keetchen, again, oui?

These days, though, umami, which translates roughly as "deliciousness," is accepted in the culinary world as an honest to goodness fifth flavor. Scientists have even identified the receptors on our tongues that allow us to taste it.

Umami may be a more subtle kind of flavor than its four brethren, but most people can easily recognize it. Describing it is a bit more difficult. Think of the flavors of these foods: parmesan cheese, bacon, soy sauce. Recognize what they have in common? A depth of flavor, a fullness that brings out the best of what they're cooked with -pasta with parmesan, spinach and bacon, rice and soy sauce.

Scientifically, umami is the taste of some of the most abundant amino acids in our diets, glutamate, inosinate and guanylate. Why humans are so hot for umami isn't completely understood, although it may have to do with the fact that glutamates are abundant in breast milk and other protein-rich foods, like meat and cheese, soy products and many kinds of seafood.

But not all umami-rich foods are high in protein. The much-maligned food additive MSG (monosodium glutamate) is umami whipped up in a lab, as are the food additives hydrolyzed soy protein, autolyzed yeast and sodium caseinate.

Vegetarians can find umami in shitake mushrooms, sweet and white potatoes, Chinese cabbage, carrots, seaweed and green tea. Even condiments can be full of umami, which may be why every region of the world seems to have a bottle of something to sprinkle on a meal. Soy sauce, fish sauce, even ketchup, are rich in umami.

The greatest fruit

At this time of year, local markets are full of one of the few umami-rich foods native to our hemisphere - tomatoes. Tomatoes may be unique in that not only are they full of umami, but they also contain all the other four flavors as well. Which is why great tomatoes, well grown and bursting with juice, are to my mind the most perfectly balanced fruit in existence.

There are a few rules about what makes a tomato great. First, a great tomato needs to be picked ripe. Which means a great tomato is either going to come from your own garden, a friend's garden or the local farmers' market.

Second, a great tomato is probably going to be an heirloom variety and not a hybrid. I've got nothing against hybrid tomatoes - I grow them every year as a hedge against disease. And the very best cherry tomato in the world, Sungold, happens to be a hybrid. But for the most part hybrid tomatoes are a stodgy lot - predictable, regular and bland.

Heirloom tomatoes, though, when well grown, are pure ambrosia. Yes, they're prima donnas. They can be stingy producers (especially in cold New Hampshire). They tend to collapse in a morass of yellow leaves if someone even whispers the word "blight"within five feet of them. But coddle your heirloom tomatoes and oh, how they will reward you.

This year we are feasting on more than a dozen varieties, all delicious, all enticing. All have a subtle bitter note - think of the potent scent of bruised tomato leaves. The yellow tomatoes - the Goldies and Pineapples, the Fargo Pear and Lillian's Yellow - are the sweetest of the lot. The greens -Green Zebra and Aunt Ruby's German Green - tease the sour receptors on the tongue. The reds and pinks - the Brandywines, German Johnsons, the Rutgers - balance the five flavors nicely. (next page »)