It's the "mmmmmmm " in meat, the pizazz in a pepperoni pizza. It's what makes people cheer for cheese.

It's "umami", and if your reaction is "u-what-ee?" you've got plenty of company. But not for long.

Chefs, nutritionists, cookbook authors and food manufacturers are salivating over the merits of umami, the so-called "fifth taste" that is neither salty, sweet, bitter or sour.

An increasing awareness of the umami quotient in food is giving everyone who cooks, from high-end chefs to home cooks, greater understanding of how adding umami flavours can perk up their culinary creations.

Nutritionists see the potential for umami to help people eat better, especially populations such as the elderly whose sense of taste may be impaired. And for food processors, boosting umami levels in their products could mean less reliance on salt, or more palatable low-sodium products.

"If we can be aware of what it is, we're going to have better-tasting food all the time," said David Kasabian, co-author of The Fifth Taste: Cooking with Umami, which he says is the first cookbook devoted to the subject.

Umami pronounced "oo-MA-mee" comes from a Japanese word meaning "deliciousness." This somewhat elusive flavour shows up in a wide variety of protein-rich foods.

It is the satisfying savour that makes people crave steak. It is the gratifying richness of grated parmesan cheese. It is the deep, comforting taste of a bowl of chicken soup.

"Yummy' is another way of saying it," said Jacqueline Marcus, a nutritionist and food consultant. "It's that salivating, lip-smacking character."

This savory taste was isolated 100 years ago by Kikunae Ikeda, a Japanese scientist who wanted to figure out what gave dashi, a Japanese seaweed soup, its distinct flavour.

Ikeda concluded that the umami flavour came from glutamate, an amino acid and protein building-block.

That means protein-rich foods such as meat and dairy products tend to be high in umami, especially when the cooking or processing of the foods gives the proteins time to break down into glutamates.

Curing, aging, browning and slow-cooking enhance the umami taste of those foods.

Although the concept has been around for a century, the fifth taste has been slow to catch on in other Western countries. For starters, there's the name, a strange-sounding foreign word.

Also, many food experts and scientists long assumed that umami was merely a combination of some of the four established tastes.

In contrast to them, "umami tastes aren't found as separate tastes," said University of Miami research scientist Nirupa Chaudhari, a specialist in the biophysics of taste. "Lemons have a very isolated sour taste. Fruit quite often has a very isolated sweet taste. Seawater has an isolated salt taste. So maybe that's why umami wasn't as well understood."

Chaudhari and her colleagues put to rest any lingering doubts about umami's status as a separate taste in 2000, when they isolated a receptor on taste buds that responds to the amino acids that impart umami flavour.

And humans are not alone in having a taste for umami. All the mammals studied so far have umami receptors, Chaudhari says. In fact, members of the cat family lack sweet receptors but have very strong sensitivity to umami, perhaps reflecting the fact that they are exclusively meat-eaters.

Chefs are increasingly incorporating umami flavours into their recipes. New York's Jean-Georges Vongerichten makes a custard with Gruyere, goat and Parmesan cheeses, topped with shaved black truffle. He calls it "an umami bomb."

And Chicago's Rick Bayless, chef-owner of the Frontera Grill and Topolobampo, recently offered an $US85 ($93) four-course umami tasting menu.

Among the items on the menu, which he billed as a "journey through deliciousness," were a duck salad with grilled onions and shiitake mushrooms, both of which are high in umami, and a grass-fed ribeye steak with a potato-and-cheese accompaniment and, for good measure, some bacon.

"My favourite umami ingredient," notes Bayless, adding that the umami dinner was his best-selling tasting menu during the past four weeks. "I think if you're a food professional and you don't know about umami, you're not very aware."

Bayless, who opened Frontera Grill in 1987, first read about umami in a restaurant trade journal about 10 years ago. As he came across more references to it, he delved more deeply into the subject because it helped explain why certain dishes have an immediate allure for diners.

"Foods that are high in sugar, or high in acid, or high in bitter tend to be more polarising," he says. "Umami is that flavour to which almost everyone is attracted, immediately and forever."

Food companies have been aware of umami for decades, even if they didn't know the word. One of the standby additives in processed food is some form of monosodium glutamate, or MSG, a flavour enhancer that is a kind of essence of umami.

But MSG came under a cloud in the 1980s, when it was linked to "Chinese restaurant syndrome," the headaches, fevers or other reactions that some people claimed were the result of eating food loaded with MSG, which some Chinese restaurants used with abandon.

Since then, rigorous scientific studies have cleared MSG of being the culprit, according to Kasabian, the cookbook author.

Even so, recognising that consumers remain suspicious of MSG, giant food companies such as Campbell Soup and Nestle are experimenting with ways to cut the salt content of food without losing flavour or adding more MSG. That can mean adding umami-rich ingredients such as cheese, mushrooms or anchovy powder, or using newly developed artificial flavours that do not contain MSG.

Marcus, the nutritionist, believes that foods with increased umami flavours could counteract the decline in the sense of taste that occurs with aging.

With a duller sense of taste, many older people lose interest in eating, opening the way to malnutrition, dangerous weight loss and vulnerability to disease.

And because umami flavours soften the bitterness of foods, some nutritionists think that an understanding of umami could lead to healthier diets for children, who have a heightened sensitivity to bitter flavours, leading to an aversion to many vegetables.

"No food is nutritious unless that food is eaten, and no food is eaten unless it tastes good," Marcus says. "And umami makes food taste good."

"Yummy is another way of saying it ... It's that salivating, lip-smacking character."