You know there's something funny about a taste when even those who spend their professional lives preparing, sampling and talking about food struggle to describe it. "Unctuous is probably quite a good description, but there's a sweetness, too, and a mouthfeel," ventures Heston Blumenthal, chef at the Fat Duck at Bray. "It's a richness, a rounding," is how Dr Peter Barham, a reader in physics at Bristol university who writes about the science of taste, puts it. Talk to a food technologist and words such as "savoury", "full-bodied" and "brothy" will crop up frequently. Even Kikunae Ikeda, the Tokyo scientist who first isolated the source of the taste, could only think to christen it "umami" which - exotic though it sounds to the untrained ear - means something close to "yummy" in Japanese.
Umami is the mysterious "fifth taste" - a flavour that has never seemed to fit into the existing categories of sweet, sour, salty and bitter. South-east Asian chefs have exploited it throughout history - it's an essential part of the taste of seaweed, among other things - and Ikeda identified it as long ago as 1908, demonstrating that the food substance usually responsible for stimulating the taste was glutamic acid; parmesan cheese, tomatoes, mushrooms and peas are loaded with it. And yet, until last week, scientists' attempts to develop Ikeda's insights - and, crucially, to find out whether or not umami really is a distinct taste - had progressed at an excruciatingly slow pace.
"We can't even be sure that we're having the same experience when we taste it," says Barham. "We all know what 'sweet' means, because we're used to tasting sugar, and we all know what 'bitter' means if we drink gin and tonic. But there's no one thing you can put in your mouth and say 'that's umami'. Parmesan certainly has a different effect to cheddar when you put it on your pasta, but it's a combination effect - and it's virtually indescribable." The debate over umami has raged within the tiny global community of taste scientists for years, and, says Barham, goes to the heart of the question of what taste itself really is. He characterises the disagreements thus: "What defines a taste? What makes something discrete and different, and what defines a taste receptor on the tongue? Does umami have a discrete receptor or not?"
Now, though, it seems that much of the mystery may have been solved. Writing in next month's edition of the journal Nature Neuroscience, biophysicist Dr Nirupa Chaudhari and her colleagues at the University of Miami announce a breakthrough: elaborating on their earlier investigations into the taste buds of rats, the researchers have isolated a protein molecule present on the tongue which, uniquely, enables us to taste umami foods - and thus to savour a vintage parmesan or a ripe tomato. Known as taste-mGluR4, it's a version of a protein also found in the human brain. On the tongue, though, it is truncated - which explains why glutamic acid must be present in relatively high concentrations before umami can be detected.
In the kitchens of the Fat Duck at Bray, umami is viewed with a mixture of enthusiasm and suspicion. "From a restaurant point of view, we're seeing something of an umami explosion - people finding all the things that have umami and chucking them all into a dish together," says Blumenthal. The taste is evoked more subtly in his own dishes, but reaches its fullest expression in the Fat Duck's roast scallop and caramelised cauliflower with marinated cep and jelly of Oloroso sherry. "Caramelising the cabbage enhances the umami taste, and scallops taste of it too, but then the sherry jelly counterbalances it and brings the whole dish together."
Perhaps unfortunately, though, Britain is best acquainted with umami via the ubiquitous gloopiness of monosodium glutamate (MSG), the powdered flavour enhancer on which your local Chinese takeaway almost certainly relies. It gives a concentrated umami hit, but it's a controversial additive - several studies have claimed that up to 30% of the population react adversely to it, displaying symptoms including migraines, hives, asthma attacks and cardiac irregulaties. Britain's Institute for Food Research insists that it's safe, but many of the smarter Chinese restaurants now spurn it, looking to natural sources of the same flavouring instead. Christine Yau, of the Ming restaurant on Greek Street in central London, runs a school for professional Chinese chefs at which MSG is a dirty word.
"A lot of cooks have misused it, and so our students are not allowed to," says Yau. "MSG is not something that you need in cooking; it enhances and brings out the flavours of good foods, but if you use it with bad materials it just makes things taste worse. We do use it occasionally at Ming, for dishes such as chicken and cashew nuts, which might otherwise be a bit bland. But the same taste occurs naturally in some kinds of vegetables, such as pak-choi cabbage and green leaves. It's a unique taste; there's a hint of sweetness there, but it's not just sweet."
There is certainly something strangely potent about the taste. Snack manufacturers have long been criticised for adding MSG to their products, at least partly because it seems to have the curious power to encourage further consumption, almost against the purchaser's will. It is being used experimentally to administer food to people suffering from cancer who lose their appetites entirely; Japanese studies of babies' facial expressions show marked signs of exceptional pleasure when they taste it.
Mysterious and exotic as umami seems, though, it is actually one of the tastes about which we know a relatively large amount: the taste receptors for sweetness and bitterness are still unknown. "It's in the west, alone, that umami has only been taken seriously in the past two or three years," says Barham. "The entire eastern world has known about it for centuries."