In the port city of Yokohama, south of Tokyo, there is a museum devoted entirely to noodle soup. It may be Japan's favourite foodie day out: one and a half million ramen fans visit the museum every year, and even on the wintry morning that I went the queue wound 50 yards down the street - young couples, mainly: cold, hungry and excited.

Inside the Yokohama Ramen Museum and Amusement Park they meet exhibitions on the evolution of soup bowls and instant noodle packets - more fascinating than you'd think, but these are not the main event. That's deep in the basement, where there's an entire street, done up to look like a raucous 1950s Yokohama harbour-front. Every shop houses a different noodle restaurant, each a clone of one of the best noodle shops of Japan. It's a culinary Madame Tussauds.

The Japanese are sentimental about their noodle soup - it's the working-class food that nourished the nation in the bleak days after World War Two. Ramen chefs are TV celebs, in a country that devotes more broadcast time to cookery than even we do. I asked the young pilgrims just what they valued above all in ramen. They sniffed the tangy air, Bisto-kid style: 'The basis of the experience is the broth,' was the consensus. In the great Japanese cod-Western Tampopo - the only movie to take noodle soup, sex and death with equal seriousness - a ramen guru announces that the key to Japan's national dish is that 'the soup must animate the noodles'.

What does chiefly animate Japanese soups and broths is an amino acid called glutamate. In the best ramen shops it's made naturally from boiling dried kombu seaweed; it can also come from dried shrimp or bonito flakes, or from fermented soy. More cheaply and easily, you get it from a tin, where it is stabilised with ordinary salt and is thus monosodium glutamate.

This last fact is of little interest to the Japanese - like most Asians, they have no fear of MSG. And there lies one of the world's great food scare conundrums. If MSG is bad for you - as Jeffrey Steingarten, the great American Vogue food writer once put it - why doesn't everyone in China have a headache?

To begin to answer this we must go back to Japan a century ago. Professor Kidunae Ikeda comes home from the physics faculty at the Tokyo Imperial University and sits down to eat a broth of vegetables and tofu prepared by his wife. It is - as usual - delicious. The professor, a mild, bespectacled biochemistry specialist, turns to Mrs Ikeda and asks - as spouses occasionally will - what is the secret of her wonderful soup. Mrs Ikeda points to the strips of dried seaweed she keeps in the store cupboard. This is kombu, a heavy kelp. Soak it in hot water and you get the essence of dashi, the stock base of the tangy broths and consommés the Japanese love.

This is the professor's 'Eureka!' moment. Mrs Ikeda's kombu is to lead him to a discovery that will make his fortune and change the nature of 20th-century food. In time, it would bring about the world's longest-lasting food scare, and as a result, kick-start the age of the rebel consumer. It was an important piece of seaweed.

Professor Ikeda was one of many scientists at the turn of the century working on the biochemical mechanics which inform our perception of the world. By 1901 they had drawn a map of the tongue, showing, crudely, the whereabouts of the different nerve endings that identify the four accepted primary tastes, sweet, sour, bitter and salty.

But Ikeda thought this matrix missed something. 'There is,' he said, 'a taste which is common to asparagus, tomatoes, cheese and meat but which is not one of the four well-known tastes.' He decided to call the fifth taste 'umami' - a common Japanese word that is usually translated as 'savoury' - or, with more magic, as 'deliciousness'. By isolating umami, Ikeda - who had picked up some liberal notions while studying in Germany - hoped he might be able to improve the standard of living of Japan's rural poor. And so he and his researchers began their quest to isolate deliciousness.

By 1909 the work on kombu was complete. Ikeda made his great announcement in the august pages of the Journal of the Chemical Society of Tokyo. He had isolated, he wrote, a chemical with the molecular formula C5H9NO4. This and the substance's other properties were exactly the same as those of glutamic acid, an amino acid produced by the human body and present in many foodstuffs. When the protein containing glutamic acid is broken down - by cooking, fermentation or ripening - it becomes glutamate.

'This study,' concluded Professor Ikeda in triumph, 'has discovered two facts: one is that the broth of seaweed contains glutamate and the other that glutamate causes the taste sensation "umami".'

The next step was to stabilise the chemical. This was easy: mixing it with ordinary salt and water made monosodium glutamate - a white crystal soluble in water and easy to store. By the time he published his paper, the professor had, wisely, already patented MSG. He began to market it as a table condiment called Aji-no-moto ('essence of taste') that same year.

It was an instant success, and when Kidunae Ikeda died in 1936 he was a rich man: he remains, as every Japanese schoolchild knows, one of Japan's 10 greatest inventors. The food chemicals giant Ajinomoto Corp, now owned by General Foods, pumps out a third of the 1.5 million tons of monosodium glutamate we eat every year - from India to Indonesia 'Ajinomoto' means MSG.

Ikeda's original paper muses a little about MSG and why it should excite the taste buds so, without arriving at any convincing conclusion. Much more work has been done since. We now know that glutamate is present in almost every food stuff, and that the protein is so vital to our functioning that our own bodies produce 40 grams of it a day. Probably the most significant discovery in explaining human interest in umami is that human milk contains large amounts of glutamate (at about 10 times the levels present in cow's milk). Babies have very basic taste buds: it's believed that mother's milk offers two taste enhancements - sugar (as lactose) and umami (as glutamate) in the hope that one or other will get the little blighters drinking. Which means mothers' milk and a packet of cheese'n'onion crisps have rather more in common than you'd think.

When you next grate parmesan cheese onto some dull spaghetti, what you will have done in essence is add a shed-load of glutamate to stimulate your tongue's umami receptors, thus sending a message to the brain which signals (as one neuro-researcher puts it) 'Joy and happiness!' Supper is rescued - and your system has added some protein and fats to a meal that was all carbohydrate.

Ripe cheese is full of glutamate, as are tomatoes. Parmesan, with 1200mg per 100 grams, is the substance with more free glutamate in it than any other natural foodstuff on the planet. Almost all foods have some naturally occurring glutamate in them but the ones with most are obvious: ripe tomatoes, cured meats, dried mushrooms, soy sauce, Bovril and of course Worcester sauce, nam pla (with 950mg per 100g) and the other fermented fish sauces of Asia.

Your mate, Marmite, with 1750mg per 100g, has more glutamate in it than any other manufactured product on the planet - except a jar of Gourmet Powder straight from the Ajinomoto MSG factory. On the label, Marmite calls it 'yeast extract'. Nowhere in all their literature does the word 'glutamate' appear. I asked Unilever why they were so shy about their spread's key ingredient, and their PR told me that it was because it was 'naturally occurring ... the glutamate occurs naturally in the yeast'.

As they put monosodium glutamate into production, Professor Ikeda and his commercial partners found that making stable glutamate from the traditional seaweed and salt was unnecessary. They developed a much simpler and cheaper process using fermented molasses or wheat - eventually manufacturers realised that almost any protein can be broken down to produce it.

The product took off, immediately, and within a few years Ajinomoto (which was now the company's name) was selling MSG across Asia. The breakthrough to America came in the aftermath of World War Two. Like pizza and vermouth, MSG was a taste American soldiers brought home with them. They weren't aware that MSG was what they'd liked in Japan - but the US Army catering staff noticed that their men enjoyed the leftover ration packs of the demobilised Japanese Army much more than they did their own, and began to ask why.

MSG arrived in America at a key moment. Mass production of processed food was booming. But canning, freezing and pre-cooking have a grave technical problem in common - loss of flavour. And MSG was a cheap and simple additive that made everything taste better. It went into tinned soups, salad dressings, processed meats, carbohydrate-based snacks, ice cream, bread, canned tuna, chewing gum, baby food and soft drinks. As the industry progressed, it was used in frozen, chilled and dehydrated ready meals. MSG is crucial in no-fat or low-fat food, where natural flavour is lost with the extraction of oils. It's now found in cosmetics, pharmaceuticals, and dietary supplements.

Ajinomoto Corp started manufacturing in the States in 1956 and in 1962 allied itself with Kellogg's. MSG sells in the States in supermarkets, under the brand Ac'cent. In Britain you will have to visit a Chinese supermarket for a supply of pure Gourmet Powder, but MSG plays a role - often in secret - in products on almost every shelf of the supermarket.

But MSG's conquest of the planet hit a major bump in April 1968, when, in the New England Journal of Medicine, a Dr Ho Man Kwok wrote a chatty article, not specifically about MSG, whose knock-on effects were to panic the food industry. 'I have experienced a strange syndrome whenever I have eaten out in a Chinese restaurant, especially one that served northern Chinese food. The syndrome, which usually begins 15 to 20 minutes after I have eaten the first dish, lasts for about two hours, without hangover effect. The most prominent symptoms are numbness at the back of the neck, gradually radiating to both arms and the back, general weakness and palpitations...'

And so was born Chinese restaurant syndrome (CRS) and a medico-academic industry dedicated to the researching and publicising of the dangers of MSG - the foreign migrant contaminating American kitchens. Shortly after Dr Ho came Dr John Olney at Washington University, who in 1969 injected and force-fed newborn mice with huge doses of up to four grams/kg bodyweight of MSG. He reported that they suffered brain lesions and claimed that the MSG found in just one bowl of tinned soup would do the same to the brain of a two-year-old.

Other scientists were testing MSG and finding no evidence of harm - in one 1970 study 11 humans ate up to 147 grams of the stuff every day for six weeks without any adverse reactions. At the University of Western Sydney the researchers concluded, tersely: 'Chinese restaurant syndrome is an anecdote applied to a variety of postprandial illnesses; rigorous and realistic scientific evidence linking the syndrome to MSG could not be found.'

Science has still not found a convincing explanation for CRS: indeed, some researchers suggest it may well be to do with the other things diners have imbibed there - peanuts, shellfish, large amounts of lager. Others say that fear of MSG is a form of mass psychosis - you suffer the symptoms you've been told to worry about.

The fact is that, since the eighties, mainstream science has got bored of MSG. Some research continues; in 2002, for example, New Scientist got very excited over a report that MSG might damage your eyesight, after Japanese scientists announced that they had produced retinal thinning in baby rats fed with MSG. It turned out they were putting 20 grams of MSG in every 100g of rat food - an amazing amount, given that, in the UK, we adults consume about four grams of it each a week. (One project took people who were convinced their asthma was caused by MSG and fed them up to six grams of it a day, without ill-effects). However, at no time has any official body, governmental or academic, ever found it necessary to warn humans against consuming MSG.

But popular opinion has travelled - spectacularly - in the opposite direction to science. By the early eighties, fuelled by books like Russell Blaylock's Excitotoxins - The Taste That Kills, MSG's name was utter mud. Google MSG today, and you'll find it blamed for causing asthma attacks, migraines, hypertension and heart disease, dehydration, chest pains, depression, attention deficit disorder, anaphylactic shock, Alzheimer's and Parkinson's diseases and a host of diverse allergies.

Thus since 1968 the processed food industry has had its own nasty headache as a result of MSG. Hundreds of processed products would have to be withdrawn if amino-acid based flavour-enhancers could not be used. They would become, simply, tasteless. By the 1980s a third of all Americans believed it was actively harmful. Crisp-buying teenagers thought MSG made them stupid and spotty. Mothers read that MSG could put holes in their children's brains.

So the food industry employed its usual tactic in the face of consumer criticism: MSG was buried by giving it new names. The industry came up with a fabulous range of euphemisms for monosodium glutamate - the most cheeky of all is 'natural flavourings' (however, the industry did remove MSG from high-end baby foods).

Nowadays the industry's PR beats a big drum. 'Natural, Tasty, Safe' is the slogan. 'Many people believe that monosodium glutamate is made from chemicals. Monosodium glutamate is a chemical in the same way that the water we drink and the oxygen we breathe are chemicals,' explains an MSG website.

MSG manufacturers are now pushing it as actively useful for health - a way to eat less salt - and they have pursued the celebrity route too. Heston Blumenthal, of the Fat Duck in Bray, is among the eminent chefs the industry has enlisted for promotion of the umami principle at conferences across the world - although he uses traditional sources like kombu.

It's not surprising that the MSG-makers are so busy on their product's image, because MSG-phobia still shows no signs of subsiding. This despite the fact that every concerned public body that ever investigated it has given it a clean bill of health, including the EU, the United Nations food agencies (which in 1988 put MSG on the list of 'safest food additives'), and the British, Japanese and Australian governments.

In fact, every government across the world that has a food licensing and testing system gives MSG - 'at normal levels in the diet' - the thumbs-up. The US Food and Drug Administration has three times, in 1958, 1991 and 1998, reviewed the evidence, tested the chemical and pronounced it 'genuinely recognised as safe.

However, there remains a body of respected nutritionists who are sure MSG causes problems - especially in children. And parents listen. Most doctors who offer guides to parents qualify their warnings about MSG - it may cause problems, it has been anecdotally linked with disorders. But public figures like the best-selling nutrition guru Patrick Holford are powerful advocates against MSG. He's sure the science shows that MSG causes migraines and he is convinced of the dangers of the substance to children, particularly in the child-grabber snacks like Monster Munch and Cheesy Wotsits .

'I'm a practitioner and there's no doubt that kids with behavioural problems react to MSG,' he says. 'I've given them the foods, and seen the different reactions. Glutamate is a brain stimulant in the way that it is given, because it enhances sensory perception in the sense that things taste much better - and some kids become very hyperactive.'

Holford admits that he has not measured this hyperactivity, or tested MSG by itself on children - his statements are based on anecdotal comparison of the effects of plain crisps versus flavoured ones. But there is some justice in his complaint that in all the acres of research on MSG, 'most is directed at the possible physiological effects, not the behavioural ones'.

Eric Taylor, professor of child and adolescent psychiatry at King's College in London, is among the leading British experts on food additives and children's behaviour. He was a pioneer of 'elimination tests' that examined food additives and their effect on children - establishing, for one, that the colouring tartrazine did contribute to hyperactivity.

Yet he does not think MSG is a culprit and he has never tested it. Why? 'There are so many substances, and there's not much funding. And, with MSG, there's no reasonable physiological theorem to justify the research.' The only investigation he has seen on children's brains and MSG, conducted in the seventies, suggested that the substance might improve reading ability.

Patrick Holford, like many of MSG's foes, also talks of its possible addictive properties and he cannot explain why 'natural' glutamate, say in cheese or parma ham, should be any less addictive, or harmful, than glutamate that's been industrially produced and stabilised with salt.

The anti-additive movement (check out the excellent and informative www.truthinlabeling.org) admits that 'natural' and 'industrially produced' glutamate are chemically the same, and treated by the body similarly. So why doesn't anyone ever complain of a headache or hyperactivity after a four cheese and tomato pizza (where there's easily as much glutamate as in an MSG-enhanced chicken chow mein)?

Their answer is that the industrial fermentation process introduces contaminants. This is possible, of course, but it ignores the fact that whole swaths of the planet - including East Asia, where I live - do not have any problem with MSG. Here in Thailand, the phong chu rot sits on the table with the fish sauce and the chilli powder where you would have the salt and pepper.

MSG has had one unarguable effect on us - and it is a benign one. It has made consumers look at the small print. In turn this kick-started the organic food movement and other, more militant consumer power groups. 1968 was a good year for rebels, and the dawn of MSG-phobia coincides with the beginning of a great shift in middle-class consumers' thinking - a withdrawal of our faith in the vast corporations that fed and medicated us. After 1968 we began to question them and their motives. Friends of the Earth and Greenpeace came next.

It is now 37 years since Dr Ho Man Kwok named Chinese restaurant syndrome, and it's plain that the case against MSG remains unproven. So either you conclude, as some will, that government, science and the mega-corporates of the food industry really are all in league with each other to poison us for profit. Or, like me, you make a different decision.

Now, I have little faith in the food industry and I'm as suspicious of food additives as the next person - I spend many hours fighting the grim battle to keep them from my children's mouths. But until new evidence emerges I am going to give MSG a conditional discharge. But would I have it in the kitchen? Well, I did. I bought a little bag of Ajinomoto from the corner shop on our Bangkok street and tried it, a gram (the tip of a teaspoon) at a time.

By itself it tasted of almost nothing. So I beat up and fried two eggs, and tried one with MSG, one without. The MSG one had more egg flavour, and didn't need any salting. I tried the crystals on my son's leftover pieces of chicken breast (definitely more chickeny). I tried it in a peanut butter sandwich (nothing). On Weetabix with milk (interesting, sort of malty) and on Weetabix with milk and sugar (thought I was going to be sick).

My friend Nic came round. He told me about a Japanese restaurant he'd been to that gave him headaches and a 'weird tingling in the cheeks' - until he told them to stop with the MSG. Then he was fine, he said. I nodded and I served him two tomato and chive salads; both were made using the very same ingredients but I told him one plate of tomatoes was 'organic', the other 'factory-farmed'. The organic tomatoes were far better, we agreed. These, of course, were the tomatoes doused with mono sodium glutamate.

Then we ate mascarpone, parma ham and tomato pizza. Nic felt fine. So did I. I had ingested, I reckoned, a good six grams of MSG over the day, and probably the same again in free glutamate from the food - the equivalent of eating two 250g jars of Marmite.

I've thrown the Ajinomoto out now. It works, but it was embarrassing - a bit like having a packet of Bisto in the cupboard. There is no need to have MSG in the kitchen. If I want extra glutamate in my food I'll use parmesan, or tomato purée, or soy sauce. Or like Mrs Ikeda, boil up some kelp.

So you think you don't eat MSG? Think again...

Some of the names MSG goes under

monopotassium glutamate
glutavene
glutacyl
glutamic acid
autolyzed yeast extract
calcium caseinate
sodium caseinate
E621 (E620-625 are all glutamates)
Ajinomoto, Ac'cent
Gourmet Powder

The following may also contain MSG natural flavours or seasonings
natural beef or chicken flavouring
hydrolyzed milk or plant protein
textured protein
seasonings
soy sauce
bouillon
broth
spices

Free glutamate content of foods (mg per 100g) roquefort cheese 1280
parmesan cheese 1200
soy sauce 1090
walnuts 658
fresh tomato juice 260
grape juice 258
peas 200
mushrooms 180
broccoli 176
tomatoes 140
mushrooms 140
oysters 137
corn 130
potatoes 102
chicken 44
mackerel 36
beef 33
eggs 23
human milk 22

For more on the MSG debate visit: www.truthinlabeling.org, www.msgmyth.com, www.msgtruth.org or www.food.gov.uk.