Why Ajinomoto Tastes So Good: Examining Umami Perception, Synergy, and the Rumors About Its Safety

According to the program, human taste was once thought to consist of four types: sour, sweet, salty, and bitter. Kikunae Ikeda challenged this. In 1908, Ikeda identified glutamic acid in kombu broth as the component responsible for a new taste, umami. The program explains that this led to the proposal that there are five tastes.

The main ingredient of commercially sold Ajinomoto is the sodium salt of this glutamic acid. The program notes that the human tongue has multiple receptors for glutamate, and states that umami physiologically exists as a taste.

On the other hand, bad rumors about Ajinomoto never seem to go away. The program cites "Chinese restaurant syndrome" as the prime example. The term refers to a set of unpleasant symptoms occurring after eating Chinese food, first reported in 1968. Symptoms listed included weakness, nausea, drowsiness, headache, numbness, and palpitations, and the monosodium glutamate in Ajinomoto was suspected as the cause.

The story appeared in 1968 in the journal The New England Journal of Medicine, and then in The New York Times, becoming perfect fodder for media coverage and attracting a great deal of attention. Although this happened more than 50 years ago, the program points out that many people still believe it.

However, according to the program, the rumor attracted the interest of many scientists, and numerous clinical studies were conducted. As a result, the symptoms were found not to be reproducible, and the claim was deemed to have no scientific basis. The program describes it as a story deserving of the label "hoax," created by the power of belief and the power of mass media.

So how is umami perceived? When monosodium glutamate dissolves in water, it dissociates into glutamate ions and sodium ions. It is the glutamate ions that produce umami: they activate multiple receptors on the tongue, and the umami signal is transmitted to the brain.

What the program highlights as interesting is the transmission pathway. Generally, each sense is conveyed to the brain through a specific nerve: vision by the optic nerve, smell by the olfactory nerve, and hearing by the vestibulocochlear nerve. Taste, by contrast, has many pathways. Taste information received at the front of the tongue and the soft palate is carried to the brain by the facial nerve, information received at the back of the tongue by the glossopharyngeal nerve, and information received in the pharynx by the vagus nerve. The program explains that this is thought to stem from the wide area in which taste can be detected, and puts it this way: the tongue seems determined to deliver the taste and umami of food to the brain no matter what.

What the program singles out as noteworthy in how umami works is its synergistic effect. Glutamate produces umami on its own, but it is known that umami is enhanced when it is consumed together with inosinate, found in katsuobushi (dried bonito), or guanylate, found in dried shiitake mushrooms.

Commercially sold Ajinomoto contains 1.25% each of sodium inosinate and sodium guanylate. The program praises this as a truly excellent formulation in terms of synergy.

The program then points out that the Japanese have long made use of this synergy: Japanese dashi, made from kombu and katsuobushi. It is a technique for drawing out the synergy between two umami components, glutamate from kombu and inosinate from katsuobushi, and the program says it is astonishing how the Japanese, in their culinary curiosity, perfected this broth before science proved the existence of umami.

The program also touches on rumors other than Chinese restaurant syndrome. One is the claim that because glutamate is a neurotransmitter, consuming Ajinomoto causes brain dysfunction. In response, the program explains that glutamate barely passes through the blood-brain barrier, the brain's protective barrier, so at normal levels of intake glutamate does not reach the brain.

The other is the blindness claim. It is based on an experiment in which mice were injected subcutaneously with 4–8 g of monosodium glutamate per kilogram of body weight, resulting in retinal damage accompanied by nerve cell death within a few hours. The program says this result must be interpreted with great caution, and gives two reasons. First, the method of administration was subcutaneous injection, not oral consumption by eating. Second is the dose: converted to a human weighing 70 kg, it corresponds to 280–560 g, an amount that would never be reached through a normal diet. The program's conclusion is therefore that applying this result directly to humans and treating Ajinomoto as dangerous cannot be called scientific.

There is also a claim that Ajinomoto is made from petroleum, which the program dismisses as simply a hoax. It is true that an organic synthesis method for glutamic acid using acrylonitrile as a starting material has been established. However, for cost reasons it is no longer used, and commercially sold Ajinomoto is produced by fermentation using microorganisms, with raw materials such as sugarcane molasses.

Regarding safety, the program notes that a large-scale review was conducted in 1987. No specific toxicity or carcinogenicity was found, and no harmful effects were identified in reproductive and teratogenicity tests either. Based on these results, the current status is that the acceptable daily intake of monosodium glutamate is "not specified." The program says it is a perfectly safe seasoning when used normally, and, regarding the endless conspiracy theories, says it can only be dismayed at the low scientific literacy of some people.

Why does the human brain process glutamate as umami? Here the program introduces one hypothesis. Umami components, glutamate among them, are abundant in well-aged meat and cooked meat. This leads to the idea that umami may function as a sensor for identifying easily digestible meats. The program considers this a reasonable guess, since fresh meat contains few umami components.

The program further extends the speculation: if this hypothesis is correct, umami may be a product of evolution arising from a desire for fermented foods, the maximization of nutritional efficiency, and probiotic-like benefits from consuming live microorganisms. This is presented strictly as a hypothesis.

If umami draws people in, many will be concerned about weight gain. The program describes a "strange duality" on this point.

On one hand, there are reports pointing toward increased food intake. In a study of head and neck patients who had undergone radiation therapy and developed taste disorders including loss of umami, monosodium glutamate was reported to increase appetite and satisfaction. There are also results showing that adding monosodium glutamate to the food of elderly people who had lost their appetite increased their intake of that food. Another report found that adding it to the meals of patients who had lost weight due to illness increased the amount they ate and improved their nutritional status. From this perspective, the program says, Ajinomoto could be an enemy of dieting.

On the other hand, results have also shown that drinking a vegetable soup containing monosodium glutamate reduces the amount eaten at the subsequent meal. From this angle, Ajinomoto could be called a savior for dieters. The program merely says that interpretations of these results are likely to differ, and that an unknown mechanism may be hidden behind them.

The program's conclusion is that the umami Ajinomoto provides is something that will continue to captivate the human tongue and brain, and that it is at the same time an extremely safe seasoning that easily brings out the umami in cooking. Current science is not opposed to giving in to that temptation, and its safety is assured when used as an ordinary seasoning, the program concludes. On the other hand, the program does not answer why its effects on body weight appear in opposite directions.