Japanese taste sensations

01 December 2011

Whether you like sushi or not, the chemistry behind the important fifth taste, umami, and the green wasabi – which is usually fake – is fascinating.

Compared with our sense of smell, our sense of taste is blunt and quite high concentrations are needed for us to perceive a taste. When we have a cold we often lose our sense of smell and only have left the basic tastes of sweet, salty, sour and bitter, as well as a fifth taste that has given researchers a real headache – umami.

The Japanese researcher Kikunae Ikeda discovered as early as 1908 that the amino acid L-glutamine, which he had found in seaweed, produced a taste that could best be described as savoury. He named it umami after the Japanese words umai (delicious) and mi (taste). For almost a century, umami’s status as a separate taste or simply an amplifier of other tastes was hotly debated. Finally, in 2002, a group of researchers were able to present evidence of taste cells that can specifically recognise L-amino acids, and umami is today considered to be the fifth basic taste.

Sushi is a good example of how umami is used to create fantastic taste combinations. Despite the fact that the rice is really the important part, we usually associate sushi with thin slices of raw fish. Fish meat is of course muscle, and the fuel for the muscles is a molecule called adenosine triphosphate (ATP).

When the fish dies, the ATP is broken down to another molecule, inosine monophosphate (IMP).

IMP can also bind to the umami receptor and therefore contributes to the taste. The really exciting thing is that IMP and glutamic acid reinforce one another strongly and the combination gives a very intense umami taste. This is where the soy sauce comes in.

To make soy sauce, wheat, salt and soya beans are mixed, then mould fungi of the species Aspergillus oyzae and Aspergillus sojae are added and the mixture is left to ferment. When fermentation is complete, the solid matter is pressed out and the liquid, which is very rich in glutamic acid, is called soy sauce. When we dip the raw fish in the soy sauce – remember that dipping the rice is unforgivable – we get a perfect combination of glutamic acid and IMP and a much stronger taste. Umai!

Sushi is usually served with wasabi, Japanese horseradish. Wasabi comes from the plant Wasabia japonica, which grows wild in streams in the mountainous regions of Japan. It is primarily the root that is used, but the leaves also contain some sharp-tasting isothiocyanates. Many of these substances are anti-bacterial and it is not impossible that wasabi was originally used with fish that was slightly past its best.

Wasabi contains a number of substances that are not sharp-tasting themselves, but when the root is grated, preferably using a dried shark skin, they are converted into allyl isothiocyanate, which gives a sharp taste. The problem is that the wasabi paste is very sensitive and should be consumed within 15 minutes.

What is more, the actual root is expensive and does not travel well, which means that the little green blob on sushi trays in Sweden is instead a mixture of normal horseradish, mustard powder and green food colouring. The taste is quite close to that of real wasabi because both mustard and horseradish produce allyl isothiocyanate when they are grated or ground. However, real wasabi contains a number of ω-methyl allyl-isothiocyanates, which give a slightly more savoury taste.

So why do we experience a tingling in the sinuses if we get too much wasabi in the soy sauce? The answer is electrophiles. Electrophile means electron lover – that is to say, a chemical substance that is happy to take in electrons. The opposite is a nucleophile, a substance that has plenty of electrons that it is happy to share with a starved electrophile, and when these two react with one another, a new chemical compound is formed.

Both DNA and proteins are mainly nucleophiles and they easily react with electrophiles. The problem is that certain electrophiles can produce mutations that could even lead to cancer. In order to protect ourselves, we have developed a system to warn against electrophiles, based on a type of receptor called TRPA1 (transient receptor potential). In TRPA1 there is an extremely nucleophilic sulphur atom that forms a compound with most electrophiles – even harmless ones like those in wasabi. When the electrophile binds, the nerves send a pain signal and the body responds with tears and coughing.

A close relative of TRPA1 is TRPV1, which instead reacts to temperatures above 43°C. In addition, some chemical compounds can bind to TRPV1, which lowers the threshold and the brain is tricked into believing it is too hot. One example is the hot substance in chilli, capsaicin, which activates TRPV1 and makes us experience a pleasant warmth throughout the body.

Ginger is the root of a plant called Zingíber officinále, which is grown in large parts of the world. In the past it was used mostly in dried form as a Christmas spice, not least in Swedish mulled wine, but with the arrival of sushi in Swedish cuisine, we have grown used to eating it in thin slices as an accompaniment to the fish. Fresh ginger contains a substance called gingerol. Like capsaicin, gingerol affects TRPV1 receptors, but gingerol is 250 times weaker than capsaicin and only produces a pleasant heat sensation. When ginger is cooked or dried, two new substances are formed, zingerone and shogaol, which make the taste sharper. The ginger that is eaten with sushi is called gari and is simply thin slices marinated in a mixture of sugar and vinegar. The idea is that the mild heat of the ginger cleanses the palate between the sushi pieces.

If you want to experience a very special food sensation, at the same time as gambling with your life, pufferfish is the choice for you. The pufferfish, Takifugu rubripes, contains an incredibly strong neurotoxin called tetrodotoxin. Unlike most neurotoxins, which affect the connection between nerves and muscles, tetrodotoxin affects something known as sodium ion channels. When tetrodotoxin binds to these ion channels, intake of sodium ions is stopped, which makes nerve signals cease and muscles become paralysed. The first symptom, which usually occurs after half an hour, is numbness of the lips and tongue, followed by stomach problems and nausea. The blood pressure falls and soon arms and legs become numb. Gradually the respiratory muscles are paralysed and the patient dies of suffocation, yet patients often remain fully conscious right up to death.

There is no known antidote to tetrodotoxin, but with the help of ventilators, the death rate has been reduced from over 50 % to around 5 %.

By far the most common cause of tetrodotoxin poisoning is the dish fugu – thin slices of raw pufferfish. Nowadays only specially trained chefs, who know how to fillet the fish to avoid large quantities of the toxin, are allowed to prepare fugu. Not all parts of the fish are toxic, but even the edible parts contain low levels of tetrodotoxin and administered in the right dose produce a tickling sensation in the lips and tongue.

Pufferfish does not produce the toxin itself, but rather gets it from its food – primarily mussels. It is therefore quite possible to produce non-toxic fugu. The problem is that it is the danger, rather than the delicious taste, which makes many people want to pay large sums of money for a plate of attractively arranged fugu. A fugu without toxin is, after all, like a samurai without a sword.

“Sushi is the world’s best food! I am convinced of it.
It has everything – it’s tasty, healthy and beautiful to behold.”  - Toshiko Lee, who opened the first Japanese restaurant in Sweden in 1973.

- Ulf Ellervik, Professor of Bioorganic Chemistry, Lund University