Umami and koku: Delving into the nuances of savoury sensations

When it comes to taste, the world has long been familiar with the basic quartet: sweet, salty, sour, and bitter. ​

However, in the early 20th century, a fifth key taste, umami, was recognised, and more recently we have been introduced to the concept of ‘koku’. But what are the subtle differences between these two concepts and how do they impact the gastronomic world?

Umami: The fifth taste​

It was Dr. Kikunae Ikeda of Tokyo University who first identified this taste. When visiting Leipzig, Germany, he found a taste in asparagus, tomatoes, cheese and meat – food which was not available in Japan at that time – which reminded him of kombu-dashi. Back in Japan Ikeda started to search for the molecule responsible for this taste and identified that it was L-glutamic acid, an amino acid. In 1909 he named the taste ‘umami’ from the adjective ‘umai’, which means delicious, nice, palatable, or savoury.¹​

Kombu-dashi is important in the Japanese kitchen. Much like bouillon is in the French cuisine, it forms the base for soups and sauces. Besides the seaweed kombu, which is rich in glutamic acid, it often contains dried bonito and/or shiitakes. Japanese chefs and home-cooks have known for centuries that this combination of ingredients have a synergistic effect on taste, without having knowledge about the chemistry behind it.

In 1918, Shintaro Kodama, a protégé of Ikeda discovered that disodium inosine 5'-monophosphate (5’IMP) was the molecule in bonito responsible for the umami taste, when combined with glutamic acid from kombu. It was not until 1960 that Akira Kuninaka discovered that disodium guanosine 5’-monophosphate (5’GMP) had a similar effect on taste.²​ It was later found in all kinds of food, but especially in shiitakes. 5’IMP and 5’GMP, also known as ribonucleotides, originate from RNA and DNA present in all cells, from plants, animals, humans and microorganisms.

The synergy of glutamic acid and nucleotides is not unique to the Japanese kitchen. Looking at Italian cuisine, one can experience the full taste from tomatoes and cheese, and other food rich in glutamic acid, in combination with meat, fish or mushrooms, rich in nucleotides.

Shizuko Yamaguchi studied the synergy between glutamic acid and other amino acids and nucleotides and developed the renowned Yamaguchi equation.³​ The nucleotides make the binding of glutamic acid to the umami receptor stronger and this provides a lingering umami taste.⁴​

Koku: Beyond just a taste​

Like umami, ‘koku’ is also a term derived from Japanese culinary lexicon. However, it doesn't just describe a basic taste. Instead, it refers to a depth, richness, or full-bodied flavour and mouthfeel in food. While umami was studied mainly in the 20^th​ century, koku is continues to be studied extensively. Although every child in Japan learns the meaning of the word koku, there is still a lot to be discovered scientifically.

While umami is mostly about the chemistry of glutamates and nucleotides, koku is more multifaceted. It is influenced by the balance of tastes, aroma, and even the textural sensation in the mouth. The definition of koku, as defined by Japanese scientists is complexity, mouthfullness, and lingering.⁵​

Techniques that provide koku are ageing, fermenting, and slow-cooking, which develop complex flavours over time. Think of the deep flavours in aged wines, preserved meats, fermented cheeses, or slow-cooked broths and stews. By these processes, larger molecules are broken down into smaller, more volatile molecules which reach thousands of different receptors in the nasal cavity. But these processes also provide di- and tri-peptides, which react with calcium-sensing receptors on the tongue giving the koku sensation. L-glutathione is one of these molecules.⁶​

Umami and koku: Siblings not twins​

Umami and fat give mouthfullness and lingering effects, two of the three koku attributes. Koku is therefore supported by umami and fat, but is provides more complexity, depth, and richness. In essence, umami is often a component of koku, but not all umami-rich foods have a pronounced koku. Moreover, koku-rich food without umami also exists, like wine, beer and butter.

The recognition of umami expanded the culinary worlds horizon, offering new dimensions to the art and science of flavour. Koku, while not as globally recognised as umami, brings another layer to this understanding, emphasising the importance of depth, complexity, texture and continuity in flavours. As the gastronomic world continues to evolve, these terms will play an instrumental role in the way we continue to perceive and appreciate food.

Find out more about Mitsubishi Corporation Life Sciences​ Europe’s umami and koku solutions.

References​

1.​ Takashi Yamamoto, Department of Nutrition, Faculty of Health Science, Kio University, Nara, Japan (Book: Koku in Food Science and Physiology 2019)
2.​ Kuninaka, A. Studies on taste of ribonucleic acid derivatives. J. Agric. Chem. Soc. Jpn. 34, 487–492 (1960).
3.​ Yamaguchi, S., Yoshikawa, T., Ikeda, S. & Ninomiya, T. Measurement of the relative taste intensity of some L-amino acids and 5′-nucleotides.​ J. Food Sci. 36, 846–849 (1971).
4.​ Zhang, F. B. et al. Molecular mechanism for the umami taste synergism.​ Proc. Natl. Acad. Sci. USA 105, 20930–20934 (2008).
5.​ Book: Koku in Food Science and Physiology, Recent Research on a Key Concept in Palatability, by Toshihide Nishimura and Motonaka Kuroda, ISBN 978-981-13-8452-3 (2019)
6.​ Takeaki Ohsu et al. (2010). Involvement of the Calcium-sensing Receptor in Human Taste Perception.​ Mechanisms of Signal Transduction. Vol. 285, 2.