Overview and Future Prospects of Studies on Kokumi Substances

This chapter provides an overview of studies on kokumi substances. A kokumi substance is defined as a substance that modifies basic tastes and flavors, such as complexity, mouthfulness, and continuity (lingeringness), when added to foods, although it is tasteless itself at the dose tested. The first kokumi substance to be reported was glutathione (γ-Glu-Cys-Gly) in garlic. Other γ-glutamyl peptides were isolated from various foods, such as edible beans and cheeses, and identified as kokumi substances. Kokumi γ-glutamyl peptides are perceived by the calcium-sensing receptor (CaSR) in humans. Screening using CaSR enabled the detection of the potent kokumi peptide, γ-Glu-Val-Gly. The effects of glutathione and γ-Glu-Val-Gly on the sensory characteristics of foods and satiety have also been discussed. Rodents exhibited a preference for kokumi peptides, such as glutathione and γ-Glu-Val-Gly, and these peptides enhanced nerve responses when added to umami solutions. The findings of functional magnetic resonance imaging revealed that the addition of glutathione to a salty/umami solution significantly activated the left ventral insula, which plays a role in processing the presence of a taste stimulus as well as its corresponding pleasantness in humans. Other kokumi substances, such as α-peptides, which are lipid-related compounds, are summarized in this chapter. Although some of the molecular mechanisms underlying the perception of kokumi substances have been elucidated, further studies in the field of physiology, neurobiology and molecular biology, and food science are needed to obtain a more detailed understanding of kokumi substances.