Kokumi substances as taste modulators: sensory properties and molecular mechanisms

Abstract

Oral perception during food and beverage consumption results from the integration of multiple sensory processes and is inherently complex. Current knowledge of oral perception is primarily derived from studies of stimuli and compounds that evoke specific sensory modalities, including taste, mouthfeel-related somatosensory sensations, and chemesthesis. Difficulties are associated with defining and characterizing each sensation because they involve multiple chemosensory systems. This review focuses on kokumi substances, taste modulators that have been increasingly examined in recent years, and oral perception, which is now globally called kokumi, and is modulated by kokumi substances. Here, we review the concept of kokumi and studies on the functionality of kokumi substances, their chemical diversity, and the molecular mechanisms of their action. We mainly investigated oral perception modified by kokumi substances using γ-glutamyl kokumi peptides, which are well-known kokumi substances, and the molecular mechanisms of action mediated by the activation of the calcium-sensing receptor, a receptor for kokumi substances. Previous findings on kokumi substances other than γ-glutamyl peptides and other receptors that may be involved in this perception are also summarized. A more detailed scientific understanding of kokumi substances may contribute to improvements in food palatability and the development of foods with less salt, sugar, and fat, providing health benefits. This may also lead to the design of new foods that contribute to sustainability. Systematic investigations of kokumi substances, their perception, and their molecular mechanisms of action will drive future research directions and investigations.

1. Introduction

Oral perception during food and beverage consumption arises from the integration of multiple sensory processes, resulting in a highly complex perceptual experience. To date, the prevailing understanding of oral perception has been shaped largely by studies of stimuli and compounds that activate specific sensory modalities, including taste, mouthfeel-related somatosensory sensations, and chemesthesis. Difficulties are associated with defining and characterizing these sensations because each component causes a single sensation and activates multiple chemosensory systems. Taste is a sensation caused by taste stimuli evoked by taste substances and modulators, whereas smell is a sensation induced by olfactory stimuli elicited by aromatic substances. Texture is a somatosensory stimulus evoked by physical structures and substances that elicit tactile sensations and smoothness on the tongue of the consumer. Although the definition remains ambiguous and open to interpretation, mouthfeel is also used to describe the varied textural and physical sensations such as viscosity and other mechanical properties (Simons et al. 2019; Wolinska-Kennard et al. 2025). Complex combinations of these sensations contribute to oral perception and food deliciousness. For example, it is challenging to describe the taste of food using only 5 basic tastes: sweet, salty, sour, bitter, and umami (Beauchamp 2019). For example, the taste of sugar (sucrose) and the sweetener sucralose are the same in terms of sweetness, whereas their sensory characteristics differ markedly (DuBois et al. 2008; Prakash et al. 2014). They differ significantly in mouthfeel and sweetness intensity over time (Servant and Kenakin 2024). In addition, compared with sugar, sucralose lacks taste-related oral perceptions such as body and richness. Individuals exhibit a high preference for sugar (Beauchamp 2016). Compared with white sugar, brown sugar is longer-lasting and rich sweetness. Therefore, sweetness alone does not describe the oral perception of sugar; other complex perceptions, such as richness, mouthfulness, lastingness, and complexity, contribute to preferences and deliciousness. Similar complex oral perceptions are characteristic of foods such as honey, aged cheese, and well-cooked stews. The use of taste modulators that mimic complex oral perceptions is promising. Taste modulators increase taste intensity and enhance additional oral perceptions, such as complexity and richness (Deepankumar et al. 2019). The use of taste modulators is beneficial not only for sugar reduction but also for decreasing salt and fat intake, and foods that utilize taste modulators may contribute to the realization of a healthy society. This is a comprehensive review that focuses on kokumi substances, which are taste modulators that have been increasingly examined in recent years and modulate the complex oral perception now globally called kokumi (Simons et al. 2019; Yang et al. 2019; Kuroda 2024c; Ramesh et al. 2025). We reviewed the historical context, definitions deduced from common insights from previous studies, and the functionality of kokumi substances in food. We also introduced various types of kokumi substances reported to date and research findings on the molecular mechanisms underlying taste modifications through the activation of the calcium-sensing receptor (CaSR), as represented by γ-glutamyl kokumi peptides. Other findings and advances in kokumi substances, as well as the progression of research on kokumi substances acting on receptors other than CaSR, are discussed, and expected future developments are summarized.

2. Kokumi substances and kokumi

In recent years, the number of scientific studies on kokumi and kokumi substances has rapidly increased (Nishimura 2024; Kuroda 2024c; Ramesh et al. 2025). However, this concept concerning kokumi and kokumi substances remains unclear, and there is no international consensus. In this section, we will explain the origin of the term, its concept and definition, the history of research on kokumi that is widely understood, and the functions of kokumi substances by referring to previous studies on kokumi and kokumi substances.

The word “kokumi” is originally a combination of 2 Japanese words, “koku” and “mi”, with “mi” meaning taste in Japanese. The Japanese word “koku” is a popular term in Japan and is used in the packaging of numerous foods, such as beer, coffee, chocolate, ice cream, curry roux, and soy sauce, to describe the positive oral perceptions of food products. In Japanese, koku means strong, rich, concentrated, deep, complex, and a lasting feeling of continuity and lingeringness. Nishimura et al. recently proposed a scientific definition of koku perception as the overall sensation perceived because of enhancements in the following 3 sensory stimuli: taste, smell, and texture, resulting in feelings of complexity, mouthfulness, and lingeringness (Nishimura 2019, 2024). Yamamoto also proposed the following attributes of koku perception: thickness (concentration, amplitude, and strength), mouthfulness (the spread of a sensation throughout the mouth), continuity (long-lasting sensory effects, including an increase in the duration of the aftertaste), roundness (smoothness, balance, and harmony), depth (richness and complexity), and punch (impact and rapid increase) (Yamamoto and Inui-Yamamoto 2023). Among the various and complex koku perceptions, kokumi, which originally had the word “mi” meaning “taste” added to koku, only refers to the taste-related perception sensed in the oral cavity. To support this, several studies from various research groups now describe kokumi as complexity, thickness, mouthfulness, and continuity of taste.

Here, we first describe how the oral perception known as kokumi differs from the canonical 5 basic tastes. Taste perception involves 2 stages: sensation, in which taste receptors detect chemical stimuli, and perception, in which the brain integrates and interprets the sensory information. The 5 basic tastes—sweet, salty, sour, bitter, and umami—are directly detected by specific taste receptors located in the taste buds on the tongue, which are activated by tastants (Chaudhari and Roper 2010). For example, sweetness is mediated by the activation of T1R2/T1R3, umami by T1R1/T1R3, and bitterness by the activation of the T2R receptor family (Chandrashekar et al. 2000; Nelson et al. 2001, 2002; Zhang et al. 2003). The binding of tastants (taste molecules) to these receptors generates neural signals that constitute the primary taste sensation (Chandrashekar et al. 2006). In contrast, kokumi substances, such as glutathione (GSH: γ-Glu-Cys-Gly) and γ-glutamyl peptides, do not elicit any basic taste on their own; instead, they enhance or modulate the intensity, thickness, continuity, and mouthfeel of other taste stimuli (Ueda et al. 1997; Simons et al. 2019; Yamamoto and Inui-Yamamoto 2023; Kuroda 2024c; Nishimura 2024; Ramesh et al. 2025). These oral perceptions are perceived in combination with other tastes. Basic tastes are recognized independently and directly sensed on their own, and brain integration may not be essential (Roper and Chaudhari 2017). Therefore, basic tastes are classified as sensory sensations. In contrast, kokumi substances are not recognized as having a basic taste on their own, but when combined with other taste stimuli, they elicit oral perception changes, such as complexity, thickness, mouthfulness, and continuity, modulating the perceived taste intensity (Ueda et al. 1990; Dunkel et al. 2007; Toelstede and Hofmann 2009; Ohsu et al. 2010; Kuroda 2024b, 2024c). Therefore, kokumi is a perceptual change that occurs through integrated cognition in the brain. Therefore, kokumi can be classified as a type of oral perception. Kokumi is an oral perception that is distinct from taste and may be described as a complex, ambiguous, and diverse taste-related oral perception that can be explained using perceptual terms like thickness, mouthfulness, continuity, roundness, richness, lastingness, and complexity. Substances that impart and enhance these oral perceptions are known as kokumi substances (Table 1).

Table 1.

Definition and classification of taste and kokumi.

	Basic Taste	Kokumi
Classification	Sensation	Perception
Definition	Taste	Oral perception (taste-related)
Sensory characteristics (Qualities and Functions)	Sweet Salty Bitter Umami Sour	Imparts and enhances taste-related oral perceptions when mixed with tastants e.g. thickness, mouthfulness, continuity, complexity, richness, roundness, lastingness, mouth-coating, and the fatty orosensation
Sensory stimulation	May be detected directly	Requires other tastants
Recognition	May be perceived independently	Requires brain integration
Receptor	Sweet→T1R2/T1R3 Salty→ENaC, +? Bitter→T2Rs Umami→T1R1/T1R3 Sour→Otop1	Functions by activating the following receptor candidates: e.g. CaSR, GPRC6A, GPR92, and GPR120
Substance	Tastant ・Has its own basic taste. Sweet→sugars, sweetener Salty→NaCl Bitter→bitter compound, e.g. caffeine Umami→umami compound, e.g. glutamate Sour→acid, e.g. acetic acid	Kokumi substance (taste modulator) No taste on its own Modulates taste-related oral perceptions when mixed with tastants 1) Imparts and enhances oral perceptions e.g. thickness, mouthfulness, continuity, complexity, and richness 2) Modulating taste intensity ・Enhances taste intensity: Sweet, salty, umami (・suppresses taste intensity: Bitter, sour) e.g. γ-glutamyl peptides, amino acids, amino acid derivatives, peptides, fatty acids, fatty acid derivatives, volatile compounds, and Maillard products

We have long been interested in kokumi, a complex and pleasant oral perception delivered by the addition of garlic and other foods, and have researched substances that impart and enhance kokumi. Ueda et al. (1990) identified the first food components that elicit kokumi, GSH, from garlic extracts and reported them as kokumi substances. In this and several subsequent studies, Ueda et al. further defined kokumi substances as substances with no basic taste but that enhance taste-related oral perceptions, such as thickness, mouthfulness, and continuity, perceived in the oral cavity when added to basic taste solutions or foods (Ueda et al. 1994, 1997). Dunkel et al. (2007) reported the same GSH functionality. Therefore, kokumi substances are defined as taste modulators (Kuroda 2024c). Based on these findings, Hofmann's group at the Technical University of Munich identified several kokumi substances in other foods that exhibit similar taste-modulating functions (Dunkel et al. 2007; Toelstede et al. 2009). With the findings of these research groups, the functionality of kokumi substances, specifically their function in enhancing oral perception (taste and mouthfeel), such as thickness, mouthfulness, and continuity, has been globally recognized (Degenhardt and Hofmann 2010; Mittermeier et al. 2018; Simons et al. 2019; Yang et al. 2019; Li et al. 2025). In recent years, research interest in kokumi substances has increased, driven by the identification of these compounds in various foods, the discovery of their taste-modulating functions, and the publication of multiple reports on the receptors involved in their perception.

3. Functionality of kokumi substances (perceptual characteristics)

Kokumi substances are taste modulators that exert taste-modulating effects at concentrations that do not independently elicit any basic taste. The functionality of kokumi substances is mainly based on 2 taste-modulating effects: 1) imparting and enhancing oral perception and 2) modulating taste intensity, typically enhancing it, although it may occasionally suppress it. The functionality of kokumi substances is described below mainly using γ-glutamyl kokumi peptides, such as GSH and γ-glutamyl-Val-Gly(γ-EVG), which are the most studied kokumi substances, as examples (Ueda et al. 1997; Dunkel et al. 2007; Ohsu et al. 2010; Goto et al. 2016; Simons et al. 2019; Yang et al. 2019; Kuroda 2024c; Li et al. 2025).

The main function of imparting and enhancing oral perception includes various taste-related oral perceptions associated with broadly defined taste and mouthfeel perceived in the oral cavity. For example, the sensory expressions of these perceptions are mainly not only based on taste thickness, mouthfulness, and continuity (Ueda et al. 1990, 1997; Dunkel et al. 2007; Ohsu et al. 2010) but also include other attributes such as complexity, richness, density, body, roundness, depth, punch, impact, fatty, mouth coating, lastingness, persistence, and viscosity (Yamamoto 2019; Yamamoto and Inui-Yamamoto 2023; Kuroda 2024c; Nishimura 2024). The addition of kokumi substances such as γ-glutamyl kokumi peptides to simple basic taste solutions (Ueda et al. 1990, 1994, 1997; Dunkel et al. 2007; Ohsu et al. 2010), mixed basic taste solutions (Ohsu et al. 2010; Goto et al. 2016), and food products (Ueda et al. 1997; Dunkel et al. 2007; Miyaki et al. 2015; Kuroda 2024b), at concentrations which do not have any basic taste themselves, has been shown to enhance these perceptions. Although the imparted or enhanced oral perception depends on the compound and its concentration of the added kokumi substance, it is also affected by the sensory properties and composition of the food components to which it is added. Taste substances are necessary for kokumi to function as taste modulators in food. However, they are not limited to specific basic taste substances and function in foods containing various taste substances. γ-EVG (0.01%) was shown to increase the thickness when added to mixed basic taste solution of monosodium glutamate (MSG) (0.1%) and NaCl (0.5%) (Ohsu et al. 2010) and foods like chicken consommé soup (Miyaki et al. 2015), hamburger steak, orange-flavored drinks, low-fat peanut butter, low-fat French dressing, and low-fat custard cream (Miyamura et al. 2015b; Kuroda 2024b). In addition, oiliness (fatty orosensation) is enhanced even in foods with reduced fat content, such as low-fat peanut butter (Miyamura et al. 2015b). As described in more detail in the next section, other previously reported kokumi substances, such as amino acids, have also been shown to impart and enhance sensory attributes, including complexity, thickness, mouthfulness, continuity, and richness of food (Ohsu et al. 2010). Therefore, this function is regarded as the main function of kokumi substances. The enhancement of these oral perceptions by kokumi substances may contribute to the modification of taste intensity, as described in 2).

Regarding the other main function, 2) modulation of taste intensity, many kokumi substances enhance basic taste, particularly salty, umami, and sweet tastes, when added to basic taste solutions and foods. γ-EVG (0.01%) was shown to increase the taste intensity when added to simple basic taste solutions such as sweet (3.3% sucrose solution), umami (0.5% MSG solution), and salty taste solutions (0.9% NaCl solution), and mixed basic taste solutions with umami and saltiness each taste after 5s in the mouth. The addition of γ-EVG (0.01%) to a mixed basic taste solution with umami and saltiness (0.02% MSG + 0.02% IMP + 0.07% NaCl) significantly enhanced taste intensity (Ohsu et al. 2010 ; Kuroda 2024b). Dunkel et al. reported that the taste recognition threshold concentration of GSH in an aqueous solution was 3.1 mM (0.092%), at which only a faint astringent sensation was observed, with no basic taste. They also reported that when GSH was added to solutions of NaCl, L-Glu, or mixtures of NaCl and L-Glu, taste recognition threshold concentrations were lower than those in aqueous solutions (Dunkel et al. 2007 ). Although no significant effect was observed in a study by Ueda et al., in which 0.04% GSH was added to a simple basic taste solution (Ueda et al. 1997), several experiments using GSH at concentrations of 0.08% (Ohsu et al. 2010 ) and 0.16% (Goto et al. 2016), both at concentrations without a basic taste, have reported similar taste-enhancing effects when added to a mixed basic taste solution with umami and saltiness (0.02% MSG + 0.02% IMP + 0.07% NaCl or 0.18% MSG + 1.0% NaCl). These results indicate that GSH enhanced the taste intensity of basic taste substances at concentrations where GSH itself elicited no basic taste. Taste-enhancing effect of γ-EVG has been detected in foods such as umami chicken broths (Miyaki et al. 2015), hamburger steak (Kuroda 2024b), and the sweetness of orange-flavored drinks (Kuroda 2024b). Furthermore, γ-EVG enhances fatty orosensation (Miyamura et al. 2015b; Kuroda 2024b) and pungency (Kitajima et al. 2022), which are oral sensations that differ from the basic tastes, exerting a broad taste-enhancing effect. In experiments using animal models, Yamamoto and Mizuta (2022) demonstrated that the addition of γ-EVG to simple taste solutions at concentrations that did not affect the intake by rats resulted in increased intakes of sweet, umami, and lipid-containing solutions.

Similar functions have been reported for kokumi substances other than γ-glutamyl peptides. For example, L-arginine and L-lysine were found to have strong saltiness-enhancing functions at concentrations at which they are tasteless (Lee 1992; Ogawa et al. 2004). In addition, the amino acid derivative N-succinyl-L-amino acids has been reported to enhance saltiness and umami taste intensities (Huang et al. 2024c, 2025). Cai et al. also reported that N-lauroyl theanine, an amino acid derivative, enhanced saltiness, umami, and sweetness (Huang et al. 2024b; Cai et al. 2024c). Furthermore, in experiments using animal models, Yamamoto et al. (2025) demonstrated that L-ornithine (L-Orn), even at concentrations that did not elicit a preference, increased the intake and preference for umami, sweet, and NaCl solutions.

Some kokumi substances have been found to suppress bitterness and sourness. γ-EVG reduces the bitterness of bitter substances (Kitajima et al. 2019; Kitajima 2024) and sourness (Sato et al. 2022). N-L-lactoyl-L-Trp, which functions as a kokumi substance, has also been found to suppress the bitterness of several bitter substances (Wu et al. 2023; Huang et al. 2024b). Yamamoto et al. (2025) demonstrated that the addition of L-Orn reduced the aversion of rats to a solution containing the bitter substance quinine hydrochloride. These findings suggest that kokumi substances inhibit the bitterness and sourness. Ueda et al. (1997) reported that the addition of GSH (0.04%) to tartaric acid did not affect sourness intensity. Therefore, the taste-modulating functions of kokumi substances are suggested to depend not only on their chemical structure and concentration but also on the taste components present in the foods or samples to which they are added. Although this may also be inferred as a secondary effect of kokumi substances that enhance oral perception and taste, such as thickness, mouthfulness, and other basic tastes, such as sweetness and umami, limited information is available on other kokumi substances. Therefore, further research on masking functions is needed.

4. Diverse kokumi substances

Kokumi substances are classified into 6 main categories based on their chemical compositions (Fig. 1). The most well-known kokumi substances are γ-glutamyl peptides (Table 2) (Yang et al. 2019; Kuroda 2024b). The addition of γ-glutamyl peptides, which function as kokumi substances, to foods enhances taste-related oral perception, such as thickness, mouthfulness, and continuity. Previous studies reported that γ-glutamyl peptides were present in foods like plants, fermented foods, and seafoods (Dunkel and Hofmann 2009; Toelstede et al. 2009; Toelstede and Hofmann 2009; Kuroda et al. 2012; Kuroda et al. 2013; Miyamura et al. 2015a; Phewpan et al. 2019; Kuroda et al. 2020; Heres et al. 2023b). These compounds may contribute to the oral perception of kokumi in foods containing γ-glutamyl peptides. The kokumi substance GSH is a γ-glutamyl peptide that has been isolated from garlic extracts and onion (Ueda et al. 1990, 1994, 1997). Hoffman and colleagues also identified various γ-glutamyl peptides in foods that function as kokumi substances by purification using column chromatography and sensory evaluations (taste breadth and complexity). They identified γ-glutamyl kokumi peptide in beans (Phaseolus vulgaris L.) (Dunkel et al. 2007), cheese (Toelstede et al. 2009; Toelstede and Hofmann 2009), and soy sauce (Junger et al. 2022). Shibata et al. used the same methodology to identify γ-Glu-Tyr and γ-Glu-Phe as kokumi substances in soybean seeds (Shibata et al. 2017; Shibata and Matsumura 2024). Some of the γ-glutamyl peptides described above have also been detected in Pla-ra, a fermented freshwater fish from Thailand, and have been shown to contribute to the mouthfulness of this fermented food (Phewpan et al. 2019; Phuwapraisirisan et al. 2024). γ-Glutamyl peptides have different potencies as kokumi substances, which depend on their peptide lengths and amino acid sequences (Ohsu et al. 2010; Amino et al. 2016, 2018). For example, the potencies estimated from the equivalent potency concentrations of enhanced thickness when added to mixed taste solutions (0.05% MSG + 0.05% IMP + 0.5% NaCl) were 0.15- and 12.8-fold higher than those for γ-Glu-Val-Gly (γ-EVG) (Ohsu et al. 2010). Although this is described in more detail in the section “Molecular mechanisms of action of kokumi substances,” γ-glutamyl peptides, which activate CaSR at low concentrations, act as kokumi substances with high potency. For example, γ-EVG functions as a kokumi substance and exerts a taste-modulating effect when added to food at a concentration of approximately 5 ppm (16 µM) in various food systems (Kuroda 2024b). Recent studies identified other γ-glutamyl peptides that activate CaSR using molecular modeling and reported their findings (Dellafiora et al. 2022; He et al. 2024; Lao et al. 2024; Perenzoni et al. 2024; Yang et al. 2024). Although these peptides require validation through in vitro and in vivo studies, including human sensory evaluation, they have been proposed to function as kokumi substances.

Figure 1.

Diverse kokumi substances found in foods.

Table 2.

γ-Glu peptides functioning as kokumi substances.

γ-Glu peptides	Taste threshold (μM)		Identified source	References
	in water	in foods
γ-Glu-Val	3,300	400a	Edible beans, fermented fish	Dunkel et al. (2007); Phuwapraisirisan et al. (2024)
γ-Glu-Leu	9,400	5.0b	Edible beans, Gouda cheese, Parmesan cheese, fermented fish	Dunkel et al. (2007); Phuwapraisirisan et al. (2024); Toelstede et al. (2009); Toelstede and Hofmann (2009)
γ-Glu-Glu	2,400	17.5b	Gouda cheese, Parmesan cheese, fermented fish, bread	Toelstede et al. (2009); Toelstede and Hofmann (2009); Phuwapraisirisan et al. (2024)
γ-Glu-Gly	1,250	17.5b	Gouda cheese, Parmesan cheese, fermented fish	Toelstede et al. (2009) ; Toelstede and Hofmann (2009); Phuwapraisirisan et al. (2024)
γ-Glu-His	2,500	10.0b	Gouda cheese, Parmesan cheese, fermented fish	Toelstede et al. (2009); Toelstede and Hofmann (2009); Phuwapraisirisan et al. (2024)
γ-Glu-Gln	2,500	7.5b	Gouda cheese, Parmesan cheese, fermented fish	Toelstede et al. (2009); Toelstede and Hofmann (2009); Phuwapraisirisan et al. (2024)
γ-Glu-Met	2,500	5.0b	Gouda cheese, Parmesan cheese, fermented fish	Toelstede et al. (2009); Toelstede and Hofmann (2009); Phuwapraisirisan et al. (2024)
γ-Glu-Tyr	3,000	5.0c	Soy beans, Parmesan cheese, fermented fish, yeast extract	Shibata et al. (2017); Toelstede et al. (2009); Toelstede and Hofmann (2009); Phuwapraisirisan et al. (2024)
γ-Glu-Phe	3,000	2.8c	Soy beans, Parmesan cheese, fermented fish	Shibata et al. (2017); Toelstede et al. (2009); Toelstede and Hofmann (2009); Phuwapraisirisan et al. (2024)
γ-Glu-Cys-Gly (glutathione)	1,303	200a	Garlic, Scallop, yeast extract	Dunkel et al. (2007); Kuroda (2024c); Liu et al. (2015); Ueda et al. (1990)
γ-Glu-Val-Gly (γ-EVG)	330	16.5d	Soy sauce, fish sauce, beer, cheese, fermented fish	Kuroda (2024b); Ohsu et al. (2010); Phuwapraisirisan et al. (2024)
γ-Glu-Glu-Tyr	4,490	1,250a	Biosynthesis	Liu et al. (2015); Yang et al. (2019)
γ-Glu-Cys-β-Ala (homoglutathione)	3,800	200a	Edible beans	Dunkel et al. (2007)
γ-Glu-Glu-Glu-Tyr	4,630	1,400a	Biosynthesis	Liu et al. (2015); Yang et al. (2019)

^aChicken broth.

^bReconstituted matured cheese extract.

^cSalty/umami solution.

^dChicken consommé.

α-Peptides have also been reported to function as kokumi substances (Table 3). Six Leu-containing dipeptides from yeast extracts have been identified as kokumi substances (Liu et al. 2015), besides γ-glutamyl peptides, which improve mouthfeel and continuity. Salger et al. (2019) identified 5 kokumi peptides in fermented cocoa beans as mouthfulness and continuity enhancers. Junger et al. (2022) examined taste-modifying peptides in soy sauce and identified 4 α-peptides as kokumi peptides. However, the concentrations of α-peptides that act as kokumi substances, at which they exert a taste-modulating effect, range from several to 100 to 1,000 μM and, thus, are less potent than γ-glutamyl peptides, such as γ-EVG, which are effective at approximately 15 to 100 μM. In addition, proteins and their degradation products have been reported to function as kokumi substances. Ohsu et al. (2010) showed that protamine and polylysine, which activate CaSR, functioned as kokumi substances. Rhyu et al. (2020) demonstrated that the peptide fraction (500 to 10,000 Da) of the typical Korean soy sauce Ganjang functioned as a kokumi material and enhanced saltiness and umami. Recent studies have reported kokumi peptides that bind to CaSR using molecular modeling (Lao et al. 2024; Perenzoni et al. 2024). Furthermore, soybean-derived peptide β51-63 (Nakajima et al. 2010) and lysozyme activate CaSR (Yamamoto et al. 2020). These peptides require further validation; however, they have been proposed as potential kokumi substances.

Table 3.

α−Peptides functioning as kokumi substances.

α−Peptides	Taste Threshold (Μm)		Identified source	References
	In water	In foods
Leu-Ala	3,100	400a	Yeast extract	Liu et al. (2015)
Leu-Glu	2,400	300a	Yeast extract	Liu et al. (2015)
Leu-Gln	1,200	600a	Yeast extract	Liu et al. (2015)
Leu-Lys	2,400	1,200a	Yeast extract	Liu et al. (2015)
Leu-Thr	1,300	700a	Yeast extract	Liu et al. (2015)
Ala-Leu	6,200	1,500a	Yeast extract	Liu et al. (2015)
Val-Pro-Ala	700	90b	Cocoa bean	Salger et al. (2019)
Asp-Trp-Pro	770	290b	Cocoa bean	Salger et al. (2019)
Tyr-Gly-Asp-Gly	>2,000	310b	Cocoa bean	Salger et al. (2019)
Lys-Asp-Gln-Pro	>2,000	230b	Cocoa bean	Salger et al. (2019)
Asn-Gly-Gly-Leu-Gln	>2,000	160b	Cocoa bean	Salger et al. (2019)
Asp-Gly-Phe-Pro	420	250c	Soy sauce	Junger et al. (2022)
Glu-Ser-Leu-Pro-Ala-Leu-Pro	771	273c	Soy sauce	Junger et al. (2022)
Glu-Val-Gly-Tyr-Gly-Tyr	166	162c	Soy sauce	Junger et al. (2022)
Met-Thr-Thr-Phe-Thr-Trp	354	279c	Soy sauce	Junger et al. (2022)

^aChicken broth.

^bModel broth.

^cPartial basic recombination of soy sauce.

Amino acids have been reported to act as kokumi substances, improving the complexity, thickness, mouthfulness, and continuity of taste at tasteless concentrations. Examples include L-methionine, L-cysteine, and L-histidine (Ohsu et al. 2010; Kuroda 2024d). These amino acids have been shown to enhance taste and thickness when added to mixed umami and salty taste solutions. Creatine and creatinine have been shown to function as kokumi substances in Japanese noodle soups (Shah et al. 2010). In addition, L-arginine and L-lysine have strong saltiness-enhancing functions (Lee 1992; Ogawa et al. 2004). L-Orn has been shown to function as a kokumi substance in animal experiments (Mizuta et al. 2021). Taste substances, such as umami substances, may be used as kokumi substances at concentrations at which they are tasteless (Yamamoto 2019). As discussed later in this review, Ohsu et al. proposed the CaSR as a receptor for kokumi substances. CaSR is activated by various amino acids with different chemical structures, including L-histidine, L-glutamic acid, L-tyrosine, L-phenylalanine, and L-methionine (Conigrave et al. 2000; Brennan et al. 2014). Therefore, various amino acids that activate CaSR may function as kokumi substances.

Besides amino acids, several N-acyl amino acid derivatives have been shown to act as kokumi substances, such as N-acetyl, N-formyl, N-succiyl, N-lactomyl, and N-lauroyl amino acid derivatives (Table 4) (Christa et al. 2022; Lin et al. 2023; Feng et al. 2024; Cai et al. 2024a, 2024b, 2024c, 2024d, 2025; Huang et al. 2024a, 2024b, 2024c, 2025). These amino acid derivatives enhance the complexity, thickness, mouthfeel, and continuity, as well as umami and saltiness, when added to foods. Christa et al. (2022) investigated the amino acid-derived kokumi substances in kimchi, a Korean fermented food. Chromatographic fractionation and purification of kimchi extracts, combined with taste detection threshold analysis based on sensory evaluation, identified N-L-lactoyl-L-amino acids and N-succinyl-amino acids as kokumi substances. These compounds had their own taste thresholds in the concentration range of 1,750 to 8,460 µM, whereas the thresholds in the model broth were 17 to 37 µM, suggesting that they acted as taste modulators. The addition of these compounds to the model broth also enhanced mouthfeel and continuity, whereas the mixture of these compounds enhanced umami and saltiness. They also indicated that these compounds contributed significantly to the overall taste of kimchi. These amino acid-derived kokumi substances have also been identified in soy sauce, cheese, and yeast extract (Hammerl et al. 2017; Wu et al. 2022; Lin et al. 2023; Feng et al. 2024). S-((4-amino-2methylpyrimidin-5-yl)methyl)-L-cysteine, which is generated by the Maillard reaction between thiamine and cysteine, functions as a kokumi substance (Brehm et al. 2019). Cai et al. (2024b) reported that N-lauroyl-L-tryptophan has the potential to bind to human CaSR, T1R1, and T1R3, which are subunits of the umami taste receptor, or multiple bitter taste receptors, T2Rs, in an in silico docking model. Further in vitro studies are needed to establish whether amino acid derivatives containing N-lauroyl-L-tryptophan bind to and activate these receptors.

Table 4.

N-acyl-amino acids functioning as kokumi substances.

N-acyl-amino acids	Taste threshold/Active concentration (μM)		Identified source	Reference
	In water	In food
N-Acetyl-Val	15,700	4a	Biosynthesis	Lin et al. (2023)
N-Acetyl-Leu	14,500	7.2a	Biosynthesis	Lin et al. (2023)
N-Acetyl-Met	13,100	6.5a	Biosynthesis	Lin et al. (2023)
N-L-Lactoyl-L-Glu	1,754	157a	Kimchia	Christa et al. (2022)
N-L-Lactoyl-L-Gln	6,434	237a	Kimchia	Christa et al. (2022)
N-L-Lactoyl-L-Asp	2,711	361a	Kimchia	Christa et al. (2022)
N-L-Lactoyl-L-Asn	4,866	373a	Kimchia	Christa et al. (2022)
N-L-Lactoyl-Phe	—	211b	Cheese	Wu et al. (2022)
N-Lauroyl-Trp	—	2.8b	Biosynthesis	Cai et al. (2025)
N-Lauroyl-Tyr	—	2.8b	Biosynthesis	Cai et al. (2025); Cai et al. (2024c)
N-Lauroyl-Phe	—	2.9b	Biosynthesis	Cai et al. (2024a)
N-Lauroyl-Theanine	—	2.8b	Biosynthesis	Cai et al. (2024b)
N-Lactoyl-Leu	—	394b	Biosynthesis	Feng et al. (2024)
N-Butyryl-Phe	—	113b	Biosynthesis	Cai et al. (2024d)
N-L-Succinyl-L-Gly	4,732	256a	Kimchi	Christa et al. (2022)
N-L-Succinyl-L-Ala	4,498	48a	Kimchi	Christa et al. (2022)
N-L-Succinyl-L-Glu	1,935	17a	Kimchi, soy sauce	Christa et al. (2022)
N-L-Succinyl-L-Gln	4,245	51a	Kimchi	Christa et al. (2022)
N-L-Succinyl-L-Val	2,971	61a	Kimchi	Christa et al. (2022)
N-L-Succinyl-L-Phe	8,460	287a	Kimchi	Christa et al. (2022); Huang et al. (2024b)
N-L-Succinyl-L-Ile	4,274	1,068a	Kimchi, soy sauce	Christa et al. (2022); Huang et al. (2024b)
N-L-Succinyl-L-Leu	—	4.7b	Biosynthesis	Huang et al. (2024c)
N-L-Succinyl-L-Trp	—	2.7b	Biosynthesis	Huang et al. (2024a)
N-(1-Oxodecanyl)-L-Tyr	627	537a	Yeast	Hammerl et al. (2017)
N-(1-Oxododecanyl)-L-Tyr	480	145a	Yeast	Hammerl et al. (2017)
N-(1-Oxomyristyl)-L-Tyr	672	160a	Yeast	Hammerl et al. (2017)
N-(1-Oxoparmityl)-L-Tyr	627	183a	Yeast	Hammerl et al. (2017)
N-(1-Oxooleoyl)-L-Tyr	446	217a	Yeast	Hammerl et al. (2017)

^aThreshold determined in the model broth.

^bTaste-modulating activity determined in salty/umami solution.

Fatty acids and their derivatives act as kokumi substances (Table 5). Mittermeier et al. (2018) detected fatty acid-derived kokumi substances in the edible mushroom golden chanterelle (Cantharellus cibarius). They identified fatty acid-derived kokumi substances through fractionation, purification, and sensory evaluation of golden chanterelle extracts. These compounds improved mouthfeel when added to a model broth at concentrations at which they were tasteless. Degenhardt et al. found that oxylipins in heat-treated avocados function as kokumi substances. They also identified 8 oxylipins as kokumi substances by chromatographic fractionation of the solvent extracts of heat-treated avocados and by sensory evaluation of the isolated compounds (Degenhardt and Hofmann 2010). They reported that the taste threshold concentrations of these substances ranged between 27 and 313 µM, whereas those in the model broth ranged between 2 and 17 µM, suggesting that the threshold for each compound in the model broth was lower than its intrinsic threshold. These findings indicate that the compounds improved the mouthfeel of the model broths.

Table 5.

Fatty acid-derivatives functioning as kokumi substances.

Compounds	Taste threshold (μM)		Identified source	References
	In water	In foods
14,15-dehydrocrepenynic acid methyl ester	648	32a	Golden chanterelles	Mittermeier et al. (2018)
14,15-dehydrocrepenynic acid ethyl ester	512	59a	Golden chanterelles	Mittermeier et al. (2018)
14,15-dehydrocrepenynic acid	531	105a	Golden chanterelles	Mittermeier et al. (2018)
(10E,14Z)-9-hydroperoxy-10.14-octadecadien-12-ynoic acid	536	38a	Golden chanterelles	Mittermeier et al. (2018)
(10E,14Z)-9-hydroxy-10.14-octadecadien-12-ynoic acid	320	69a	Golden chanterelles	Mittermeier et al. (2018)
(10E,14Z)-9-oxo-10.14-octadecadien-12-ynoic acid	639	79a	Golden chanterelles	Mittermeier et al. (2018)
(9Z,15E)-14-oxo-9,15-octadecadien-12-ynoic acid	228	19a	Golden chanterelles	Mittermeier et al. (2018)
(9Z,15E)-14,17,18trihydroxy-9,15-octadecadien-12-ynoic acid	>1,000	59a	Golden chanterelles	Mittermeier et al. (2018)
1-acetoxy-2,4-dihydroxyheptadeca-16-ene	34	9b	Thermally processed avocado	Degenhardt et al. (2010)
1-acetoxy-2,4-dihydroxyheptadeca-16-yne	27	5b	Thermally processed avocado	Degenhardt et al. (2010)
1-acetoxy-2-hydroxy-4-oxoheptadeca-16-ene	88	11b	Thermally processed avocado	Degenhardt et al. (2010)
1-acetoxy-2-hydroxy-4-oxoheptadecane	313	17b	Thermally processed avocado	Degenhardt et al. (2010)
1-acetoxy-2-hydroxy-4-oxooctadeca-12-ene	70	5b	Thermally processed avocado	Degenhardt et al. (2010)
1-acetoxy-2-hydroxy-4-oxoheneicosa-5,12,15-triene	70	2b	Thermally processed avocado	Degenhardt et al. (2010)
1-acetoxy-2,4-dihydroxyheneicosa-12,15-diene	92	2b	Thermally processed avocado	Degenhardt et al. (2010)
1-acetoxy-2-hydroxy-4-oxoheneicosa-12,15-diene	121	8b	Thermally processed avocado	Degenhardt et al. (2010)

^aPartial basic recombination of Golden chanterelles.

^bModel broth.

Other volatile flavor compounds such as aldehydes (Kitajima et al. 2023) and lactones (Nobuhiro 2018; Kitajima et al. 2025) also function as kokumi substances. We previously showed that these flavor substances exert significant taste-enhancing effects under olfactory deprivation conditions using nose clips and impart taste thickness. Ahmad et al. (2025) reported that GSH-derived Maillard reaction products (MRPs) activated CaSR, whereas Yang et al. (2025) showed that the MRPs of γ-glutamylated beef protein hydrolysates and xylose enhanced kokumi. This suggests that volatile MRPs may act as kokumi substances.

5. Molecular mechanisms of action of kokumi substances

In this section, we describe the molecular mechanisms of action of kokumi substances in the periphery that have been elucidated, with extensively studied γ-glutamyl peptides as examples. Although several groups have reported on kokumi substances, their relationship with taste receptors remained unclear during the 2000s. To examine the physiological effects of amino acids, we focused on CaSR, a class C G-protein-coupled receptor (GPCR), because it is activated by calcium and L-amino acids (Conigrave et al. 2000; Brennan et al. 2014). In 2006, it was reported that CaSR is activated not only by L-amino acids but also by peptides, including the kokumi substance GSH. (Wang et al. 2006). Based on the findings, Ohsu et al. (2010) investigated the relationship between CaSR and kokumi substances. The findings revealed that CaSR activators such as Ca-lactate, the basic proteins protamine and polylysine, the amino acid L-His, and cinacalcet, a pharmaceutical drug for functional hyperparathyroidism, had the same sensory functionality as GSH, defined as a kokumi substance (enhancements in thickness in a mixed basic taste solution with umami and saltiness (0.1% MSG + 0.5% NaCl) at concentrations that do not elicit a taste). To clarify the relationship between CaSR activation and sensory properties of kokumi substances, we synthesized various γ-glutamyl peptides related to GSH, selected 5 γ-glutamyl peptides, including GSH and γ-EVG, and investigated the relationship between CaSR activity and sensory property potencies of these γ-glutamyl peptides. To evaluate the sensory property potencies, each γ-glutamyl peptide was added to a mixed basic taste solution with umami and saltiness (0.05% MSG + 0.05% IMP + 0.5% NaCl). The GSH concentration perceived as equivalent in thickness to this solution sample with each concentration of GSH was selected and subjected to probit analysis to obtain the point of subjective equality and relative activity to GSH. We found significant positive correlations between CaSR activity and sensory properties (Fig. 2) (Ohsu et al. 2010; Kuroda and Miyamura 2015). These findings suggest that substances with high CaSR activation potential are kokumi substances with strong sensory potency. In addition, Ohsu et al. (2010) reported that the addition of NPS-2143, a selective antagonist of CaSR, attenuated the taste-modulating effects of GSH and γ-EVG. We also recently demonstrated that short-chain aliphatic aldehydes and medium-chain lactones, which are common volatile flavor components in foods, function as kokumi substances by activating the CaSR using a similar approach that combines the CaSR receptor assay and sensory evaluation (Kitajima et al. 2023, 2025). Various kokumi substances, such as γ-glutamyl peptides, amino acids, α-peptides, volatile compounds, and minerals, appear to exhibit their functions by activating CaSR.

Figure 2.

The relationship between the CaSR activity and taste-enhancing activity of various γ-glutamyl peptides. CaSR and taste-enhancing activities were measured using a previously described method (Ohsu et al. 2010). (Adapted from (Kuroda and Miyamura 2015)).

Previous studies have investigated the binding and activation modes of kokumi substances in the CaSR (Kitajima et al. 2023; 2025; Yang et al. 2024). We demonstrated that CaSR-activating kokumi substances, such as γ-glutamyl peptides, act as positive allosteric modulators of CaSR. We also demonstrated that CaSR-activating kokumi substances enhanced the responsiveness of CaSR to Ca²⁺, an orthosteric ligand of CaSR, particularly at low concentrations, in a calcium mobilization assay (Fig. 3) (Kitajima et al. 2023; 2025). We studied the binding sites of the potent kokumi substance γ-EVG to CaSR using cryo-EM structural analysis of their complex and a functional assay with CaSR point mutants (Fig. 4). The findings showed that γ-glutamyl peptides, including γ-EVG, bound to CaSR at allosteric sites close to the calcium-binding region (orthosteric region) of the extracellular Venus flytrap domain, where L-amino acid binding was reported (Yamaguchi et al. 2025). This binding site is the same region that had been estimated in previous molecular models of CaSR and kokumi substances (Dellafiora et al. 2022; He et al. 2024; Lao et al. 2024; Perenzoni et al. 2024). In addition, we found that highly potent kokumi substances, such as γ-EVG, appeared to have multiple binding residues on CaSR, resulting in a high CaSR activation ability (Yamaguchi et al. 2025). Other substances that activate the CaSR and function as kokumi substances, such as protamine, poly-Lys, cinacalcet, and calcium, which act as CaSR agonists, have been reported to bind to other regions of the CaSR (Ahmad and Dalziel 2020; Diao et al. 2021). Therefore, CaSR activation elicits its kokumi function independent of the binding sites and activation mode.

Figure 3.

γ-EVG acts as a positive allosteric modulator of human CaSR. Dose–response curves of CaSR to calcium chloride were obtained in the presence and absence of γ-EVG (0.5 µM). PEAK ^rapid cells transiently transfected with human CaSR cDNA were stimulated with increasing concentrations of calcium chloride in the presence of DMSO (open circles, control) and γ-EVG (0.5 μM, red circles). Changes in fluorescence (y-axis, ΔF/F) are plotted against agonist concentrations (x-axis, logarithmically scaled). γ-EVG enhanced the response of CaSR to calcium chloride at lower concentrations. (Adapted from Kitajima et al. 2023).

Figure 4.

Binding of γ-EVG to CaSR. (a) Cryo-EM map of the CaSR/γ-EVG complex. Overall density map of CaSR (left) and cartoon model of active CaSR (PDB: 7m3G). (b) Focus-refined map of the VFT domain of the CaSR. (c) Interaction between CaSR and γ-EVG. Hydrogen bonds are shown as green dotted lines in ChimeraX. CaSR is shown as a cartoon model (cyan), and the side chains of residues that interact with γ-EVG are shown as stick models (orange). (Adapted from (Yamaguchi et al. 2025)).

CaSR expression has been examined in the tissues responsible for taste perception (San Gabriel et al. 2009; Bystrova et al. 2010; Maruyama et al. 2012). San Gabriel et al. (2009) reported that CaSR is expressed in the taste buds and taste cells of rats. Maruyama et al. investigated the expression of CaSR in mouse taste bud tissues and detected it in type II and type III taste cells. In addition, they demonstrated that CaSR-expressing cells differ from umami or sweet-taste receptor cells that express T1R3, a subunit of sweet and umami taste receptors. We also detected CaSR expression in taste cells expressing the bitter taste receptors T2Rs (Kitajima et al. 2017, 2019; Kitajima 2024). Collectively, these findings indicate the involvement of CaSR expressed in type II and type III taste cells of rodent taste buds in the perception of kokumi substances. Furthermore, CaSR expression has been detected in the taste cells of cats and chickens, which are non-rodent animals (Laffitte et al. 2021; Omori et al. 2022). In these studies, HEK cells expressing animal CaSR genes responded to diverse kokumi substances, such as calcium, amino acids, and γ-glutamyl peptides. In these animals, CaSR has been suggested to function as a receptor for kokumi substances and may be involved in oral perception.

In a study on the responsiveness of taste cells from mice to kokumi substances, Maruyama et al. demonstrated that taste cell subsets exhibiting calcium responses to γ-EVG differed from those that responded to the umami substance monopotassium glutamate or the sweet substance SC45647 (Maruyama et al. 2012; Maruyama 2024). These findings revealed that the taste cells activated by kokumi substances differed from the umami and sweet-taste receptor cells, suggesting that the taste-enhancing effect of kokumi substances is exerted by a molecular mechanism different from that of the basic tastes reported. Maruyama et al. indicated that the neurotransmitter acetylcholine (ACh) secreted from taste cells activated by kokumi substances is sensed by neighboring T1R3-expressing umami or sweet-taste receptor cells, leading to an enhancement in the intensity of the taste signal within these cells. Therefore, an increase in ATP release from these taste cells contributes to the enhancement of taste intensity (Fig. 5) (Maruyama 2024). Based on their findings, they proposed the following mechanisms: 1) ACh is secreted from CaSR-expressing taste cells activated by stimulation with the kokumi substance γ-EVG; 2) secreted ACh activates muscarinic receptors, such as M3 receptors, expressed in neighboring sweet-taste receptor taste cells; 3) the activation of muscarinic receptors further enhances the response (taste signal) intensity of sweet-taste receptor cells to SC45647; 4) ATP release from sweet-taste receptor cells increases depending on the enhancement of the taste signal intensity with these cells; 5) the increased release of ATP results in a taste-enhancing effect (Maruyama 2024). This is an interesting hypothesis that may explain the functionality of kokumi substances in enhancing the intensity of basic tastes at concentrations at which they do not elicit basic taste. Yamamoto et al. reported that the response of the chorda tympani to sweet, umami, and fatty stimuli in rats was enhanced by the addition of γ-EVG (0.02%). Regarding the enhancement of umami by kokumi substances, since glutamate has been shown to activate CaSR (Conigrave et al. 2000), its effects as a kokumi substance may act synergistically with CaSR to enhance umami intensity. Conversely, this may contribute to the differences in the additive effects of MSG and IMP in animal preference tests (Yamamoto and Mizuta 2022). However, no model has yet explained the salt-enhancing function of kokumi substances. In experiments using rat fungiform taste cells, Rhyu et al. showed that γ-glutamyl peptide did not act on amiloride-sensitive epithelial Na⁺ channels (Bigiani and Rhyu 2023). The modulation of salty taste intensity through other receptor mechanisms has also been suggested.

Figure 5.

Hypothetical model of CaSR activation by kokumi substances enhancing sweet-induced ATP release from sweet-taste receptor cells via cholinergic cell-to-cell signaling within a taste bud. Proposed model for cell-to-cell communication between CaSR- and T1R3-expressing taste cells in the taste buds. Ligand activation of CaSR by kokumi substances induces acetylcholine secretion. The secreted acetylcholine then acts on nearby cholinergic taste cells (e.g. T1R3-expressing cells) to enhance ATP release following basic taste signaling (e.g. sweetness).

Regarding the mechanisms by which kokumi substances suppress bitterness, the activation of CaSR by kokumi substances has been suggested to inhibit the activation of coexpressed T2Rs through their direct protein-protein interactions (Kitajima et al. 2019). In this study, CaSR expression was observed in taste cells expressing the bitter taste receptor, T2Rs. Furthermore, both receptors bind to and interact with each other in vitro. Moreover, in functional evaluations, γ-EVG, which activates CaSR, and CaCl₂ significantly suppressed the bitterness of branched-chain amino acids or quinine (Kitajima et al. 2019; Kitajima 2024). These results suggest that CaSR activation by kokumi substances contributes to bitterness suppression. Meanwhile, Huang et al. (2024b, 2025), using in silico docking models, suggested that succinyl-amino acid-kokumi substances may bind not only to the CaSR but also to bitter taste receptors (T2Rs) and other taste receptors, including the umami receptors (T1R1-T1R3) and sweet-taste receptors (T1R2-T1R3). However, because these findings are obtained from simulations using in silico models, further in vitro validation is required to determine whether the reported kokumi substances actually suppress off-tastes by acting on the CaSR or on other taste receptors. Furthermore, the enhancement of saltiness, umami, and sweetness attributed to kokumi substances may secondarily reduce the perceived intensity of bitterness and sourness through a taste contrast effect (Nakamura et al. 2002). In addition, the enhancement of oral perceptions, such as thickness and mouthfulness, by kokumi substances may influence taste integration in the brain by rounding the taste or improving its taste balance, suppressing the perceived excessive bitterness and sourness of the food. Further studies with other kokumi substances and coadditions with taste substances, such as bitter and sour compounds, are required for further confirmation.

Besides the modulation of taste intensity, a more detailed understanding of the molecular mechanisms underlying the enhancement of oral perceptions, such as thickness, mouthfulness, and continuity, is needed. However, no clear explanation of this phenomenon has been established. Ohsu et al. reported that various kokumi substances that activate the CaSR consistently enhance the oral perception of thickness. A significant correlation was observed between the degree of CaSR activation and the thickness-enhancement intensity. Furthermore, this effect was suppressed by the CaSR antagonist NPS-2143, suggesting that the increase in thickness was mediated by CaSR activation (Ohsu et al. 2010). However, studies investigating the downstream signaling pathways following CaSR activation in taste cells and their relationship with oral perception are limited. Maruyama et al. reported that stimulation with the kokumi substance γ-EVG induced calcium responses in isolated mouse taste cells. However, whether kokumi stimulation leads to ATP release from activated taste cells has not yet been reported. In experiments using an ATP biosensor, Maruyama (2024) reported that stimulation of isolated mouse vallate taste cells with γ-EVG alone did not induce ATP secretion. Although the cells used in this study responded to the sweet substance SC45647, the findings appeared to be derived from sweet-taste receptor cells that did not express CaSR. However, this is the only study to examine ATP secretion in response to stimulation with a kokumi substance; therefore, further research on ATP secretion by taste cells that show a calcium response to γ-EVG is warranted. In our study, we observed CaSR protein expression in taste cells that also expressed the bitter taste receptors T2Rs (Kitajima et al. 2019). This suggests that taste-related signaling, possibly involving an unidentified signal originating from bitter taste cells, may contribute to kokumi-related oral perception. Furthermore, Maruyama (2024) reported that kokumi stimulation leads to the release of acetylcholine from taste cells. These findings imply that oral perception may be mediated by unknown mechanisms and signaling molecules, including acetylcholine, although further elucidation of these molecular mechanisms is needed.

Besides taste buds, CaSR expression has been detected in trigeminal nerve endings in the tongue tissue. Akiyama et al. (2023) recently examined and confirmed the expression of CaSR in mouse trigeminal ganglion (TG) cells using RT-PCR. Furthermore, using calcium imaging of isolated TG cells, they found that TG cells responded to γ-EVG and their responsiveness to allyl isothiocyanate (AITC), an agonist of TRPA1, was enhanced by pretreatment with γ-EVG. Maruyama et al. (2018) detected CaSR-positive trigeminal nerve endings in the keratinous layer of mouse fungiform papillae. They also suggested that γ-EVG penetrates the keratinous layer, potentially reaching trigeminal nerve endings and affecting nerve activity. These findings suggest the involvement of CaSR in the perception of nonbasic tastes, such as pungency, by the TRPA1-expressing trigeminal nerve. Therefore, we investigated the effects of CaSR-activating kokumi substances, γ-glutamyl peptides, on pungency in the presence of AITC. CaSR-activating γ-glutamyl peptides, such as GSH and γ-EVG, significantly increased the pungency of AITC, whereas anserine, a peptide without CaSR activity, did not. The increase in pungency induced by GSH was suppressed by NPS-2143. Collectively, these findings suggest that γ-glutamyl peptides increase pungency by activating CaSR (Kitajima et al. 2022). Therefore, the activation of CaSR, which is expressed in the trigeminal nerve endings, may be involved in the perception of pungency (Fig. 6).

Figure 6.

Hypothetical model of enhancements in the pungency of AITC via CaSR activated by kokumi substances in trigeminal nerve endings. Ligand activation of CaSR by kokumi substances increases the action potential of trigeminal nerve endings and enhances AITC-induced pungency.

Other receptors, besides CaSR, have been suggested to play a role in the perception of kokumi substances. Yamamoto et al. reported that GPRC6A, a class C GPCR to which receptors such as CaSR and T1Rs belong, might function as a receptor for kokumi substances in rats and mice (Yamamoto et al. 2025). They detected the expression of GPRC6A in type II taste cells of these animals. They also demonstrated that the coaddition of L-Orn, a ligand for GPRC6A that itself does not elicit a taste or induce a preference, significantly enhanced gustatory neural responses to basic taste substances such as umami and feeding preferences in 2-bottle preference tests (Mizuta et al. 2021). Based on these findings, the authors proposed that GPRC6A functions as a kokumi receptor in these animals (Yamamoto et al. 2025). Another study showed that the addition of the feline GPRC6A activator, L-Orn, increased food intake in cats in 2-bottle preference tests (McGrane et al. 2024). GPR92 may also function as a kokumi receptor. Haid et al. (2012) reported the expression of GPR92 (also named GPR93; LPAR5), a receptor for protein hydrolysates in the gastrointestinal tract, in mouse type II taste cells, and that it highly colocalized with T1R1, a component of the umami taste receptor. Gibbs et al. (2023) found that the addition of peptone, a protein hydrolysate derived from animal samples, to water significantly increased the feeding preference in cats. When taken together with protein hydrolysates and peptone, not eliciting basic tastes, these findings suggest the involvement of GPR92 in the perception of kokumi substances, such as protein hydrolysates and α-peptides.

GPR120, a medium-long-chain free fatty acid receptor, may be involved in the perception of fatty acids and kokumi substances associated with fatty acid perception (Kuroda 2024a). Matsumura et al. reported the expression of GPR120 in mouse type II taste cells. (Galindo et al. (2012) detected its expression in human taste bud tissue. Yasumatsu et al. (2018) also suggested the involvement of GPR120 in the taste perception of fats and oils based on an analysis of gustatory neural responses in mice. We recently reported that the addition of several synthetic GPR120 agonists, such as TUG-891 and medium-long-chain fatty acids, including oleic acid and linoleic acid, to emulsion solutions and foods containing fats and oils, at concentrations that did not elicit a taste, enhanced the fat-like mouth coating sensation, which is like lastingness, in human sensory evaluations (Iwasaki et al. 2021, 2022, 2024, 2025). Based on these studies and the finding that free fatty acids and GPR120 potent agonists do not elicit a basic taste, GPR120 may be a receptor for free fatty acids and kokumi substances associated with taste-related oral fatty orosensations. Yasumatsu et al. (2018) demonstrated that mice exhibited clear neuronal responses to GPR120 agonists. These responses differ from those elicited by other taste substances and may involve novel taste signaling pathways and mechanisms, including those activated by compounds that stimulate candidate kokumi receptors, such as CaSR and GPRC6A.

6. Future perspectives

Although the molecular mechanisms of action of kokumi substances, including their receptors, require further detailed research, the taste-modulating effects of kokumi substances improve the taste of food (Yamamoto and Inui-Yamamoto 2023). In recent years, new plant-based food products have been developed to replace or reduce animal-derived materials and their environmental impact. This approach is important for realizing a sustainable society. However, the taste perception of plant-based foods, such as soy-based meat substitutes and milk-free plant-based dairy products, in terms of taste thickness, mouthfulness, continuity, and mouthfeel, is significantly weaker than that of animal-derived materials (Wang et al. 2022; Tachie et al. 2023; Saffarionpour 2024; Wolinska-Kennard et al. 2025). This weakened taste perception makes it difficult for consumers to purchase plant-based foods. Regarding cultured meat, for which research and development are ongoing, it is easy to assume that its composition consists mainly of tasteless proteins and that it is difficult to perceive the complexity of the original meat taste (Fraeye et al. 2020), which has a positive perception, kokumi, and palatability (Yamamoto and Inui-Yamamoto 2023). Because kokumi substances function as taste modulators, meaning that they do not add any additional taste to food but enhance taste perception by increasing thickness, mouthfulness, and other characteristics, they contribute to improvements in complex positive taste perceptions with kokumi, similar to animal-derived raw materials (Valerón 2024; Wolinska-Kennard et al. 2025). This may have also enhanced its flavor. This is supported by previous findings showing an increase in γ-glutamyl peptide content and enhancement of CaSR activation in dry-cured ham (Kim et al. 2022; Valerón et al. 2023; Heres et al. 2023a, 2023b). Plant-based meats, such as those made from beans, are often bitter (Karolkowski et al. 2023). Some kokumi substances have been reported to suppress bitterness, which may be useful in masking the off-taste of alternative foods (Kitajima et al. 2019, 2024; Huang et al. 2024a). Therefore, the use of kokumi substances in new foods, such as plant-based foods and cultured meat, may contribute to the realization of a sustainable society. However, the functionality of kokumi substances, such as the enhancement and modulation of taste, mainly depends on the food to which they are added because kokumi substances are taste modulators, and it is difficult to clearly predict the effects they may exert when added to food systems. This effect may also depend on the chemical structure of the kokumi compound. Several kokumi substances in foods have yet to be identified, and their functionality remains unclear. Therefore, to identify the novel compounds and reveal the functionality of each kokumi substance, continuous research is required to discover new and useful kokumi substances and examine their perceptual properties. The global sharing of knowledge and development of kokumi substances as new food ingredients and their supply to the market are encouraged.

Because kokumi substances enhance taste intensity, they are considered useful for addressing social health issues, such as reducing salt, fat, and sugar intake. The functionality and usefulness of kokumi substances in savory foods, such as chicken broth and meat products, have been extensively examined, possibly because GSH, the first kokumi substance to be discovered, has a meaty aroma that matches well with umami and salty tastes. γ-EVG has been reported to enhance the taste of sweet products, such as beverages, custard cream, and peanut butter, due to its ability to increase the taste intensity of sweetness and fatty orosensation (Kuroda 2024b). Collectively, these findings suggest that the utilization of kokumi substances will meet the demands for salt, sugar, and fat reduction in both sweet and savory food categories, which is a global concern reported by 22% of the population (GlobalHeadquarters 2024). Therefore, the increased use of kokumi substances may contribute to the resolution of public health issues and improve the deliciousness of food.

Kokumi substances are considered useful for resolving several social issues. However, the detailed mechanisms underlying how kokumi is perceived have not yet been elucidated. Although γ-glutamyl peptides, such as γ-EVG, exert their functions by activating CaSR, limited information is currently available on other kokumi substance receptors, such as GPRC6A (Yamamoto et al. 2025), and their molecular mechanisms of action. Further identification and functional analysis of kokumi substances, which are present in a diverse range of food products, and research to elucidate the mechanisms by which they are perceived will greatly contribute to the discovery and design of new and useful kokumi substances. These further studies may also help elucidate the physiological importance of kokumi perception. This will require research in fields such as chemosensory, food chemistry, and nutritional sciences. We hope that new knowledge will be obtained through research on kokumi substances in multiple research fields and that these findings will be used to resolve social issues.

Author contributions

Seiji Kitajima: Writing—review and editing, writing—original draft, review and editing, conceptualization. Ryusei Goda: Investigation, review, and editing. Kuroda: Writing, reviewing, and editing.

Funding

The authors gratefully acknowledge the grant support from Ajinomoto Co., Inc., which had no influence on the research conducted or the publication of this study.

Data availability

No new data were generated or analyzed in support of this research.

Declaration of generative AI in scientific writing

The authors declare that generative AI and AI-assisted technologies were not used in the writing process, and that they take full responsibility for the content of the publication.