With the discovery in 2000 of the first umami receptor, taste-mGluR.4, scientists were finally in a position to investigate the molecular basis of umami, which Professor Ikeda, as early as 1908, had already determined could be elicited by glutamate.
The peculiar name of the receptor, taste-mGluR4, tells us that it is related to the already known mGluR4 in the brain, which is sensitive to glutamate when it acts as a neurotransmitter. The difference between taste-mGluR4 and mGluR4 is that the former is a truncated version of the latter; the part of the taste receptor that projects from the taste cell is only about half as big as that of brain-mGluR4. As the outermost part of the receptor molecule is exactly the site where glutamate is bound, the consequence is that the taste receptor is much less sensitive than its larger counterpart in the brain. Actually, taste-mGluR4 is more than one hundred times less sensitive than brain-mGluR4. It is, however, not important for the taste receptors to be nearly as sensitive as those in the brain because the concentration of glutamate in food is much greater than the concentration of glutamate in the neural cells.
Not all the details of how the taste receptor taste-mGluR4 works are known, but seemingly it is activated when glutamate is bound to the part of the protein that projects from the membrane. This part is very flexible, almost like a hinge. As soon as the glutamate molecule binds, the hinge partly closes around it, thereby holding it more strongly. It turns out that this closing mechanism is also the secret behind the synergistic interaction of the ribonucleotides inosinate and guanylate, which enhances umami. We will return to this later.
When glutamate is bound to the binding site in the receptor, a signal is sent through the protein, so that a G-protein is bound to the receptor on the other side of the membrane, which faces toward the inside of the cell. This indicates that the signal has been received and triggers a cascade of sequential biochemical processes: Certain ion channels open, which results in a drop in the membrane’s electrical potential, which again sends an impulse through the nerve cell to the brain. The taste impression has arrived at its destination.
Schematic illustration of a receptor embedded in the cell membrane of a sensory cell.
Two years after the identification of the taste-mGluR.4 receptor, scientists found a second, more complicated type of umami receptor. It is from a family of receptor cells, called T1R-receptors, found uniquely in the sensory cells. And like the taste-mGluR4 receptors, they are distantly related to the glutamate receptors in the brain.
This new receptor reveals a little more about the nature of umami and how the perception of this taste is possibly related to that of sweetness. Furthermore, as discussed below, this receptor is also, and quite surprisingly, a key to understanding synergy in our experience of umami; that is, the way in which some ribonucleotides enhance the perception of glutamate.
In 2002, two groups of researchers working independently discovered that members of the T1R family of receptors could assemble into pairs, forming two related complexes, T1R1/T1R3 and T1R2/T1R3, which are sensitive to umami and sweet tastes, respectively. The similarity between them may be an indicator that the biochemical pathways for the recognition of sweetness and umami are very closely connected. In fact, they may be so intimately related that it would explain why rats experience umami as a sweet taste and stop eating sugar when they have ingested too much glutamate. It may also explain why umami can enhance the perceived sweetness of some foodstuffs.
In rats and mice, the T1R1/T1R3 pair is sensitive to a broad spectrum of left-turning amino acids, but in humans it responds most strongly to glutamate and aspartate. On the other hand, this receptor complex is not sensitive to right-turning amino acids or other substances. Only about 70 percent of the T1R1 in rodents is identical to that in humans. The binding site for glutamate in the T1R1/T1R3 receptor is found in the T1R1 part, and it has the same molecular structure as in the mGluR4 and taste-mGluR4 receptors. In contrast to taste-mGluR4, however, T1R1/T1R3 is not sensitive to L-aspartate, which has only a little umami.
It is also interesting that, although glutamate binds exclusively to T1R1, the receptor system works only if T1R1 is paired with T1R3 and if T1R3 is intact. Genetic variations expressed in the T1R3 receptor can, for example, have an effect on the ability to detect umami. In addition, substances that bind to T1R3, and in this way suppress sweetness, can also suppress umami. An example of this effect is the commercial attempt to use the substance lactisole as a taste modifier to diminish the taste of certain natural and artificial sweeteners. Because a lactisole molecule binds itself to T1R3, it has an effect on the sensitivity of the receptor pair T1R2/T1R3 to sweet things. But, in so doing, lactisole also has an impact on the function of T1R1 and, as a totally unforeseen side effect, also suppresses umami.
An essential feature of the discovery of the T1R1/T1R3 receptor complex as it relates to umami is that its sensitivity to glutamate is strengthened in a very robust way by inosinate and guanylate, a very important aspect of umami synergy. In contrast, the receptor cannot be made to respond to right-turning amino acids with the help of inosinate. As well, the receptor is not sensitive to inosinate on its own, which is also characteristic of umami.
Schematic illustration of the umami receptor T1R1/T1R3 embedded in the cell membrane of a sensory cell.
In summary, T1R3 is a common partner for the perception of both sweetness and umami. This suggests that in forming pairs with T1R1 and T1R2, respectively, T1R3 itself is neutral with respect to tastes but nevertheless has an effect on the sensitivity of the coupled pairs to these two tastes.
More recently, yet another mGlu glutamate receptor, which is related to taste-mGluR4, has been discovered. This brings the total number of receptors for umami to at least three, the two mGlu receptors and T1R1/T1R3. Although it is not known for sure, there is some indication that the signaling pathways for the three types of receptors are different, even though they may involve the same G-proteins (possibly gustducin). It has been suggested that mGluRs and the T1R1/T1R3 pair have different functions in the perception of umami. According to this theory, T1R1/T1R3 plays a major role on the front part of the tongue in the preferential selection of food that has umami, whereas the mGlu receptors are important on the back part of the tongue with regard to discriminating between umami and other tastes.
The discovery of how the taste receptors T1R1 and T1R3 work in tandem in the combination T1R1/T1R3, which can be activated by the substances that synergistically induce umami, paved the way for further research. The scientists were at last on the way to finding out what cooks the world over have known for a very long time, namely, that meat soup with vegetables tastes good. The meat contributes inosinate and the vegetables have glutamate. Or what the Japanese have also known for centuries: Dashi is best when konbu, containing glutamate, is combined with katsuobushi or shiitake, which contribute inosinate or guanylate, respectively.
Schematic illustration of the umami receptor T1R1/T1R3 embedded in the cell membrane of a sensory cell. The T1R1 part binds a glutamate ion (the green sphere) and a nucleotide (the blue triangle). This triggers a signal through the receptor that a G-protein (the blue ‘jelly bean’ shape) is to be bound on the other side of the membrane.
Inosinate and guanylate do not activate T1R1/T1R3 by themselves, but can do so in conjunction with glutamate. Conversely, other ribonucleotides that do not bring out umami have no effect on the receptor. From recent research it is possible to explain this synergy with the help of a mechanism that resembles the trapping mechanism in the Venus flytrap, an unusual carnivorous plant.
The activity all takes place in the T1R1 part of the receptor complex, which has a hinge-like structure. L-glutamate binds near the place where the hinge will bend. As in the Venus flytrap, this causes the hinge to snap shut, trapping the glutamate molecule securely. Inosinate or guanylate, on the other hand, binds to a place at the edge of the hinge, where there is a spot that resembles a cleft. This increases the trapping power of the hinge and results in an even stronger binding of the glutamate molecule. Stabilizing the glutamate in this way is equivalent to making the receptor more sensitive with respect to glutamate. On the other hand, neither of the two ribonucleotides would be able to stimulate the receptor on its own in the absence of glutamate. Chemists refer to this effect as allostery, which in this case means that the effect of the protein’s function is dependent on something that happens in a place on the protein other than its active binding site.
Recent research has shown that the allosteric action is manifested in the dynamics of the Venus flytrap: Without bound glutamate, the flytrap is very dynamic; when glutamate binds, the dynamics are slowed down significantly; and when a nucleotide also binds, the dynamics are extremely slow.
Furthermore, it turns out that glutamate is also involved in stabilizing the actual pair formation of T1R1/T1R3. The whole process, which results in umami taste, is therefore a truly cooperative effort that depends on a number of separate entities coming together to work as a team.
The molecular mechanism of the umami receptor (T1R1/T1R3), illustrating the synergistic action by simultaneous binding of glutamate and a 5’-ribonucleotide (in this case guanylate).
Schematic comparison between the dynamic functioning of the umami receptor and Pac-Man. Without glutamate in his throat, Pac-Man's jaws are very dynamic; when glutamate is bound, the jaw motion is slowed down; when, in addition, a nucleotide binds at his lips, the dynamic motion stops altogether and he is silent.
The twenty different naturally occurring free amino acids can be divided into groups according to their tastes, as shown in Table 1. There are two amino acids that are considered to be tasteless or neutral in taste, aspartic acid and asparagine. The salt of aspartic acid, aspartate, has a slight umami taste. Some sweet amino acids, such as lysine and proline, taste bitter in large quantities. One of the amino acids, methionine, tastes both bitter and sulfurous. The taste of the different amino acids can vary with pH.
Molecular structures of MSG (monosodium glutamate) and MSA (monosodium aspartate).
TABLE 1: THE TASTE OF AMINO ACIDS
| Umami |
Bitter |
Sweet |
Sour |
Neutral |
| aspartate
glutamate
(glutamic acid) |
arginine
histidine
isoleucine
leucine
methionine
phenylalanine
tryptophan
tyrosine
valine |
alanine
glutamine
glycine
lysine
proline
serine
threonine |
cysteine |
asparagine
aspartic acid |
Source: Belitz et al. Food Chemistry. Springer, New York, 2004, p. 34, Table 1.12.
Table 2 shows the taste thresholds as a percentage by weight for substances that impart an umami taste in pure water. The given values are subject to a considerable degree of uncertainty.
TABLE 2: TASTE THRESHOLDS FOR UMAMI
| MSG
IMP
GMP
MSG+IMP
MSG+GMP |
0.03%
0.012%
0.0035%
0.0001%
0.00003% |
MSG = monosodium glutamate; IMP = inosine-5’-monophosphate;
GMP = guanosine-5’-monophosphate.
Source: Maga, J. A. Flavor potentiators. Crit. Rev. Food Sci. Nutr. 18, 231–312, 1983.
Tables 3 to 11 list the content of bound and free glutamate, as well as of 5'-ribonucleotides, in a variety of raw ingredients and prepared products. The data is drawn from a number of sources, and it should be stressed that there can be considerable variation in currently available values for the same foodstuff. In addition, the content of these substances can, to a large extent, depend on the specifics of a given sample, such as the species, place of origin, degree of ripeness, storage conditions, and so forth. Regrettably, there is often no information in the technical and scientific literature regarding certain raw ingredients or processed foods.
There can also be considerable variation between the content of free and of bound glutamate in a particular raw ingredient. The amount of free glutamate is indicative of the foodstuff’s innate ability to impart umami taste. On the other hand, the bound glutamate content is indicative of the ingredient’s potential to develop more umami taste substances if it is treated in such a way that the proteins are broken down into free amino acids. Examples of processes used to do so include cooking, fermenting, curing, dehydrating, and marinating.
TABLE 3: BOUND AND FREE GLUTAMATE IN A SELECTION OF RAW INGREDIENTS
| Food category |
Bound glutamate (mg /100 g) |
Free glutamate (mg /100 g) |
| Meat
Beef
Pork |
2,846
2,325 |
33
23 |
| Poultry
Duck
Chicken
Eggs |
3,636
3,309
1,583 |
69
44
23 |
| Fish
Cod
Mackerel
Salmon |
2,101
2,382
2,216 |
9
36
20 |
| Vegetables
Green peas
Corn
Carrot
Spinach
Tomato
Potato |
5,583
1,765
218
289
238
280 |
106
130
20
48
140
102 |
| Milk and cheese
Parmigiano-Reggiano
Cow’s milk
Human breast milk |
9,847
819
229 |
1,680
1
19 |
Source: Ninomiya, K. Natural occurrence. Food Rev. Int. 14, 177–211, 1998.
TABLE 4: COMPARISON OF AMINO-ACID CONTENTS IN DASHI AND VARIOUS SOUP STOCKS
The table lists data for the content of umami free amino acids (glutamic acid and aspartic acid) and a sweet amino acid (alanine) in different preparations of dashi and soup stock. The recipe calls for 1.8 L water and 30 g seaweed. The seaweed is soaked in the water and extracted at 60°C for 1 hour. The Rausu-konbu dashi and the dulse dashi are prepared by using about twice as much seaweed per liter of water as for the Rishiri-konbu dashi and ichiban dashi.
| Dashi or soup stock |
Glutamic acid mg/100 g |
Aspartic acid mg /100 g |
Alanine mg /100 g |
| Rishiri-konbu dashi
Rausu-konbu dashi (traditional)
Rausu-konbu dashi (sous-vide)
Ichiban dashi
Dulse dashi
Western chicken stock
Chinese tang (chicken base) |
22
100
145
25
40
18
14 |
16
60
85
18
27
6
4 |
1
7
20
4
25
11
8 |
Sources: Mouritsen, O. G., L. Williams, R. Bjerregaard & L. Duelund. Seaweeds for umami flavour in the New Nordic Cuisine. Flavour 1:4, 2012; Ninomiya, K. Unpublished data from the Umami Information Center; Kurihara, K. Glutamate: from discovery as a food flavor to role as a basic taste. Am. J. Clin. Nutr. 90, 719S-722S, 2009; Ozawa, S., H. Miyano, M. Kawai, A. Sawa, K. Ninomiya, K. Mawatari & M. Kuroda. Changes of free amino acids during cooking process of chicken consommé (in Japanese) presented at the 58th Congress of the Japanese Society for Nutrition and Food Science, Sendai, May 21–23, 2004, p. 322; Ozawa, S., H. Miyano, M. Kawai, A. Sawa, K. Ninomiya, K. Mawatari & M. Kuroda. Changes of free amino acids during cooking process of Chinese chicken bouillon (in Japanese) presented at the 59th Congress of the Japanese Society for Nutrition and Food Science, Tokyo, May 13–15, 2005, p. 322.
TABLE 5: FREE GLUTAMATE IN RAW INGREDIENTS
| Food category |
Free glutamate (mg /100 g) |
| Meat and poultry
Ham (air-dried)
Duck
Chicken
Beef
Pork
Eggs
Lamb |
337
69
44
33
23
23
8
|
| Fish and shellfish
Anchovies (marinated)
Sardines
Squid
Scallops
Sea urchin
Oysters
Mussels
Caviar
Alaska crab
Sardines (dried, niboshi)
Shrimp
Mackerel
Dried bonito
Dried tuna
Salmon roe
Salmon
Crab
Cod
Lobster
Herring |
1,200
280
146
140
140
130
105
80
72
50
40
36
36
31
22
20
19
9
9
9 |
| Tea
Green tea
Green tea (roasted) |
450
22 |
| Vegetables
Tomato (sun-dried)
Tomato
Potato (cooked)
Potato
Corn
Broccoli
Green peas
Lotus root
Garlic
Chinese cabbage
Soybeans
Onion
Cabbage
Green asparagus
Spinach
Lettuce
Cauliflower
White asparagus
Carrot
Marrow
Green bell pepper
Cucumber |
648
200
180
102
130
115
106
103
99
94
66
51
50
49
48
46
46
36
20
11
8
1 |
| Milk
Human breast milk
Goat’s milk
Cow’s milk |
19
4
1 |
| Fungi
Shiitake (dried)
Shiitake
Button mushroom
Truffle |
1,060
71
42
9 |
| Fruits and nuts
Walnuts
Strawberries
Apple juice
Pear
Avocado
Kiwifruit
Red wine grapes
Grapefruit
Apple |
658
45
21
20
18
5
5
5
4 |
| Dried seaweeds
Konbu (Saccharina japonica)
Nori (Porphyra yezoensis)
Wakame (Undaria pinnatifida) |
1,400–3,200
1,378
9 |
| Cheese
Parmigiano-Reggiano
Roquefort
Gruyè de Comté
Stilton
Cabrales (goat’s milk blue cheese)
Danish blue
Gouda
Camembert
Emmenthal
Cheddar |
1,000–2,700
1,280
1,050
820
760
670
460
390
308
182 |
Sources: Ninomiya, K. Umami: a universal taste. Food Rev. Int. 18, 23–38, 2002; Ninomiya, K. Natural occurrence. Food Rev. Int. 14, 177–211, 1998; http://www.umami-info.com; http://www.msgfacts.com; Özden, Ö. Changes in amino acid and fatty acid composition during shelf-life of marinated fish. J. Sci. Food Agric. 85, 2015–2020, 2005; Löliger, J. Function and importance of glutamate for savory foods. Amer. Soc. Nutr. Sci. 130, 915S-920S, 2000; Giacometti, T. Free and bound glutamate in natural products. Glutamic Acid: Advances in Biochemistry and Physiology (L. J. Filer, Jr. et al., eds.) Raven Press, New York, s. 25–34, 1979; Komata, Y. Umami taste of seafoods. Food Rev. Int. 6, 457–487, 1990; Maga, J. A. Flavor potentiators. Crit. Rev. Food Sci. Nutr. 18, 231–312, 1983.
TABLE 6: FREE GLUTAMATE IN FERMENTED FOOD PRODUCTS
| Food category |
Free glutamate (mg/100 g) |
| Soy sauce
Korea
China
Japan |
1,264
926
782 |
| Fish sauce
Japan
Vietnam
China |
1,383
1,370
828 |
| Modern garum
Garum
Quick-and-easy garum |
623
217 |
| Fermented beans
West African soumbala (Parkia biglobosa)
Chinese douchi (soybeans)
Miso
Tempeh
Japanese nattō (soybeans) |
1,700
1,080
500–1,000
985
136 |
| Fermented rice
Sake |
186 |
Sources: Ninomiya, K. Umami: a universal taste. Food Rev. Int. 18, 23–38, 2002; Ebine, H. Miso preparation and use. Food Uses of Whole Oil and Protein Seeds. (E. W. Lusas, D. R. Erickson & W.-K. Nip, eds.) American Oil Chemists’ Society, Champaign IL, pp. 131–147, 1986; Murata, K., H. Ikehata & T. Miyamoto. Studies on the nutritional value of tempeh. J. Food. Sci. 32, 580–586, 1967; www.kikumasamune.co.jp; glutamate content of garum and quick-and-easy garum measured by Niels O. G. Jørgensen.
TABLE 7: INCREASE OF FREE GLUTAMATE IN A RIPENING TOMATO
The table lists the free glutamate content (mg/100 g) in a field tomato, from the green to the overripe stage. It should be noted that the content varies from one part of the tomato to another, with the innermost part often having up to five times as much as the outer surface. Free glutamate content also varies by species, with cherry tomatoes having the most.
| Green |
Ripe green |
Partly red |
Pale red |
Red |
Fully ripe |
Overripe |
| 20 |
21 |
30 |
74 |
143 |
175 |
263 |
Source: Ninomiya, K. Natural occurrence. Food Rev. Int. 14, 177–211, 1998.
TABLE 8: INCREASE OF FREE GLUTAMATE IN AGING CHEDDAR
The table lists the free glutamate content (mg/100 g) in cheddar cheese from the time it is made until it has been aged for eight months.
| Aging time (months) |
0 |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
| Free glutamate (mg/100 g) |
11 |
22 |
36 |
54 |
78 |
112 |
121 |
160 |
182 |
Source: Ninomiya, K. Natural occurrence. Food Rev. Int. 14, 177–211, 1998.
TABLE 9: INCREASE OF FREE GLUTAMATE IN A CURING HAM
The table lists the progression, over a period of eighteen months, of the free glutamate content (mg/100 g) in ham that is being air dried. The curing process involves a complicated cycle of salting and drying at different temperatures and humidity levels. The ham loses about 30 percent of its initial water content.
| Curing time (months) |
0 |
1 |
2 |
4 |
6 |
12 |
18 |
| Free glutamate (mg/100 g) |
6 |
11 |
35 |
46 |
142 |
207 |
337 |
Source: Ninomiya, K. Natural occurrence. Food Rev. Int. 14, 177–211, 1998.
TABLE 10: 5'-RIBONUCLEOTIDES IN RAW INGREDIENTS
Content of free 5'-ribonucleotides in a variety of raw ingredients. The data is drawn from different sources, and there can be considerable variation in available values for the same foodstuff. Moreover, the content can, to a large extent, depend on the specifics of a given sample, such as the species, place of origin, degree of ripeness, and storage conditions.
| Food category |
IMP (mg /100 g) |
GMP (mg/100 g) |
AMP (mg /100 g) |
| Fish and shellfish
Katsuobushi (dried bonito)
Niboshi (dried sardines)
Anchovy paste
Sardines
Scallops
Sea urchin
Mackerel
Tuna
Salmon
Lobster
Cod
Shrimp
Squid
Crab |
687
863
300
193
-
2
215
286
154
-
44
92
-
5 |
5
-
2
-
-
-
-
-
-
5 |
52
6
172
10
6
6
6
82
23
87
184
32 |
| Vegetables and fungi
Tomato (sun-dried)
Tomato
Potato (cooked)
Green peas
Shiitake
Shiitake (dried)
Matsutake
Morel (dried)
Enokitake
Porcini mushrooms (dried)
Oyster mushrooms (dried)
Green asparagus |
-
-
-
-
-
-
-
-
-
- |
10
-
2
-
16–45
150
65
40
22
10
10
- |
21
4
2
4 |
| Meat and fowl
Chicken
Pork
Beef |
201
200
70 |
5
2
4 |
13
9
8 |
| Seaweeds
Nori (Porphyra yezoensis) |
9 |
5 |
52 |
| Milk
Human breast milk |
0.3 |
|
|
IMP = inosine-5’-monophosphate; GMP = guanosine-5’-monophosphate; AMP = adenosine-5’-monophosphate; - = below measurable threshold; blank = no available data.
Sources: Ninomiya, K. Umami: a universal taste. Food Rev. Int. 18, 23–38, 2002; Ninomiya, K. Natural occurrence. Food Rev. Int. 14, 177–211, 1998; Goral, D. M. Flavour-enhancing food additive. US patent 7,510,738B2, 2009; Maga, J. A. Flavor potentiators. Crit. Rev. Food Sci. Nutr. 18, 231–312, 1983.
Molecular structures of the nucleotides IMP, GMP, and AMP.
TABLE 11: CHANGES IN CONTENT OF UMAMI SUBSTANCES IN MEAT AND FOWL AFTER SLAUGHTERING
The table lists the content (mg/100 g) of free glutamate and free IMP (inosine-5’-monophosphate) in the period following slaughter, at a storage temperature of 4ºC.
| |
Beef |
Pork |
Chicken |
| 4 days |
12 days |
1 day |
6 days |
0 days |
2 days |
| Glutamate |
6 |
10 |
4 |
9 |
13 |
22 |
| IMP |
90 |
80 |
260 |
226 |
284 |
231 |
Sources: Ninomiya, K. Umami: A universal taste. Food Rev. Int. 18, 23–38, 2002; Ninomiya, K. Natural occurrence. Food Rev. Int. 14, 177–211, 1998.
