How Umami Multiplies

Kombu broth on its own is polite. So is a broth of bonito flakes. Combine them into dashi and the result turns suddenly deep, a kitchen rule of thumb that laboratories have since reproduced in numbers and explained down to the receptor. Start with the numbers.

Multiplication, out of addition

Glutamate combined with the nucleotide inosinate produces a synergistic umami far stronger than either one alone.¹

How much stronger is far stronger? There is an experiment that sat people down and measured it.

In humans the umami intensity of a glutamate solution is about 8 times larger when inosinate is added, whereas in rats the neural response is about 1.7 times larger, and this multiplicative synergy is the basis of dashi, which pairs glutamate-rich kombu with inosinate-rich katsuobushi. The 8 times and 1.7 times figures are measured at particular glutamate and inosinate concentrations, and the size of the synergy moves with the ratio of the two.²

The eightfold figure comes with its caveat, measured at particular concentrations, but the direction is unambiguous. Mixing the two families produces a magnitude that piling on more of one ingredient never reaches.

What happens at the receptor

The multiplication happens on the tongue's umami receptor, and the heart of the mechanism is that the two molecules sit in different spots on the same receptor.

Umami synergy arises when free glutamate and a free ribonucleotide such as inosinate, guanylate, or adenylate bind simultaneously to the Venus-flytrap motif of the T1R1 and T1R3 receptor, so the nucleotide stabilizes glutamate binding and produces a super-additive rather than merely additive amplification of umami intensity.³

The name is literal: the receptor closes over glutamate like a flytrap over a fly. What the nucleotide contributes becomes clearest at the level of mutation experiments.

In a published review of the umami taste receptor, inosine monophosphate and guanosine monophosphate are described as binding to a distinct site of the TAS1R1 Venus-flytrap domain adjacent to the glutamate pocket, and the review states that inosinate binding alone does not close the domain but stabilizes the glutamate-bound domain in the closed active conformation; mutation of residues H71, R277, S306 and H308 in TAS1R1 abolished the enhancing effect of inosinate without preventing glutamate binding. This is why a broth needs both classes of ingredient rather than more of either one.⁴

Inosinate cannot close the trap by itself. Its job is to hold the door shut once glutamate has sprung it. That is the molecular reading of why extra bonito flakes alone hit a ceiling, and why the kombu side of the broth has to be there first.

A ranking among the nucleotides

In a published review of the umami receptor, guanosine monophosphate is described as more potent than inosinate while adenosine monophosphate and xanthosine monophosphate have weaker effects, and a separate study gives relative umami potency against monosodium glutamate as 0.18 for adenylate, 1.0 for inosinate and 2.3 for guanylate, with taste thresholds of 12.5 milligrams per 100 grams for guanylate and 50 for adenylate. Guanylate carrying more than twice the weight of inosinate is the quantitative argument for putting a dried mushroom into a stock built on dried fish.⁵

The habit of slipping a dried shiitake into a simmering pot is this ranking put to work. Within the nucleotide family itself, guanylate does the same job at a smaller dose.

The kitchen proved it first

Japanese dashi pairs glutamate-rich kombu with inosinate-rich bonito to create synergistic umami.⁶

The kitchen had the answer long before the receptor was mapped. Once the principle is visible, the same pairing can be built anywhere, well outside dashi.

• When a broth tastes flat, check for the missing family before adding more of anything. One of the two axes, glutamate or nucleotide, is usually empty.
• Reread the Western habit of anchovies or a parmesan rind in tomato sauce through this lens. It is the same pairing.
• In a meat stock, try a small amount of a glutamate-rich ingredient like kombu or tomato. The meat already covers the inosinate axis.
• Keep shiitake soaking water and pour it into the pot. It is the cheapest way to fill the guanylate axis.
• Season only after the pairing is complete. Synergy is a problem of foundation, and salt cannot fix a missing axis.