Some of you may have taken one look at the subtitle to today's post and instinctively shrunk away from your screen at the mention of biology. "I came here to read about food, not biology!" you may be thinking. Well this post will still talk about food, but I do hope you'll stay for the biology as well, as I think it is rather fascinating.
A few days back, I was making tteokbokki, the Korean dish made of rice cakes simmered in a spicy broth. While living alone, I've found it to be a simple, reliable, and tasty dish that can be easily adapted to the ingredients I happen to have on hand. In particular (for those of you who would like a recipe), the dish involves first making an anchovy-kelp stock (more on this later), which is used to cook the rice cakes along with vegetables--I like green onion, napa cabbage, and enoki mushrooms. The flavoring comes from a combination of finely ground gochugaru (Korean chili powder) and gochujang (fermented chili paste) in approximately a 1:1 ratio, supplemented with garlic, a touch of soy sauce, and a generous spoonful of sugar (traditionally you will also see oligodang, a syrup made of oligosaccharides and used to thicken and sweeten dishes).
Now, if you're not familiar with Korean food, you might be wondering about this anchovy-kelp stock. It turns out to be a rather interesting component of tteokbokki, despite its simple preparation. The stock is traditionally made from dried anchovies (myeolchi, or Japanese anchovy) along with dried kelp (dasima, frequently of the species Saccharina japnonica). To extract the maximum flavor, I like to soak the kelp in cold water for several hours (or even for multiple days) before bringing to a bare simmer (approximately 170 degrees Fahrenheit) and then adding the anchovies. I think strictly speaking it's best to remove the innards from the anchovies, though I am frequently too lazy to do this since I don't mind the fishy taste they give the broth. After cooking the anchovies for about five minutes, you can strain and use the broth immediately. Also, the leftover kelp and anchovies can be used to create a weaker, "second broth" (this is quite similar to niban dashi in the Japanese version). Even simpler than cooking, however is to simply leave all the ingredients in a large container in the fridge for several days and strain as needed, since the cold infusion works plenty well given enough time (and some may argue even yields more refined results). (As a side note, the anchovy-kelp stock used in Korean cooking is very similar to the kombu-niboshi stock used in Japanese cooking. However, as I learned recently, the niboshi used there are in fact baby sardines, not anchovies, as are often mislabeled in grocery stores. On the other hand, much of the same discussion below on glumates applies exactly the same to dashi.) The question I'd like to delve into today is what the role of anchovy stock (or dashi) is in Korean cooking and what exactly gives it its uniquely savory character.
As I'm sure you've guessed by now, the key to anchovy stock is the presence of compounds called glutamates, which are frequently cited as the effectors of the umami or savory taste. For those who've seen glutamates mentioned in recipes or various food shows, you may be wondering what exactly they are and why they're so important. Glutamates are simply one of the many amino acids we have in our body, amino acids being the components that make up proteins. This, you might guess, is why savoriness is associated with meaty foods. Outside of its role in taste, glutamate functions in the brain as a neurotransmitter. You may also see "glutamic acid" in place of glutamate, which is simply the acidic form of glutamates (if you can recall from general chemistry, glutamate and glutamic acid are an acid-conjugate base pair, so they differ structurally in only one proton). In processed foods, we encounter glutamate most frequently as monosodium glutamate (MSG), which essentially justs attaches the negative charge on glutamate to the positive charge on sodium (Na+) for stability in salt form.
Anchovies and kelp are very good sources of glutamates (up to 1200 mg/100 g for anchovies and 3400 mg/100 g [!] for kelp1) and as such they form a remarkable backbone of savoriness on which many recipes build their flavors. Of course, there are other oceanic, vegetal, iodine-like, and fishy flavors which these two ingredients contribute, and hence their effect cannot be replaced by simply dissolving MSG in water, but, on the whole, their presence boosts the umami flavors of any dish.
Besides glutamate, there are in fact two other compounds identified to produce a similar savory flavor--inosinate and guanylate. Inosinate is more well-known as inosine monophosphate (IMP) and guanylate as guanosine monophosphate (GMP) in biology. They both are similar to adenosine triphosphate (ATP), which you probably remember from biology class, and are classified as nucleoside monophosphates, which means that they are composed of a nitrogen-containing base (i.e. inosine or guanine), a sugar (ribose in this case, which gives the name to ribonucleic acid or RNA), and a single phosphorous-containing group. You might recall adenine, guanine, cytosine, and thymine (or even uracil) from a previous class, since those are the canonical bases used in DNA (or RNA), but inosine is interesting in that it occurs during the biosynthesis of these other bases, but plays other roles such as tRNA synthesis. The point here is not to belabor the nucleotide biochemistry, but to note that inosinate and guanylate both contribute to the umami flavor in similar ways to glutamate. However, the most interesting part is not even that there is more than one umami flavor compound, but that these three compounds work synergistically--that is, the combined effect of two compounds working in concert is more effective than a single compound (given that the total concentrations are the same). The strength of this synergism by inosinate has been measured as increasing glutamate sensitivity by up to 15-fold!2
The reason why is this relevant to anchovy broths is that kombu contains many glutamates, but not so many inosinates, where as sardines (used in niboshi dashi), for example, contain high levels of inosinates.1 Dried shiitake mushrooms, also used to enhance savoriness, contain higher levels of guanylates. Altogether, these ingredients combine to create flavors stronger than the sum of their parts, and hence may serve to explain the classic pairings of kelp with anchovies or with dried mushrooms in Korean (and Japanese) cuisine.
So now we have reached the final level of today's discussion, where we dive one step deeper and begin to consider what exactly is happening at the molecular level to generate the umami taste and especially the syneristic effect I mentioned before. Interestingly, in the course of my research for this post, I came to the realization that we characterize umami as a distinctive taste because it corresponds to specific cells and receptors on our tongue. (On the other hand, capsaicin binds to a receptor associated with heat sensation, which should qualify spiciness as a flavor, though I've never seen it listed as one the canonical five.) In particular, several researchers discovered in 20023 a group of receptors (T1R1 and T1R3) which are activated by glutamate as well as inosinate. More specifically, these are part of a class of receptors called G-protein coupled receptors, or GPCRs. GPCRs are among one of the most diverse group of cellular proteins, and a large proportion of them are targets for pharmaceuticals modulation.
The aspect of these proteins I wanted to focus on is the so called "umami taste synergism." A paper by Feng Zhang and colleagues published in 20084 seems to suggest that there is a specific molecular mechanism that explains this glutamate-inosinate synergy. The binding of molecules to receptors can, in a simplified view, be thought of the fitting of a lock and key. The shape of the protein's binding pocket, which possesses a unique three-dimensional structure, is particularly tuned to the complementary shape of the binding molecule. As a wrinkle to this picture, the binding molecule often induces a change in the shape of the receptor itself to enhance the complementary fit, in a model known as "induced conformational fit." In this model, you can think of the protein being in essentially two different shapes, one which corresponds to the unbound state, and one which appears once the binding molecule (here, glutamate) gets close and induces a change (through geometry or other pathways). These two states are in a delicate balance (like a see-saw), where the presence or absence of various molecules will alter the equilibrium and change the properties of the receptor. In the case of T1R1, computer modeling of the protein and various functional studies seem to indicate that inosinate stabilizes the glutamate-binding form of the receptor, thus increasing its activity. In our induced fit model, you might imagine this as tipping the scale towards the complementary shape to which glutamate more easily binds This finding is quite incredible, in my humble opinion. With the techniques of modern biochemistry and molecular biology, we are now able to pinpoint down to the atom the exact mechanism which operates to generate these taste effects. From an outside perspective, it is rather impressive that researchers can identify and characterize the exact protein that makes glutamate and inosinate such good partners, and indirectly, why tteokbokki tastes so delicious.
Finally, have reached the end of this long and arduous journey. If you've stuck around until the end, I thank you for your persistence (I just realized this post clocks in at over 1700 words!), since I know many of my readers have a relatively sparse background in biology and chemistry (a fact that I often forget to the detriment of my writing). I hope, though, that this blog post has shown the remarkable depth of connections ranging from the cultural (use of anchovy stock in Korean cuisine) all the way to the molecular level. As always, I appreciate your readership and I'll see you next time!
1. Kurihara, Kenzo. Umami the Fifth Basic Taste: History of Studies on Receptor Mechanisms and Role as a Food Flavor. Biomed Res. Int. 2015, 189402 (2015).
2. Li, X., Staszewski, L., Xu, H., et al. Human receptors for sweet and umami taste. PNAS 99, 4692-4696 (2002).
3. Nelson, G., Chandrashekar, J., Hoon, M. et al. An amino-acid taste receptor. Nature 416, 199–202 (2002).
4. Zhang, F., Klebansky, B., Fine, R. M., et al. Molecular mechanism for the umami taste synergism. PNAS 105, 20930-20934 (2008).