posted by Dave Arnold
Kombu, a variety of giant kelp (seaweed), is a cornerstone of Japanese cooking. Kombu is particularly high in free glutamic acid—the umami maker—but its flavor is otherwise rather delicate, so it is an excellent source of MSG. By the way, glutamic acid + sodium = MSG. (You’ll get much more on MSG and umami in our upcoming post: Umami Nation: MSG the Superspice—the Headache is in Your Head.) This post is about kombu broth.
Several types of Kombu are used in Japanese cooking. For a discussion of kombu, I refer you to this website or this book. We used ma kombu which was harvested—as many good things are—in the cold waters off Hokkaido, Japan’s northernmost main island. Yuji Haraguchi of True World Foods, a premier supplier of fish and Japanese products (we buy our fish for ike-jime from them), brought us the kombu.
Most traditional books will instruct you to place a piece of kombu in cold water and then heat that water until it just begins to bubble, upon which you immediately remove the kombu. But recent work in Japan has shown that optimal glutamate extraction is actually obtained by steeping kombu in 65° C water for about an hour. Several years ago, when we first heard about temperature regulating kombu stocks, we told a group of Kaseiki chefs that they might want to use an immersion circulator. They looked at us like we had three heads (in fact we had only two). Undaunted, we vowed to thoroughly test kombu stocks in the circulator. Two and a half years later, we finally got around to it.
Setting Up the Test
The tests: we would make one broth traditionally, and one like the Kaiseki do—by attempting to regulate water temperature to 65° C on a typical stove. After carefully reading the dashi sections of some our favorite books, we settled on 10 grams of kombu per liter of liquid—though intuitively this seemed low to us. For a third test we would circulate the kombu in water at 65° C.
We could think of two possible reasons the circulated broth wouldn’t be as good (or better) than other techniques: 1) if the air churned in by circulation had a negative effect, or 2) if the crappy heat control of the stovetop was actually a benefit; i.e., if temperature fluctuation somehow made the stock better. To control for the air problem we added a test of some kombu circulated in a vacuum bag. It didn’t take long for Nils and I to realize the bag was probably a good idea anyway, because we would get some vacuum-infusion benefits.
Yuji from True World also suggested we try some cold infusion, so we added a 10 gram/liter overnight infusion to our test, a 10 gram/liter overnight vacuum infusion, and a 20 gram/liter overnight vacuum infusion. We never heated these infusions with the kombu in them. As a final test, Yuji suggested we heat the regular overnight infusion in the traditional way (until bubbles start to form), with the kombu in it. We ended up with 8 different kombu broths, and we tasted them blind.
What We Didn’t Test
- We didn’t test a temperature range. We didn’t do 60° versus 65° versus 70° C. Next test.
- We didn’t test the level of free glutamate in the broth. Most literature will tell you this number is important. But we can’t measure free glutamate here, and we thought it much more important to judge the broths on taste alone.
- We didn’t test the interaction of the kombu broths with other umami-producing ingredients like bonito flakes (katsuobushi) or shitake mushrooms. Bonito flakes have inosinic acid, and shitakes have guanylate, both of which act synergistically with the glutamic acid in kombu to create a lot more umami than kombu alone.
- Often a kombu is re-used to make a secondary dashi after the primary broth is made. Some of the extraction methods we used rendered the leftover kombu virtually tasteless, which would make a secondary dashi impossible. So we didn’t test secondary dashis.
Vacuum bagged 65° C circulated kombu stock reigned supreme!
The verdict was decisive. Unanimous and unequivocal.
In the future, we will test vacuum bagged 65° C kombu at 15 g/liter and at 20 g/liter, because 20g/liter won the cold infusion category. We should also try cold-vacuum-infused-then-heated broth. We think it would win.