Whether it's lox, or salt cod, humans have been preserving fish using salt for hundreds of years. While the methods and seasonings may vary, the basic idea of curing is to use salt to draw out water from the fish through osmosis. In the same way that dried fruit resists spoiling, reducing the water content of fish also slows the growth of microbes. The salt left behind further inhibits the growth of bacteria by drawing out water from the microorganisms themselves.
These days with modern refrigeration, we don't have as much need for these preservation methods anymore, but the fact that they're still around is a testament to the fact that curing makes food taste better. The main reason is that because you've lowered the water content of the food, the stuff that makes it taste good has been concentrated. The other reason is that salt has synergistic effects with umami producing compounds in the food, making the concentrated good stuff taste even better.
There are a number of ways to cure fish, and I've previously shared with you my method for makin lox, using a combination of salt, sugar and spices. One of my favorite methods uses a different technique called kobujimé developed in Japan before refrigeration to enable the transport of fish inland from the sea. The basic idea is to sandwich a fillet of fish with salt between two sheets of konbu (kelp). The konbu not only helps absorb some of the liquid coming off the fish, it also infuses the fish with glutamic acids, which helps to boost the natural umami from the inosine 5'-monophosphate(IMP) in the fish. While there are many foods (like tomatoes and corn) that are a rich source of glutamic acids, konbu also has the added benefit of being fairly neutral in flavor, allowing you to amp up the umami of a mild fish without obscuring its subtle flavor.
While you can make kobujime with any fish that's safe to eat raw (see my post on that here), my favorite is Tai (red sea bream). It's a delicate white meat fish that takes on a marvelously firm texture when cured, and because of its hefty dose of IMP, the taste is simply divine.
Once cured, you can use the kobujime for sushi, sashimi, or in a crudo such as the one I did above with ume paste, olive oil and salted konbu. Kobujime also works fabulously in ochazuke where the cured fish is placed on hot rice and covered with green tea, which extracts the umami from the Tai, creating a broth, while gently poaching the fish.
- Because kobu varies in thickness, you want to go by size rather than weight. Chose a few pieces of kombu that are slightly smaller than the surface area of each side of fish (they'll grow considerably as they rehydrate.
- Rehydrate the pieces of konbu in room temperature water. How long they take to rehydrate will depend on how thick they are, but you just want them just soft enough so they are no longer brittle. If you soak them too long, the goodness in the kombu will leach out into the water.
- Once the konbu is ready to go, drain it, and then place the pieces on a large piece of plastic wrap.
- Dust both pieces of konbu with a generous sprinkle of salt. How much salt you add will affect the length of preservation as well as the saltiness of the fish. This is something you'll need to experiment with to get the right balance for you, but you're going to want to use more salt than you would if you were just going to pan fry it.
- Place the fish on one of the pieces of konbu and then flip the other piece of konbu onto the fish so that the salted side is in contact with the fish.
- Wrap tightly with plastic wrap.
- At this point you need to make sure the konbu is making good contact with the fish. One way is to place the wrapped fish on a tray and then place another flat tray on top with some weight on it. The better way is if you have a vacuum sealer, you can put the wrapped fish in a bag and vacuum seal it.
- Let the fish cure overnight in the refrigerator.
- The next day unwrap the fish and remove and discard the konbu. You can slice the fish using a sharp knife and then serve it as sashimi, make a carpaccio drizzled with olive oil, or use it in an ochazuke (tea rice).