The power of kombu Fluke Kobu-Jime

With its chewy yet delicate translucent flesh and a clean, subtle taste thanks to its low fat content, the fluke* is one of the best loved of high-quality fish. In order to best enhance these natural features, the fluke, when cooked as part of Japanese cuisine, is often treated to a preparation known as Kobu-Jime, or placing the fish between sheets of kombu. Kobu-Jime is an extremely simple technique, but is credited with increasing the perceptible umami of the fish flesh to a great extent, giving the resulting sashimi a superb taste.

It certainly tastes like the amount of umami in a fluke prepared by Kobu-Jime is far greater than that of an untreated fish. We should still ask whether there is actually an observable difference in the level of umami present in treated and untreated fish.

The graph below displays a comparison of the levels of various amino acids and nucleotides found in standard fluke sashimi and in fluke sashimi which has been treated using Kobu-Jime. Click here to display graph

Results of the analysis showed that 100g of untreated fluke sashimi contains 12 mg of glutamate. Compare this with 100g of fluke sashimi treated with Kobu-Jime, which contains 320mg of glutamate. The Kobu-Jime treated fluke also contains greater levels of asparagine, the amino acid contained in kombu. On the other hand, no discrepancy was seen between the levels of inosinate found in the two kinds of fluke, the nucleotide which contributes to giving a food umami.

The difference in the levels of glutamate found in the two kinds of fluke sashimi can be explained when we consider that 100g of kombu (makombu) is known to contain 3000mg (3g) of glutamate. When the fluke is placed between sheets of kombu during Kobu-Jime, this glutamate is then able to thoroughly infiltrate the flesh of the fish. This is the truly marvellous power of kombu.

*The fluke is also known variously as flatfish,flounder,and halibut.

Kobu-Jime fluke sashimi - method

1. Fillet the fluke, cutting either four fillets (upper and lower tail and head end) only or six, by also cutting two smaller engawa, or porch, fillets from around the fins on both sides
2. Wipe the dust and impurities off the dried kombu with a moist cloth
3. Place each fillet of fluke between two pieces of kombu, one on top and one underneath
4. Tightly wrap each fillet in cling film and leave to refrigerate overnight
5. Cut the fillet into fine sashimi pieces

* If you are using pre-cut fluke sashimi, wrap each slice in cling film between two pieces of kombu and refrigerate for 2-3 hours.