The most important contributor to umami in fish sauces is glutamate. The content of free glutamate can be very great, close to 1,400 mg per 100 g in Japanese ishiri and the Vietnamese nuoc mam tom cha, or a little more than in soy sauce. Usually, the salt content is also very high, all the way up to 25 percent, which is quite a bit more than the 14–18 percent found in soy sauce. The combination of umami and salt works synergistically to enhance the saltiness of fish sauce.
Interestingly, in contrast to such other taste additives as anchovy paste and certain original Asian fish extracts, fish sauce has no significant content of free 5’-ribonucleotides. In ancient Japan, a concentrated fish extract, irori or katsuo-irori, was produced by reducing to a paste the liquid in which bonito (katsuo) had been cooked, using the same techniques as those now used to make katsuobushi. Katsuobushi is the component of classical Japanese dashi that contributes the 5’-ribonucleotides that interact synergistically with the glutamate from konbu to impart a strong umami taste. As katsuobushi dates back only to the 1600s, it is a much newer arrival on the culinary scene than irori.
Although irori disappeared from Japanese cuisine during the Meiji era (1867–1912) and is no longer on the market, a similar product, senji, is still made on the island of Kyushu in the southern part of Japan. When produced according to the traditional recipe, senji contains 900 mg of glutamate per 100 g and no less than 786 mg of inosinate per 100 g. Comparably high levels of inosinate are found in another traditional fish extract, rikakuru, from the Maldives. Both of these sauces are much richer in inosinate than katsuobushi, which has only about 474 mg per 100 g.
In Asia, fish sauces are generally incorporated into cooked dishes or served as a condiment with, for example, rice. They are much more widely used as salt substitutes and taste enhancers than as true sauces to be poured over food when it is served. The fish sauces can be seasoned with other ingredients, such as chile and lime juice.
The quality, and with it the content of substances that impart umami, of all of these fish sauces and pastes is very dependent on the raw ingredients that go into them and the method of production. Many of the commercially available products contain a great deal of salt and added MSG, and they do not live up to the high standard that is the hallmark of traditionally made fish sauces.
Cut the whole mackerels, innards and all, into smaller pieces and place them in a crock. Mix in the salt. Cover the crock with a loose-fitting lid and place it outside where it is warm, preferably in the sun. Allow the fish to ferment for a couple of months, turning the fish pieces in the salt once in a while. Then strain out the liquid, first in a sieve or colander to remove the large pieces and then through cheesecloth to ensure that the liquid is completely clear. Store the finished sauce in the refrigerator in a bottle with a stopper. If you wish, you can dilute it with a little leftover wine. For this recipe, you can also use just the entrails and blood from mackerels.
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