Mentsuyu is a Japanese flavoring sauce made mainly from soup stock, soy sauce, mirin (sweet sake [Japanese liquor] for cooking) - or sake itself - and sugar. It is mainly served as soup with noodles, such as somen (Japanese vermicelli), soba (Japanese noodles made from buckwheat), udon (Japanese noodles made from wheat), and hiyamugi (thin udon noodles served cold).
In the Muromachi period, before kaeshi (the flavoring sauce served with soba for dipping, using soy sauce as an ingredient) existed, but after udon came to exist, there was a flavoring sauce called taremiso. Taremiso was made by adding water to soybean paste, boiling it down, and straining it through a cloth bag. Taremiso has two categories, namatare (the raw taremiso) and ninuki which is made by boiling down both namatare and shaved pieces of dried bonito together. Incidentally, namatare was abbreviated to "tare." And today's tare seems to have originated from namatare.
It says in 'Ryori Monogatari' (Tale of cooking), a book written before 1643, that soba dipping sauce can be made by adding the juice of piquant "daikon" (Japanese radish) to taremiso (for udon) or to ninuki, and that one can add other ingredients to the sauce for flavor, such as, shaved pieces of dried bonito, grated piquant daikon, chive, mustard, and Japanese horseradish.
Also, 'Soba zensho' (All About Soba), a book written in 1751, introduces two kinds of production methods for soba dipping sauce.
How to make soup with taremiso
You should boil down taremiso, sake, and shaved pieces of dried bonito together.
You should strain them, and then add salt and thick soy sauce for flavor.
The above-mentioned taremiso, after being warmed, is what was really used at soba shops in Edo at that time.
How to make soup with soy sauce
You should boil down soy sauce, sake and water together on a low flame.
This is the soup that Yukyoshi NISSHINSHA, the writer of 'Soba zensho' used for his homemade soba. He himself did not use shaved pieces of dried bonito, because he favored Shojin-jiru (soup that does not include animal protein). However, he also wrote in his book that those who favored the animal protein could add the broth extracted from the bonito pieces to the above-mentioned soup. After that, the soup that used soy sauce and mirin (or sake with high sugar content) became popular.
Mentsuyu today on the market
Mentsuyu today on the market is, in most cases, used after being diluted with water, because most of it is concentrated. It is, of course, served as soup with noodles. It is also used as the flavoring for nimono (boiled foods), the soup stock for nabemono (foods cooked in a pot), or the dipping sauce for tempura (Japanese deep-fried dishes). It is sometimes poured on foods, such as bean curd served cold and grated daikon, without being diluted. Or, it is sometimes combined with other flavorings into a new dressing or a new Japanese style sauce. It can also be used, instead of soy sauce, with tamago kake gohan (boiled rice topped or mixed with raw egg, commonly with soy sauce poured on it), along with many other uses. As it uses sugar and soy sauce as its main ingredients, it spoils easily, so after opening it, one should keep it in a cool, dark place and use it up as soon as possible.
About the soup stock used in mentsuyu
Basically, mentsuyu is made by adding sugar, soy sauce, and soup stock to mirin (or sake) that has had its alcohol content reduced by heating. The soup stock is made from sea kelp, shaved pieces of dried bonito, shiitake mushroom (Lentinus edodes), dried small sardines, and so on. The soup stock of mentsuyu on the market is mostly made from sea kelp and shaved pieces of dried bonito.