In Japanese culinary terms, “nitsumeru” (verb) refers to the process of reducing the liquid content of a sauce containing seasonings by boiling off the moisture, thereby concentrating the flavor and increasing its thickness. Both “eel sauce” and “conger eel nitsume” are sauces made by reducing the liquid content through boiling.
First, you need to understand the difference between eel and conger eel.
In Japanese, the word “unagi” generally refers to a freshwater eel. The scientific name is Anguilla japonica Temminck & Schlegel, 1846. On the other hand, the word “anago” generally refers to the conger eel. Its scientific name is Conger myriaster (Brevoort, 1856).
While both eel sauce and conger eel nitsume are made by reducing liquid through boiling (the process known as nitsumeru), only the latter is traditionally referred to as “nitsume” in sushi restaurants.
So, what is the fundamental difference between them?
Eel sauce (known as unagi sauce or unagi no tare) is a rich, sweet soy sauce-based sauce that is ideal as a dip or drizzling sauce. It does not actually contain any eel ingredients, but it is a sweet, sticky sauce that is perfect as a topping for sushi rolls and other dishes.
“Eel sauce” can refer broadly to both mass-produced sauces and artisan sauces made by eel restaurants. The basic method for making eel sauce is to mix mirin and soy sauce and heat it. Various ingredients such as sake, sugar, tamari soy sauce, water amber, honey, dashi, and potato starch (commercial products often contain thickening agents) are added to create a unique flavor.
Incidentally, most traditional eel restaurants make their eel sauce using only high-quality mirin and soy sauce. This gives the eel a beautiful glaze and subtle sweetness when grilled, as well as a clean aftertaste, which is why it is considered a craftsmanship.
When making kabayaki, the process of applying eel sauce is repeated multiple times. The components of the eel sauce combine with the umami of the eel to create a deeper, more complex flavor. In addition, applying the sauce and grilling it causes chemical reactions such as the Maillard reaction and caramelization, which further enhance the appetizing flavor.
While you can easily buy eel sauce at the market, it’s incredibly easy and tasty to make at home. Eel sauce can be used as a seasoning for other dishes if there is any left over, but it is primarily a sauce specifically made for making kabayaki. By the way, kabayaki is a fish dish where the long-bodied fish is filleted, the central bone removed, skewered, grilled, and then coated with a thick sauce before being grilled again. In Japan, when people say “kabayaki,” they are usually referring to “unagi kabayaki.” Other ingredients used include hamo and anago.
Nitsume is brushed onto nigiri sushi made with anago, shako, shellfish, and other toppings. In sushi restaurants, it is often abbreviated as “tsume.” There are several methods for making nitsume, including reducing the cooking liquid from simmered conger eel, or simmering the head and bones of conger eel to extract the broth, then adding seasonings and reducing it further. In other words, nitsume contains components of conger eel.
Compared to anago, unagi is more flavorful, fatter, and meatier. Moreover, unagi is considered a higher-quality delicacy in Japan and is therefore more expensive. In terms of taste, unagi has a richer, more intense umami flavor. The common opinion is that anago, because of its lighter taste, harmonises better with soured sushi rice and is therefore the preferred choice for making nigiri sushi.
As a side note, while nigiri sushi with eel is common in the Kansai region, eel is not used as a sushi topping in Edomae sushi. Therefore, Edomae sushi restaurants do not serve unagi nitsume.
In summary, unagi (eel) sauce is made by eel restaurants, while anago (conger eel) nitsume is made by sushi restaurants. Eel sauce does not contain eel components, while conger eel nitsume contains conger eel components.