Miso, which we think of as Japanese, probably originated in China. The base is made from soybeans or other beans, grain (usually rice or barley), and salt. Then a starter (koji) — usually one that includes a mold called Aspergillus oryzae — is added to begin fermentation. High-quality, naturally made miso may go through a cycle of fermentation and aging that lasts as long as three years, sometimes in wood barrels that add flavor (just as they do in winemaking). With more industrial or “quick” miso, this process is hurried and usually includes pasteurization.
Like good-quality yogurt, traditionally made miso is a living food, full of enzymes, micronutrients, and active cultures. Since pasteurization kills beneficial microorganisms along with undesirable ones, and at least some of the taste, it’s best to buy unpasteurized miso whenever possible; these may be Japanese or, increasingly, domestic.
There are dozens of subcategories of miso, but most fall into three depending on the main ingredients. (Miso is also sometimes identified by color, regardless of ingredients.) When soybeans are fermented with white or brown rice the result is kome-miso, usually white or light beige in color and smooth in texture, with a hint of sweetness. You may also see it labeled “mellow miso” or “sweet white miso.” This is best used in dressings and light sauces, and as a dairy substitute.
Mugi-miso is made from barley and soybeans and is often referred to as yellow, medium, or mild miso. It is usually smooth textured and ranges from golden to reddish brown; the flavor varies a bit but is best described as earthy. Consider this an all-purpose miso.
Hatcho miso contains all soybeans; these are the richest, darkest, deepest-flavored misos. Some are chunky; some are smooth. Either way, this is the best type of miso for heartier soups and stews, full-bodied sauces, and glazes.
Miso must be stored refrigerated, where it will keep for months. To keep it from spoiling, always use a clean spoon when you remove some from the container.
Unless you are a miso aficionado, you’ll probably have just one miso in the fridge at a time. It may not be traditional, but all may be used interchangeably in recipes calling for miso. There is one rule, however, that should not be broken: Don’t boil it. You will not only deactivate the beneficial cultures, but you will dramatically reduce the complex flavor.