Miso is a fermented bean product usually made from soy, although it can also be cultured from other varieties of beans such as chickpeas or azuki beans. Grains such as rice or barley may also be used for additional flavor. Miso can be an artisan product, cultured and packed by hand in small batches, or it can be industrially manufactured in factories. Miso has a long shelf life, with some aficionados even claiming that it lasts indefinitely. Kept refrigerated, miso should last at least a year, even if its expiration date is shorter.
Beneficial and Harmful Microorganisms
Miso is produced through a process of lacto-fermentation, which involves introducing beneficial bacteria to make the nutrients in the beans and grains more easily available to the human body. Miso can ferment for six months to three years before it arrives on the shelf of the grocery or natural foods store. Refrigeration slows down this fermentation process once the miso has reached a suitable flavor and color, but some fermentation continues even at lower temperatures. Unlike the beneficial organisms involved in fermentation, mold is a biological process that can make miso inedible. Although you can simply scrape mold off the surface of a container of miso, it is safer to discard the entire container if you see any.
Mellowing with Age
The beneficial bacteria that digest miso's starter beans and grains through fermentation continue to do their job while your miso is sitting in your refrigerator. This fermentation process gives miso its deep and mellow flavor, and this flavor continues to ripen and deepen as the miso ages, losing some of its sweetness. The color of the miso changes as well, darkening over time until the paste eventually turns completely black. According to the South River Miso Company, even miso that has turned black is safe to eat, although it should not be consumed if it smells sour or spoiled.
Pros and Cons of Pasteurization
Artisan miso is often unpasteurized; that is, heat has not been used to control the spread of dangerous microorganisms. Because the fermentation process that produces miso also depends on microorganisms -- albeit beneficial ones -- pasteurization can kill helpful, healthful bacteria as well. According to Mark Bittman of the New York Times, pasteurization may increase the shelf life of refrigerated miso, although it does also lessen and simplify its flavor.
When cooking unpasteurized miso, don't heat it past the boiling point of water. Boiling kills off beneficial microorganisms created during the fermentation process. If you are preparing a miso soup, cook the other ingredients, turn off the heat, and then add the miso. Although using miso this way keeps the beneficial bacteria intact, miso dishes and soups are nonetheless extremely perishable. The other cooked ingredients usually have short shelf lives, even in the refrigerator. Store cooked leftovers containing miso in the refrigerator and discard them after several days.
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