If refrigerated, Miso will last a very long time. Exact shelf life depends on the salt and moisture content of the miso. It is not uncommon for miso to be stored for over one year in a refrigerator without any ill effects on taste or quility.
There are several ways to preserve miso, whether by using additives and preservatives, dehydrating or freeze drying, or storing in a refrigerator or freezer. The Japanese first began dehydrating miso in the 1950’s through a process called “spray-drying” where miso mixed with water is sprayed into an atomizer, and quickly solidifies into fine powder. Freeze drying however is the preferred method of preserving miso which involves quick-freezing miso on large trays which are then vacuum-dried for several hours. Though slightly more expensive and labor intensive, freeze-drying preserves more of the flavor in miso.
Miso is produced through a process of fermentation, which can act as a natural preservative under the right conditions. Quick cook miso is usually pasteurized before packaging to prevent the continual production of carbon dioxide which sometimes causes the packaging to pop. Varies chemicals and synthetics including food coloring, bleaches, ethyl alcohol are sometimes added to aid in the aging process. Naturally produced miso is largely kept in large barrels by local farmers to maintain nutritional and widely-held medicinal value. The most wildly recognized methods of packaging however usually involve Styrofoam cups or polyurethane bags with allow producers to include freeze-dried ingredients like vegatables and beef.
At home, refrigerate miso in an airtight container. Use the light-colored miso within nine months and dark miso within eighteen months. Packets of additive-free, freeze-dried instant miso soup are available in natural food stores, easy to store in bulk, and are good convenience items. They are also great for travel and quick lunches.