Red miso, or Akamiso (赤味噌), is salty with some astringency with umami. It is strong-tasting, and the depth of color depends on the formula of the soybeans and the quantity that is used. Red miso is aged, for a year or more.
Miso, or みそ, is a traditional Japanese seasoning produced by fermenting soybeans with both salt and koji (fungus Aspergillus oryzae) and sometimes with rice, barley and seaweed. Miso is salty, but it's flavor and aroma depends on the ingredients used or the fermentation process. There is a wide variety of Japanese miso, it is typically classified by grain type (barley, buckwheat, rice etc), color (white, brown, black & red), taste (sweet, salty, earthy, fruity etc) and background (where it's from). The most common miso flavor categories are: Shiromiso (white), Akamiso (red) and Awasemiso (mixed). Miso is a rich source of protein.
Miso is high in sodium, a known carcinogenic, but is safe to consume as the carcinogenic effects of the salt are counteracted by the anti-carcinogenic effects of the soy. As for risks of hypertention, the salt in miso is squeezing blood pressures up, but the soy protein in miso is relaxing blood pressures down. All in all, the anti-hypertensive effects of the soy in the miso exceed the hypertensive effects of the salt.
Fermentation time ranges from as little as five days to several years. The color of fermented miso changes from white, to red or black, due to the Maillard reaction, a chemical reaction between amino acids and reducing sugars that gives browned food its distinctive flavor. Miso typically comes as a paste in a sealed container requiring refrigeration after opening. It is used in soups (miso soup, ramen, nabe, udon etc), traditional confections (mochidango) and to make pickes (misozuke).
Cooking notes: As natural miso is a living food, it contains many beneficial microorganisms (like Tetragenococcus halophilus) which can be killed by overcooking. To better conserve these precious microorganisms, miso should be added to soups or other foods just before they're removed from heat. Eating raw miso is even better.