The flavor of miso is complex and built with a harmony of sweetness, saltiness, umami, sourness, and bitterness. To make a tasty miso soup, all of the components must be high quality and blended for the proper balance.
Miso’s sweet flavors come from koji; the more koji, the sweeter the miso. Miso’s umami flavors come from the breakdown of proteins into amino acids, which happens during the fermentation process. The longer miso is aged, the stronger the umami flavor will be. As the umami flavors get stronger, the saltiness is rounded out and softened. So, the longer miso is aged, the less salty it will taste.