Aged sake, or Koshu, so called if they are aged for more than 3 years in the sakagura, today constitute a small slice of the national production of sake, but they are constantly growing. More and more producers are experimenting with new aging methods or replicating ancient production methods that require a maturation phase in wooden barrels.
Aging in "taru", the Japanese wooden barrel, is still the most common method, but you can find different sake matured and aged in steel tanks or directly in the bottle.