- New sake, old sake
There are no clear legal definitions of the terms shinshu and koshu, written respectively with characters meaning “new sake” and “old sake”. The Japanese Brewing Year indicates the twelve months between July 1st and June 30th the following calendar year, and all sake made in the period of the current Brewing Year is called shinshu. Sake made during the preceding Brewing Year is called koshu, and that brewed still earlier is called o-go-shu (long first” o”, written with the character 大 (for big or great ) before the two characters of koshu).
In some cases, too, unpasteurized sake following pressing is sometimes referred to as shinshu in the period until it undergoes low-temperature sterilization (hi-ire). There are also cases where the sake, brewed in winter and pasteurized, then matured over the summer, is already referred to as koshu when it is shipped in the autumn.
The pale yellow colour of sake darkens during storage, eventually taking on amber colour and showing great differences in taste and aroma. Though varying in degree according to storage conditions, sake stored for several years takes on these characteristics conspicuously. Although the bulk of sake is sold within six months-to-a-year of being pasteurized, that stored for longer to give greater variation of flavour and colour is also sometimes called koshu.