Koshu/熟成古酒 means “old” or “aged sake.” This informal term refers to sake that’s been aged at the brewery for some time before release.
In general, sake tends to get darker, more savory and more mellow with age. How the sake is aged has a huge impact on the final results.
It may be refrigerated throughout this period, or allowed to mature at room temperature. It may be aged in a metal tank, a wooden barrel, or after bottling. Some breweries may even employ a combination of the methods above, or blend together koshu brewed in different years to achieve a desired flavor profile.
It has well-balanced sweetness, acidity, bitterness and umami (savoriness) while still containing the characteristics of Japanese Sake. When poured, it sends out sweet aromas like nuts, caramel and vanilla. Its golden color, thick texture and long finish are also its uniqueness.
It is best to be consumed at room temperature, just like whiskies and other spirits. A Sake cup / glass shaped like a wine glass is recommended to swirl and enhance its rich aromas, like red wine or brandy.