As we all know, sake is a perishable product that is pasteurized and does not use the long lasting aging agents such as sulfites found in wine. Wine of course wins the battle of being able to lay bottles down for years and years, but once you open a bottle of wine you are only talking hours rather than days for drink-ability. Because of pasteurization sake keeps longer once the bottle is opened – much longer! Now what about aging sake if it is perishable? Like brewing sake itself, the act of aging sake is an art form pure and simple.
Aging sake is part of the sake making process. Historically sake was made in the winter and spring months. It was "laid down" either in large vats or in bottles during the summer and released in the fall. This so-called aging period was intentionally used to mellow out the flavor of freshly brewed sake, which is actually quite lively, raw and crisp. Let's just say they used the "aging period" to take the edge off. Each brewery uses their own aging schedule, which they feel works best for their sake. That said the average aging/mellowing period is roughly 6 months, at which time they bottle and/or release the sake to the public. If you put a gun to a sake maker's head and said "how long will the sake be good for after you bottled and released it?" they would answer, "help!" But if you asked nicely they would say "6 months." And by that time figure they are guesstimating that the sake will not change much during that period.
Aged sake is called Koshu. And there are many ways to achieve this "aged-sake" status from keeping it refrigerated at sub-zero temperatures to forgetting about leaving a bottle in your closet for years. The results vary. Some breweries specialize in Koshu and they use various refrigeration techniques, and others use room-temperature storage techniques. Low temperature or freezing aging techniques yields a mellowed yellow color sake or even clear if it is cold enough. And room temperature techniques yield a brownish colored sake bordering on soy sauce looking. They all look and taste quite differently. I know of one brewery that blends their 25,15, and 10 year-old Koshu into one sake.
Koshu sake is still quite a mysterious subject. Many feel it is a bit of a taboo topic, but more feel that aging sake is an entirely new and pleasurable way of enjoying this great beverage. Typically Koshu is served after a meal, as they tend to be full-bodied with deep and rich flavors. Many earth tones such as grains, nuts, mushrooms, rice, and straw become pronounced, and deep fruit tones and honey-like qualities also appear. I will be the first to say that Koshu is not for everybody, but once you try it – it is a broad new terrain to travel. I also make it a policy and store invitation that I will trade you new and fresh sake for any old sake that you may stumble across in your closet. But be brave and try it yourself. The worst thing that can happen is that it tastes bad and you chuck it. I have never heard of anybody dying from bad Koshu. That said I have heard of people dying for good Koshu!