Recently opened a bottle of sake? Wondering how long it will keep its "freshness"? Want to know what happens to the flavor over time? How about storing it once its been opened?
Sake is a wonderful thing in that it is pasteurized. This beats the pants off of preservatives such as sulphites, which die in mater of hours rather than days. Now there are some folks out there who say that sake is ruined once you open it. I say BS. We all cannot live in a brewery now can we? I like to point out the fact that some Japanese sushi restaurants will keep a large bottle of sake out in the open air until it is sold and gone. This could be up to three weeks!
Sake that has been opened (seen air) starts to decay. Yuk! No not like cavities. It decays as it oxidizes. But unlike wine that goes grotti when air hits it, sake tends to get softer and at times even silkier with a muted acidity. The flavor of these open older sakes tends to compact and get thin in the middle and round on the edges. Sometimes the acidity takes hold and it gets even more brisk/sharp. You never know. It's like Forrest Gump's box of chocolates. I swear to the Bible of how sake is made! I ask the Tojis (head brewers) themselves. And they say that sake should be consumed as soon as it is opened to achieve the flavor and feeling that they wanted you to taste. That is in a perfect world. But we don't live in that world at all times. So I say that you will not really taste a significant flavor change in a sake for three to four days in the refrigerator (this is a generalization as all sakes die differently).
It also depends on the type of sake too. For instance, Dai Ginjos and as such change far less than other categories of sake as they have fewer impurities that go wacky. These sakes actually get a little smoother a couple days after the fact. So the answer is that there is no real answer. But rest assured most sakes (99%) are drinkable 3-4 days after opening for certain, and then it is a matter of taste. If you find that an opened bottle doesn't agree with you 5 days down the road then try warming it. Or even making it hot!
For storage think cool and dark. And try to consume the sakes between 12-15 months after the bottling date.