We call unpasteurized sake Namasake. Brewers pasteurize sake in order to stabilize its quality and halt fermentation. For this type of sake, However, brewers do not conduct pasteurization, and that allows fermentation to proceed during storage and after bottling too. So, compared to pasteurized sake, this kinds of sake has unstable shelf-life. Therefore, you should drink it as soon as possible after you buy it, keep in mind it always needs to be chilled. This kinds of sake is not pasteurized, so you can enjoy its fresh taste