Koje' are microbes, similar to those used in blue cheese production, that are good for the health. In ancient Japan, sake' production was confined primarily to the imperial court and to large temples and shrines. That's why sake' is often associated with religious rituals and festivals. Even today, sake' is always served at traditional ceremonies in Japan. Since sake' is an extremely delicate alcoholic beverage that is especially sensitive to heat and light, it must be stored in a cool, dark place.
Refrigeration is recommended.
Exposing sake' to sunlight can change its taste as well as its color. Exposure to air can reduce the quality of sake' over time. This means that once you have opened a bottle, it's best to finish it fairly soon. You can drink it either way. It's also perfectly acceptable to drink it at room temperature.