Most Americans enjoyed their first cup of Sake served in a style the Japanese call atsukan, or very hot. While it is certainly pleasant to drink warmed Sake on a chilly night (some brewers even craft their Sake specifically to be served slightly warmed or nurukan style ) in order to fully appreciate the subtle aromas it is best to enjoy fine sake just slightly chilled.
Modern rice-milling techniques, carefully isolated yeasts, and newly crossed “heritage” strains of rice all collaborate to offer a broad diversity of flavors in Sake, most of which are muted when the wine is heated. All those delicate melony notes and earthy, herbal influences will simply evaporate with the introduction of heat. If you do wish to enoy sake served warm try to choose a premium sake that is dry, robust and with less distinctive bouquet. This type of sake might also offer pleasing umami notes when served warm.
For chilled sake there are plenty of seasonal styles to consider such as Nigorizake, an unfiltered Sake style now gaining popularity outside of Japan. The word nigori, which means “cloudy,” refers to this style allows the rice remnants that did not fully ferment in the process of brewing to remain in the bottle. In more refined Sake, these remnants would have been removed, but in Nigorizake the remnants are allowed remain with the wine in order to enhance both its texture and flavor.
Another unique Sake experience, only available for a brief period at the start of each year, is freshly-pressed Namazake or unpasteurized Sake. Nama means raw or fresh, and this type of Sake has never undergone the brief heating process designed to kill off enzymes and stabilize the drink for longer bottle-life. In fact, heating-up a bottle of Namazake would defeat the entire point of drinking it fresh and unpasteurized. For this reason unpasteurized Namazake must be kept exceptionally well-chilled in order to prevent the enzymes from activating and spoilage to occur.
By the way if spoliage ever did occur you’d very likely notice it by a disturbingly yeasty aroma and a taste both cloyingly sweet and unpleasantly acidic. The best way to avoid this risk is to religiously finish-off every bottle of sake you decide to open no matter the cost involved, perhaps accompanied by some fresh shellfish or a delicately-steamed spring-roll served with a bright and citrusy dipping sauce.