The Nikkei Style site features an article on how sake can be better hot, increasing its aroma and umami factor.
Kicking off with the obligatory seasonal reference, it points out that winter is the season for enjoying your sake warm, particularly together with dishes like nabe or stews that will ward off the chill.
Despite the success in recent years of light and fruity sake best drunk cold, there are still plenty of labels that benefit from kan-agari (燗上がり), the revelation of their inherent characteristics through heating.
Inokichi Shinjō, 7th kuramoto of the Suehiro brewery in Fukushima Prefecture, enthusiastically claims that sake is one of the only alcoholic drinks that gets better when heated. While the West does heat some drinks, like hot wine or whisky, sake is the only one whose flavour changes significantly with temperature.
Shinjō recommends yamahai or kimoto to those who are new to heating sake. Both are started with natural lactobacilli, so more lactic remains in the end product than with standard fast-starter (速醸, sokujō) brewing. And according to Shinjō, it’s the lactic acid that increases flavour when heated.
The brewery was founded in 1850, around the end of the Tokugawa Shogunate, and according to the article established the yamahai style of brewing. The ancient kimoto method involved putting the steamed rice into a wooden barrel (木桶, ki-oke) and crushing it with wooden poles, a process called yama-oroshi (山おろし). The name yamahai is a combination of yama-oroshi and the term haishi (廃止), to abolish. The yamahai method developed out of experiments by brewing researcher Kinichirō Kagi at the Suehiro brewery in the early Taishō period.
The recommended serving temperature for their flagship product, Denshin Yamahai Junmai Suehiro, is nuru-kan (ぬる燗), around 40°C (104°F), or atsu-kan (熱燗), 40-55°C (104-131°F). Heating brings out a rich, mellow fragrance similar to hōji-cha (roasted tea). Flavours unfold gently in the mouth, with a gradually rising level of umami.