It is a fragrance derived from esters, especially ethyl caproate and isoamyl acetate, generated by sake yeast in the process of low-temperature fermentation like ginjo brewing. (Refer to "Moromi.")
吟醸造りのような低温発酵時に酵母が出すエステル類、特にカプロン酸エチルや酢酸イソアミルに起因する香りである（参照：醪）。