Iwanoi is the flagship of Iwase Shuzo, a brewery founded in 1723 in Onjuku on the Boso Peninsula of Chiba Prefecture, drawing on one of Japan's hardest brewing waters at roughly 240 degrees of hardness from mineral-rich shell-layer aquifers. The brewery practices yamahaimoto fermentation for a significant share of its output, and its yamahai junmai daiginjo scored 95 points in the Japanese equivalent of the Parker point scale in 2017, ranking second among 800 entries.
