Ryuka-shu (sulfur odor)

Ryuka-shu (硫化臭, sulfur odor) is a distinctive smell that can occur during sake brewing, mainly while the shubo (yeast starter) is bubbling up or while the moromi (fermentation mash) is foaming. The smell is caused by hydrogen sulfide and resembles rotten eggs. It is unpleasant, but it usually disappears over time. Even if a sulfur odor is noticed temporarily, there is no need for concern. This phenomenon can occur naturally in sake brewing, and with proper management and handling it does not affect the final product.