What type of water does the term kosui refer to?

Kosui specifically refers to hard water that has a high mineral content, which is significant in the context of sake brewing. The mineral composition of the water used in brewing can greatly influence the flavor profile and overall quality of the sake.

Hard water typically contains higher concentrations of minerals like calcium and magnesium, which can enhance the fermentation process and impact the resulting sake's texture and mouthfeel. Different types of sake, such as Junmai or Ginjo, may benefit from varying water profiles, but the presence of hardness in the water can help in developing richer flavors, making it particularly desirable for certain styles of sake.

Understanding the characteristics of kosui is important for brewers, as the choice of water is one of the critical factors in the sake-making process.