Usukawa (thin foam film on the moromi)

Usukawa (薄皮) is one of the phenomena observed during the fermentation of sake moromi (fermentation mash). It refers to the state in which a thin, film-like layer of foam forms on the surface of the moromi as fermentation progresses. It is also evidence that the yeast and enzymes in the moromi are fermenting actively, and the condition of the foam serves as an indicator of how far fermentation has advanced. Usukawa means that light foam is visible on the surface of the moromi, but a solid body of liquid lies beneath it. As fermentation proceeds, the moromi passes through other foam stages (such as sujiawa 筋泡, mizuawa 水泡, iwaawa 岩泡, and takaawa 高泡) and finally enters the stage in which the foam subsides. Usukawa holds an especially important place in this process. By observing the condition of the moromi, brewers can follow the progress of fermentation and carry out the necessary operations at the best time. The appearance of usukawa is one of the important indicators in making high-quality sake.