Yasei-kobo (wild yeast) refers to all kinds of yeast which originally live in the nature including the air, the soil, and plants while, in the fermentation process of Japanese sake, refers to all kinds of yeast other than those that are used to obtain the desired sake quality.
Though not in the concept of 'yeast' defined by the modern science, the human being have been, since the ancient times, explicitly and implicitly aware of the presence of yeast which variously influences our daily life, have been selected the yeast based on our empirical rules, and have been used only the yeast which is suitable for the purpose of ours. Yeast for seishu (clear rice wine) has also been screened and handed down by the human being in such a history.
When yasei-kobo proliferates in a bucket or a tank, the sake produced itself does not have the quality which is aimed at even though the right yeast for seishu has been added.
Therefore, in order to prevent yasei-kobo from being mixed in, a dedicated space under strict management is provided when shubo (the main material of sake) is produced. This is referred to as shubo-shitsu (shubo room) or motoba. In general, the room temperature of shubo-shitsu is controlled to be at 5 centigrade.
When study tours to sake producers are conducted, some sake producers show the visitors their motoba.