Long before the days of thermometers, hydrometers and barometers, brewers relied entirely on their senses to gauge the progress of a fermenting tank of sake. They might not have known the scientific causes, but experience and intuition told them how to interpret what they saw, tasted and smelled.
One of the most reliable of these empirical yardsticks -- one that is still used today -- is the appearance of foam on the top of the moromi (fermenting mash). Throughout the 18- to 35-day ferment, the foam will change appearance quite regularly -- and very clearly reflect what is taking place inside the tank. Over the centuries, names were given to the foam at each stage, making it easy to assess and convey the status of the work in progress.
After the yeast starter has been created, and after water, rice and koji have been added over four days, foam will begin to develop as the yeast cells process the sugars in the tank and give off carbon dioxide. This will rise to the surface, often dragging yeast cells with it. This is why the foam has 2 1/2 times as many yeast cells in it as the liquid beneath.