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The main show, the grand performance, where all the work is being done thanklessly by the yeast.  The moto has created a large soup of mostly yeast ready to get a move on.   Since sake is a huge brew, we must break down the moromi into three steps, or additions.  The idea is to allow for a exponential progression of growth.   We can't just hit the gas and move into high gear instantly.  In sake, the sugars are not all available at once.  The rice solids help keep the yeast suspended and in contact with food.  The koji's enzymes will keep breaking down the starch into sugar, but we throttle the food quantity by adding more koji and more starch (rice).   At every step, not all the starch is converted, but the amount of enzymes get doubled.  This allows the yeast to constantly adjust to the environment as well as creating a secondary starter.  The moto is the first build up of yeast.  When adding each addition, more oxygen is also added along with food.  The majority of the yeast will start to reproduce again.  Since the colony is fairly large, the exponential growth is great.  At each addition, you are coaxing the yeast to grow another generation of cells.  By the third time comes around the final generations of growth are finishing up and now there is enough enzymes and food for a constant feeding of the whole colony.  It maximizes every generation as much as possible.  Since yeast consumes sugar, a high concentration of it will stress the yeast.  In sake, sugar is eaten by the time then enzymes make it available.   The temperature at which sake is fermented is low allowing for the enzymes to work slower.  Optimal temperatures for the enzymes are around 113-148F, which most likely will kill yeast.  The ferment continues cool to keep organisms other than yeast in slumber, delaying the spoilage.  At the end of the ferment, the sake is removed from the lees or left over rice solids and pasteurized.  The whole principle is to allow the good functions of the spoiling organisms to be effective, suspend them, and then kill them off at the end to prevent spoilage.