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Moto

Sake is brewed technical like a beer, but yet has process similar to wine.  Either by natural process selection or methodical choices, this beverage makes use of organisms that are usually ruinous and creates a symbiotic cause and effect relationship.  Domesticated mold grows onto a substrate such as rice, creating various useful byproducts and flavors.  The mold's growth is stopped at a peek time for aesthetic and wholesome quality.  More rice is steamed and added to clean impurity free water, usually being soft and free of metallic elements such as iron, zinc and manganese.  What is left of the rice dissolves into the water along with enzymes responsible for breaking down proteins, fats and most importantly complex carbohydrates.  When more steamed rice is added, this enzyme laden water is absorbed into the gelatinized starch.  The enzymes go to work creating sugar, and amino acids.  

The world is filled with life, usually well balanced.  What is toxic for one organism is food for another.  Ultimately, yeast converts simple sugars into co2, flavors, and alcohol.  Yeast requires no competition for its food and expects a proper growing environment.  Sake is a very demanding process and without the yeast doing a proper job, the end product won't be very impressive.

Yeast need a slightly to moderate acidic environment for growth.  Yeast tends to help lower the pH.  Lower pH also stuns competing organisms.
The moto, or yeast starter sets up the environment for stable yeast production, flavors and a most impressive result.

When a source of food is made, organisms tend to find it.  The moto is such a mechanism.  By creating a food source and encouraging the right kind of organisms to grow, the moto becomes acidic and starts to promote more helpful organisms and stuns others.  If left alone too long, these organisms take over just like the mold used for kome-koji would do if left to grow for too long.  The process of the moto slows down at the peek of readiness.  Yeast is added and now the environment is excellent for yeast growth.  By selecting most useful control ranges, yeast becomes the dominate organism.  The rest of the time is allowing the yeast to gain the majority population, but still let the other beneficial organisms contribute a little bit.  By the end of the moto, yeast being dominate , a fairly substantial alcohol content, and acid being present causes all other organisms to practically slumber, if not die off completely.  All the while, slowly working are organisms that will convert alcohol into acetic acid.  These bugs start to come forward if sake is left alone too long without pasteurization.

So to recap, the moto is a yeast starter that uses spoiling bugs to our advantage.  Some forms of moto can have lactic acid added directly, making the environment ready sooner, but will change the flavor complexity later on.