Greetings, I am a rice brewer in Quebec Canada; Starch is the fermentable that is used in cereal ferments. Starch is converted to sugars, enzymatically (Saccharification), whether the enzymes are sourced from Bacteria, Rhizopus (yeast balls), Aspergillus (Koji), or embryonically from germination (malting) , starch converted to sugar, provides the base for fermenting. Starch has two constituents amylose, a substance affected by the enzyme amylase to create readily fermented sugar, and the constituent amylopectin pectin referring to a protienaceous gel type of starch that is less fermentable than amylose.
Saccharification is the conversion of starch to sugar. In Africa and Asia the bacteria Rhizopus tends to be a common saccharifier in cereal ferments. Aspergillus is the saccharifier for sake. Rhizopus favors wheat, there are many varieties of both aspergillus and rhizopus. Rhizopus tends to be predominant in rice ball jiu. Unfortunately rhizopus can be mucilaginous, producing a polysaccharide, something like mother of vinegar, a pellicle, and have high ester profiles, fumaric acid being one of them. Thus a bitterness and potential sliminess. Aspergillus does not have a polysaccharide tendency, less protein produced. Proteins tend to create a cloudiness and tendency to off flavor.
Controlling the response of protein in the ferment can create better results. The Japanese polish their rice removing as much protein as possible so that the sake ferment is dry clean and clear. Protienaceous sugars tend to be less fermentable and lead to sweeter results, cloudier, and thicker in mouth feel. Higher temperature ferments tend to be faster, clearer, thinner mouth feel, and dry less sweet.
It is quizzical to me that for the most part the rice varieties chosen for ferment are all glutinous types (higher protein) , one wonders if that is cultural and only because that is all that was available.