Advantage: rice can be stored for long time
Disadvantage: not a fermentable sugar
・Reduce the unneccessary ingredients (proteins, lipids, vitamins and minerals)
・Adjust the moisture content
・Gelatinisation for the enzymes to consume

Polishing machine was developed around 1930, widely spread around 1980, it leads 'ginjo boom'.
For 600kg of rice, it takes 10hours to 70% polishing racio, 45 hours to 50%.
After polishing rice is rested 3-4 weeks to reduce the heat and reabsorb the moisture to avoid cracking.

To remove remaining nuka from polishing.
Premium sake: traditionally handwashing with small batch, also the machine to moisturise evenly
Futsu-shu: large scale washing tank 

Moisture should rise to 30-35%, water temperture 8-15c.
If the Moisuture level is too high koji mould grows too quickly without producing the appropreate levels of enzyme. Also the fermentation breaks up too quickly. 
Less polished (70% polishing racio) rice hardly absorb more than 30% of water. Highly polished rice(50% or less) water easily abosorb more than 30% so more precision required, soaking should be done ・in small batches ・cold water to slow down the abosrption・timing the soaking

Steaming is to sterilises the rice, change the structure of starch molecules (gelatinise), and to raise the moisture level to 40%.
The rice should be cooked evenly, firm on the outside and soft on the inside.
Koji rice should be less moisture so the top layer of the rice is used, lower layer is used for the starter or main fermentation. 
Usually traditional method takes about 1hr to steam, continuous steamer takes 30-40 min.