Japanese sake is normally subjected to hi-ire (pateurization) twice (once when stored and once before shipment). However, namazake is not heated at all. Therefore, it has a fresher flavor. The quality of namazake changes depending on the storage condition. Namazake is unstable compared with standard sake. Therefore, they make two type of sake that has only one pasteurization (heat-treatment), to balance the freshness and stable, called Nama-chozo shu and Namazume shu.
Namazume shu has gone through the first pasteurization process but not the second. It is usually matured after the first pasteurization and then shipped without pasteurizing, giving it a Namazake-like flavor.
Namachozo Shu has gone through the second pasteurization process but Not the first. Usually contains more umami flavor than Namazume Shu.