Interesting stuff here.
Koshihikari rice from near Griffith in southern NSW. Notes from the brewers; The crux of what we try to do in the brewery is make the best sake we can from Australian rice. This = serious limitations in rice types and processing options we can use which = blessing and curse. Blessing in that we can focus on getting results from what we have. We use two different rice types, both of Japanese heritage, both intended for eating (table rice). Table rice = more nutritious than sake rice = more non starch nutrients = more prone to robust, savoury, umami flavours. We can't polish the rice as the machinery is specialised and $$$$ = more non starch nutrients = more prone to robust, savoury, umami flavours. So basically the rice we use wants to become super savoury porridge. We balance that out by using low and slow fermentation temp + aromatic yeast strain to try and bring some brightness and freshness. Post fermentation we don't dilute, pasteurise or charcoal filter, unlike 99% of sake's. Both Sakes are brewed EXACTLY the same way. 90% Polishing Rate (10% removed - basically white rice you would eat). 25% Koji Ratio. Yeast 9 - aromatic and floral. Low-temperature fermentation so the yeast has a chance to build some delicacy and freshness around the robust and rich riceyness that 90% polished gives.. So the differences you are seeing are purely from the rice.