I am looking at a paper, which in part, discusses the impact of rice proteins on sake aroma and flavor. And admittedly, I may be stepping over the boundary between sake brewer and geek with this discussion, but I think you may find it interesting to see some of the underlining interactions that make sake what it is.
In this paper the authors brewed five sake variants, four of which include rice enriched with rice derived proteins. They then assessed the relative impact of these proteins on flavor and aroma.
The protein in the rice consists of roughly of 20% prolamin and 70% glutelin with several other proteins filling out the mix. The four tested in this study were prolamin, glutelin, globulin (2% each of total rice mass enhancement) and albumin (1% total rice mass enhancement).
While the sake with added prolamin has alcohol levels and acidity close to that of the control (no added proteins) the rest have lower alcohol levels and higher acidity. All brews have higher amino acids than the control. Not surprising because proteins are broken down by enzymes from the koji into amino acids. However, both the koji and yeast also produce amino acids which could provide unexpected outcomes.
Because some of the flavors and aromas come from the koji enzymes braking down the proteins into amino acids that contribute to the overall taste and aroma of the sake. Yeast also produces aroma and flavor (more aroma than flavor) but in this study the yeast was held constant and used in all 5 brews.
Isoamyl alcohol, which is known for its contribution to unpleasant aroma, had higher levels in each brew as compared to the control. On the other hand, all but one of the enriched brews had lower levels of isoamyl acetate and ethyl caproate which give a fruit-like aroma to sake. The single exception was the brew with the addition of globulin which had the highest levels of isoamyl alcohol, isoamyl acetate and ethyl caproate as well as the highest amino acids and close to the highest acid levels.
In the sensory scores for aroma and flavor (conducted by a seven person panel) no brew using protein enriched rice did better than the control and only one did as well as the control. The brew enriched with albumin matched the control in both flavor and aroma. The authors conclude “In sake brewing, enrichment with rice protein extracts at the beginning of the fermentation had a significantly unfavorable effect on the sensory properties of sake. The aroma of sake deteriorated significantly with the addition of prolamin.”
This analysis shows just part of the story about why the milling of sake rice is so important. Other elements in the bran and concentrated in the outer layers also contribute negatively to sake quality, but that is a different post J
 Influence of Rice Proteins on Eating Quality of Cocked Rice and on Aroma and Flavor of Sake by Sachiko Furukawa, et al. 2006