Sake Brewing Science Vol. 1 - The Significance of Rice Polishing Ratio

Realizing the importance of studying sake brewing in greater depth, I have decided to launch a project to decipher sake from the perspective of brewing science.

We begin with the raw material: rice.

vol.1 - The Significance of Rice Polishing Ratio

Sake has a curious set of rules where its classification changes based on the rice polishing ratio. For instance, rice polished to 70% or less is called "Honjozo," 60% or less is "Ginjo," and 50% or less is "Daiginjo."
Why is the rice polishing ratio so important?

The Definition of Rice Polishing Ratio

According to the National Tax Agency, "rice polishing ratio refers to the weight of polished white rice as a percentage of the original brown rice." If 1kg of brown rice is polished down to 600g of white rice, the polishing ratio is 60%; if it is milled down to 200g, the ratio is 20%. It is simply a percentage, as the term implies.

Why Polish Rice?

According to the National Tax Agency, "the germ and outer layers of rice contain proteins, fats, minerals, and vitamins. Excessive amounts of these components can negatively affect the taste and aroma of sake. Therefore, when used as a raw material for sake, white rice is used after these outer layers have been removed by polishing."
In short, the prevailing opinion is that "excessive nutrients are off-flavors." But is this really true? In the world of beer, to borrow the term, brewers use malt with a 100% polishing ratio—the barley is crushed whole—and I have never heard it said that it is better to polish the surface of malt.

Where, then, lies the true intent behind polishing rice?

The Significance of Polishing - Inhibiting Ester Formation

Unlike beer, sake has fewer raw materials and cannot utilize flavor-packed ingredients like hops. Furthermore, since rice itself does not have a particularly intense aroma, the generation of fragrance through fermentation becomes extremely important.

The two primary scent compounds generated by fermentation are esters and thiols.
Setting aside thiols for a moment, it appears that ester formation is linked to the polishing of rice.

Rice Polishing Ratio and Ester Formation - Proteins

It is documented that the outer layers of rice are rich in proteins and lipids. Although this research dates back to the 1970s, it indicates that polishing rice to a 60% ratio reduces protein content to below 20% and lipids to below 5%.
Proteins are eventually broken down by koji mold into amino acids, which serve as essential nutrients for yeast. Whatever is not consumed as a nutrient has a high potential to contribute to the umami and overall flavor profile.

It is known in the world of brewing that an excess of amino acids can inhibit ester-forming enzymes. It is also understood that this leads to the increased production of higher alcohols—often described as "alcoholic" odors—making it difficult to control the fermentation process.

From this perspective, the significance of polishing away excessive protein becomes clear.

Rice Polishing Ratio and Ester Formation - Lipids

It seems that polishing away lipids, not just proteins, is also crucial. In beer, lipids are a cause of off-flavors known as oxidation odors, and since sake is also constantly exposed to oxygen, one would expect this to be a concern.

In terms of ester formation, it appears to be related to the production of ethyl caproate, a fatty acid ester.

Ethyl caproate is a compound formed by the reaction of caproic acid and ethyl alcohol. When caproic acid is abundant, it dissolves well into the sake, resulting in that signature "Ginjo aroma"—often affectionately called "capu-echi"—that defines premium sake.

Caproic acid is a fatty acid. When there is an abundance of lipids within the yeast cell membrane, the yeast determines there is no need to produce its own, and consequently, production remains low. By polishing the rice to remove these oils, the yeast "thinks" it must produce its own, leading to the increased synthesis of caproic acid.

Caproic acid also seems to be produced in abundance during low-temperature fermentation. It appears the yeast actively generates it to keep the cell membrane from becoming too rigid in the cold, as caproic acid is essential for maintaining membrane fluidity.

That is the gist of it. Isn't the rice polishing ratio fascinating?