Sake Brewing Science vol. 2 - What happens when the rice is different?

I realized that it is important to study sake brewing thoroughly, so I have planned a series to decode sake from an enological perspective.

First, starting with rice, the raw material.

vol.2 - What difference does the rice make?

Rice is very familiar to Japanese people. It seems there are two main types grown in Japan: "table rice" for eating and "sake rice" for drinking. I would like to reconsider what differences these types create.

What are the differences in sake rice?

Sake rice (sake-brewing suitable rice) has characteristics that differ slightly from table rice varieties like Koshihikari or Akitakomachi, which is why it is considered suitable for sake brewing.

① Large grains with a high rate of shinpaku (white core).
② Easy to polish and resistant to cracking.
③ Low in protein and lipids.
④ Appropriate hardness.

There are other factors like harvesting performance, but since that doesn't relate to the flavor, I will omit it. (Disease resistance is often mentioned for hops, but I feel brewers should focus on more pure elements.)

Large grains with shinpaku - As covered in vol. 1, sake brewing involves polishing the rice to remove the outer layers. Larger grains are preferred because they allow for more leeway in how much of the outer layer can be removed. Shinpaku is the white, starchy core seen in the rice. It is said that when there are gaps, light enters and diffuses, making it appear white. Rice with a high probability of this composition allows koji mold to easily take root in those gaps, making it easier to create good koji.
Easy to polish and resistant to cracking - The same as point 1. However, it seems if the shinpaku is too large, the rice becomes brittle at a certain point, making it unsuitable for high-polishing.
Low in protein and lipids - Some rice types remain high in these even after polishing, and since they tend to produce richer flavors, they are considered unsuitable for styles like Junmai Daiginjo. However, that ultimately just means they produce a different style of sake, so I personally don't think "suitable" or "unsuitable" matters that much.
Appropriate hardness - My chosen theme for today. "Hardness" seems to be an indicator of "difficulty in dissolving," meaning it is less susceptible to enzymatic reactions in the mash. In terms of composition, it refers to rice with high levels of amylose, which has a hydrophobic structure. Rice with high amylose is dry and lacks space for moisture to penetrate.

What happens when it's hard?

You might wonder why we need to discuss whether the rice is hard or not. How does that affect the taste?

As I mentioned earlier, hard rice creates a state where enzymes have a harder time working. Therefore, in sake brewing—where enzymatic action by koji and alcohol production by yeast happen simultaneously—this means a slower supply of amino acids and fermentable sugars.

In fact, the relationship between hardness and enzymatic efficiency has been confirmed in various studies. A 2020 study reaffirmed that "the time required for saccharification was shorter for glutinous rice than for table rice." Glutinous rice can be recognized as extremely "soft rice" with 0% amylose content.

It is also said that high protein content can increase the bonding between sugar molecules, raising hydrophobicity and contributing to hardness. In any case, the important point is that rice described as "hard" often results in a slower supply of nutrients to the yeast, leading to a longer fermentation period.

This is getting interesting.

What happens when the fermentation period becomes long?

Generally, what happens when the fermentation period is long?
In the case of beer, it overlaps with "storage," and up to a certain point, it contributes to the reduction of off-flavors, which is sometimes described as careful crafting.

How about in sake?
I have often seen articles and texts stating that hard rice correlates with Ginjo aroma. Is there a connection?

The Ginjo aroma, which we also covered in vol. 1, is caused not only by rice polishing but also by low-temperature fermentation. It is highly likely that yeast produces abundant caproic acid to increase cell membrane fluidity even at low temperatures, and as a result, these react with alcohol to produce ethyl caproate (the apple-like Ginjo aroma).

It would be interesting if hardness and low-temperature fermentation were related.

The correlation between hard rice and low-temperature fermentation

I couldn't quite grasp the logic behind how hardness leads to Ginjo aroma, but it seems plausible if low-temperature fermentation is involved. It is easy to understand that hard rice, where enzymatic reactions by koji do not progress easily, could slow down the nutrient supply to yeast and reduce the fermentation rate. This could lead to less fermentation heat, and if the ambient temperature is low, the fermentation temperature could drop further, leading to a potential spiral of further slowing down.

But then, one might think, "Why not just ferment at a high temperature?" I thought so too. But as far as I have researched, a style of "hard rice x high temperature x long-term fermentation" doesn't really appear. Maybe I'm just searching the wrong way.

However, compared to beer, sake involves keeping the tank open for longer, and if fermentation is prolonged, the risk of contamination by unwanted bacteria increases significantly. Therefore, it is quite understandable that low-temperature management is paired with long-term fermentation.

Could this be increasing the production of ethyl caproate?

"Hard rice results in long-term fermentation, so it must be managed at low temperatures."

"If you ferment at a low temperature, a lot of ethyl caproate is produced."

"Hard rice produces a lot of Ginjo aroma."

Could this be the syllogism? What shall we do next?